54280995 the Self Reliant Potter Refractories and Kilns 1987

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  • 8/6/2019 54280995 the Self Reliant Potter Refractories and Kilns 1987


    MlCROFICHEREFERENCELIBRARYA project of Volunteers in Asia

    The Self . . l.Rew Potter. Re&gtonemd lQ&By: Henrik Norsker

    Published by: Friedr. Vieweg & SohnVelagsgesellschaft mbHBraunschweig, Germany

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    Henrik NorskerThe Self-Reliant Potter:I Refractories and Kilns

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    Deutsches Zcntrum fik Entwicklungstechnologien - GATEDeutsches Zentrum fiir Ent\U&mgstechnologien GATE stands for German Appro-priate Technology Exchange. It was founded in 197Xas a special division of the DeutschcGesellschafi fiir Technische Zusammenurbeit fC;TZ) GmbH.GATE is a centre for the dissemination and promotion of appropriate technologies fordeveloping countries.GATE defines ..Appropriate technologies as those which are suitable and acceptable n thelight of economic. social and cultural criteria. They should contribute to socio-economicdevelopment Lvhilst ensuring optimal utilktion of resources and minima! detriment to theenvironment. Depending on the case at hand ;I traditional. intermediate or highly-devclopcdcan be the ,.approprinte one. GATE focusses ts work on three key areas:- Twh~~~/r~~~~~klr~ur~~~~:ollecting. processing and disseminating information on technolo-gies appropriate to the needs of the developing countries; asctxtaining the technologiciilrequirements of Third World countries: support in the form of personnel, material ;rndeqcipmont to promote the development and adaptation of technologies tix developingcountries.Rr.wtrrc*h ml/ I)odttptttcwr: ~ollductinp and or pr~~lllotillg rcSWrd1 N-IL! d~?vc!opnicn1work in approyri;tic rxhnologies.(f~optwlicdr itr ?;~c~lttrokt.~ rc.~rl~c~lr~lc~~ttttc~tt~.~cN:iXJrilti~~fl in the Itxm 0fJolnt projects \\.it hrelevant institutions in developing countries and in the l-e:JcruIRcpuhlic 1>1krllliIn>.For scvcrd years GATE HIS !XX~ itI iIcti\.e supporter ofthc SATIS network (Socii~ll~Appro-priate Technology Information Services) ttld has entcrcd into cooperiiticln qrcements with ;Inumber of technology centres in Third World countries.C;A!X ofterst free information serviceon appropriilte technoiogks for ii11 public and priviltcdwelopment institutions in dewloping countries. dealing with the development. ildapt;ition,introduction and upplic:~tion of technologies.Deutsche Cesellschaft fiir Tcchsische Zusammenarbeit (CTZ) GmbHThe povernment-owned (;T% operates in the Iicld of Technical Coupcri~tkln. 2 200 (krmanevpcrts ;Ire \\orking together with partners Iwni about I00 countries of .4IriCil. ASiiI andLatin .4mcrica in pwjccts cavering priICtiCaI!y every sector of agriculture. fkxtry. ccunomic

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    Henrik NorskerThe Self-Reliant Pot.ter:Refractories and Kilns

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    The Author:Henrik Norsker has been making pottery since 1970. He left his pottery workshop inDenm:.irk in 1976 to establisl; ;: pottery school in ~1 illage in Tanzania. Since then hehas continued working in developing countries uith the promotion of modern pot-tery. Hesides Tanzania he has been involved in ceramic projects in Nepal, !ndia andBangla.lesh. He is presently working on ;t pottery project in Burma.

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    This book about how to construct and tire kilns addresses rainees and practising potters indeveloping countries. It intends to assist potters to become more self-reliant by providingadvice on the optimum use of locally available raw materials and by explaining techniqueswhich will help potters to increase their production and their income.The first section of the book describes he whole sequenceof producing refractory items. Thesecond section deals with different kiln types and their functioning principles.The various methods of constructing kilns are treated in a practical way with comprehensiveillustrations. Finally instructions are provided L:? the loading and tiring of kilns and ondifferent methods of measuring temperature, including a thorough description of how pot-ters can make their own pyrometric cones.

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    Preface ......................................................l.Refractories .................................................




    fntroduction. ............................................Refractory raw materials. ....................................1.2.1 Kaolin ...........................................1.22 Fireclay ........................................12.3 Aluminous materials .................................1.2.4 Bauxite ..........................................1.2.5 Laterite ........................................12.6 Silimanitc, kyanite. andalusitc ..........................12.7 Zircon ..........................................1.2.8 Silica ...........................................1.7.9 fow to get refractory materials ...........................Production of refractory items ................................1.3.1 Clay cleaning .......................................1.3.2 Grog ............................................Kiln furniture ...........................................1.4.1 Saggars nd slabs ....................................1.4.2 Thermal shock ......................................1.4.3 Saggars .........................................1.4.4 Shaping .........................................1.4.5 Kiln shelves ......................................



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    2.1.2 Sinde up-draught ki!n .................................2.1.3 Bangladeshup-draught kiln ..............................2.1.4 Permanentup-draught kilns .............................2.1.5 European up-drauy- kilns ..............................2.1.6 Down-d+++ t .IVS ..................................2.1.7 Khurja kil ....................................2.1.8 Mayangoneali ......................................2.1.9 Bujora down-draught ..................................2.1.10 Cross-draughtkilns ...................................2.1.11 Tubekilns ..... ...................................2.1.12 Chinesechamber kiln .................................2. I .I 3 Champaknagarchamber kiln .............................2. .14 Sumvecross-draughtkiln ...............................2.2 Choice of fuel ...........................................2.2.1 Firewood .........................................2.2.2 Agricultural waste ..................................2.2.3 Peat .............................................2.2.4 Lignite ...........................................2.2.5 coal ............................................2.2.6 Oil products ....................................2.3 Combustion and fireboxes ...................................2.3.1 Combustion ........................................2.3.2 Firewood tirebox ....................................2.3.3 Sawdust firebox .....................................2.3.4 Coal fireboxes ......................................2.3.5 Oil drip firing. ......................................2.3.6 Pressure urner system ................................

    2.4 Heat transfer and draught ....................................2.4.1 Transfer of heat through air .............................2.4.2 Transfer of heat through solids ...........................


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    2.5.12 Maintenance of kilns .................................. 1042.6 Loading and setting of the kiln ................................ 1052.6.1 Loading biscuit firing ................................. 1062.6.2 Loading glaze iring. .................................. 1072.7 Kilnfiting .............................................. 1122.7.1 Biscuit firing ....................................... 1132.7.2 Glaze firing ........................................ 1162.8 Temperature measurement ................................... 1192.8.1 Thermometers ....................................... 1192.8.2 Colour ........................................... 1192.8.3 Test draw ......................................... 1192.8.4 Cones ............................................ 1202.8.5 Pyrometer ......................................... 1212.8.6 Self-madecones ..................................... 122

    Appendix .................................................... 126Tables of weights and measures ................................... 126Table of Segercones ........................................... 128Table of Orton cones. .......................................... 128Averageproperties and measures ................................... 129Mohs scaleof hardness. ........................................ 130Temperature conversion ......................................... 13 1Useful formulas. .............................................. 132Bibliography. ............................... : ................ 134

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    The idea of writing a ceramic book specifi-cally to suit conditions in developing coun-tries originated from my personal experienceand associatedproblems whilst 1 was strugg-ling to set up modem pottery production ina Tanzanianvillage en years ago.When1was apotter in Denmark,ceramic raw materialsandkiln refractories had only been a question ofwhich supplier to contact whereas u Tanza-nia we had to find our own clay and glazeminerals, produce firebricks and kiln slabs,and construct the equipment locally. Fromthat experience I realized the shortcomingsof my former training and how difficult itwas to extract appropriate technology fromcurrently available ceramic literature. Thisliterature mainly addresses tself to a marketcomprising amateurs, art potters and indus-trial engineers n developedcountries. Cener-ally, the hobby books are too basic and theengineering books are too advanced to be

    The aim of this book is not to enable some-body without practical pottery experienceto start up modem potttry production onhis own. The book is mainly written forthe benefit of potters already involved withmodem pottery, and for teachers and stu-dents involved with the growing number ofpottery training centres and institutes indeveloping countries.GATE is planning to publish more technicalbooks on ceramic technology and thesewould cover the subjects of glazing, clay pre-paration and shaping methods. GATE invitesusers of this book to forward their com-ments and any suggestions regarding theplanned future seriesof ceramic books.

    AC:-: owledgments

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    who has also contributed to the book with Finally I owe thanks to my wife Tin Tinhelpful suggestions nd photographs. Moe for her encouragement and patienceKaung Kaung 00 has helped with working with the writing of the book.drawing; for some of the kilns.Peter Nauman has produced the majority of My thank.; to all of you.the drawings and has had the tedious task ofcorrecting my English and proof-reading themanuscript.The manuscript has been typed and retyped Rangoon, 27th December 1985several imes by Nan Win Moe. Henrik Norsker

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    ls Refractories

    1.1 IntroductionFor the constructian of k&s it is necessaryto use bricks and mortars which will endureintense heat. For glaze firings it is also usu-ally necessary o have materials for stackingpottery in the kiln chamber Saggars.kilnshelvesand props are examplesof kiln furni-ture.Industrial srandardBy industrial standardsa clay is called refrac-tory when it does not soften below 1580 C.However, in most caseswe will have to besatisfied with clays that soften at a muchlower temperature because real refractoryclaj;. may not bc available or is too expen-sive. In any case, most potters will not bringtheir kiln above 1250 C and will only main-tain the maximum temperature for a shortperiod.

    Potter k refractoryFor the purpose of this book the term re-fractory will cover clays and materials thatare suitable to be used n a potters kiln firedup to 1250 C. In case he kiln is to be tiredat a lower temperature, it might be possibleto use ordinary building bricks and saggarsmade by less refined methods than thosedescribed below. However, the principles re-main the same and the additional effort willoften be rewarded by a longer life for thekiln and kiln furniture.

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    1.2 Refractory raw materialsin most cases efractory items for ordinarypotteries will have o be made of clay.

    1.2.1 KaolinKaolin, also calied China clay, is the best re-fractory ciay type. A pure kaolin clay wiUnot soften below 1750 C. Kaolin has beencreated by the decomposition of feldspar(fig. l-2).Prirnqv clavPure kaolin is found at the site of its parentrock (primary clay) and has not been mixedwith impurities which would reduce its re-fractoriness and change its colour. Kaolinclays possess ittle plasticity due to theirlarge clay particles.PorcelainPure white burning kaolin is much in de-mand for making porcelain and is thereforeexpensive. However. for the production ofrefractory items, kaolin firing to a buff co-lour is acceptable.

    siderable amount of sand which is left be-hind when the parent rock has changed ntoclay. Sometimes only a small part of the pa-rent rock is changed into clay ad in othercases raw kaolin occurs in pockets amongstgranite rocks. The raw kaolin is normallya) Ancient times

    bl Ancient times - weathering

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    white but some types of rock produce a pink-ish colour which may still be a suitable.re-fractory clay. Solid firebricks can often bemade from raw unwashed kaolin. Kaolin isalso used in the production of paper andrubber.

    1.2.2 FireclayFireclays are produced in the same way askaolin but have been transported away fromthe location of the parent rock (secondaryclay). Fireclays are also refractory, but oftenmore plastic than kaolin. The colour of rawfireclays varies from white to yellow, brownor grey, and the sand content can be morethan 50%.Sometimes the term fireclay is used only forthe clays lying below and between coal-seams. Such clays do not occur under allcoal-seams nd they might not always be re-fractory. However, there is a good chance offinding a suitable fireclay where coal-seamsare ocated. Even under inferior coals such aslignite it is sometimes possible to find suit-able clays.

    Bauxite is the raw material from which themetal aluminium is produced. It is found inmany places though only a few deposits areutilized. Even deposits which are not suit-able for aluminlum production may be use-ful to the potter.Bauxite grogRed bauxite is less refractory than white orgrey bauxite. The bauxite has no plasticityand needs a binding clay. The raw bauxiteshould be ground, mixed with 25% plasticclay, shaped n rough bricks which are firedto about PO0 C, and then crushed. This ma-terial can then be used as ordinary grog in arefractory body. A standard mixture is 75%bauxite grog and 25% fireclay. Bauxite grogcan also be used as a substitute for a portionof the grog in the production of ordinary re-&actor-y bodies.CalcinationThe process described above of firing therough bricks of bauxite is called c&nation.The raw bauxite cannot be used withoutcalcination (calcination at 700-900 C;above 1000 C bauxite becomes hard to

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    1.2.6 Silimanite, kyanite, andalwiteAI: OS Si02Melting point: 1850 CDensity: 3.2-3.6 g/mlHardness: 6-7Silimanite is found mainly in India whileandalusite and kyanlte are more wide-spread. Although these materials may costtoo much for mos! potters, they are good re-fractory materials and produce long-lastingkiln shelvesand saggars. n India some small-er potteries have started to use silimanitefor their saggars nd found this to be econo-mical as it has extended the life of the sag-gais. The three materials are rather similarexcept that silimanite and andalusite can beused raw while kyanite needs to be calcinat-ed above 1350 C at which temperature itexpands by 17%. (If intended for use below1350 C the calcination may be omitted.)The materials are non-plastic and can beused in mixtures as grog. A saggar bodycould be 60% silimanite, 30% fireclay and10%plastic clay.

    1.2.7 ZirconZrSi04

    shelvesand saggars.The kiln wash is madefrom either pure zircon mixed with water orwith the addition of kaolin. The wash pre-vents glazed ware from sticking to the set-tings.

    1.2.8 SilicaSi02Melting point: 1710 CDens%-: 2.6 g/mlHardness: 7Silica is found as part o f rocks and clays andit is so common that it makesup 60% of allmaterials n the crust of the earth.OccurrenceAs a free mineral, not combined in clays androcks, it occurs as quartz rock, silica sand,sandstone, Qint pebble and as semi-preciousstones such as agate, opal and asper.Refractory


    The addition of silica makes a clay mixturemore refractory. However, items exposed tosudden temperature changesshould containas ittle free silica as possible.Some forms of

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    r uartztemperature C

    Fig. l-3: Three forms of silica with different rates of expansion. The cr;~.&ls of crystoballite expand sudden-ly at 220 C and the quartz crystals expand suddenly at 573 C. \vhzf! silica has no crystal fo rm as in silicaglassor ash t expands evenly.

    of small quartz crystals but B particular sandmay contain many other minerals whichmay reduce its refractoriness. n general, hewhiter the sand, the purer it is. White beach

    C.33ulogikalnstitutesOther sources of information are geologicalinstitutes and mining corporations but

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    Fig. II: Clay mining in Bangladesh

    poses such as whitewashing houses. Alsocontact loca! well-sinkers who should havesome knowledge of what soils are hidden be-low the surface. Apart from kaolin, fireclayand silica sand, width can usually be foundlocdly, the other refractory materials listedabove normally have to be purchased from asupplier.

    1.3 Production of refractory items1.3.1 Clay cleaningSome refractory clays can be used as dug forthe production of firebricks but usually.

    Fig. l-5: The day is stirred in the pond to theback. It is then poured through a screen into thesmall p it from where it runs to the fwo lower sett-ling ponds.


    For the clay cleaning, two or more shallowponds (for example 4 x 2 metres and 1/2me-tre deep) should be dug in the ground closeto the clay source. The sidesof the pond canbe made of brichwork but simple wicker-work plastered with clay will do. The pondis half-filled with water and clay is added un-til the pond is filled. The , rw clay is stirredwith a shovel until a!1 he clay particles areseparated from the coarser sand. With coars-er types of clays, like kaolin. the stirringmay not take more than 30 minutes but

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    Fig. l-b: After setthnp the washed kaolin clay isdried in the sun.

    led the clear water on top should be run offcautiously without stirring the settled clay.The water can be reused by transferring it tothe first pond by the help of a pump or bybucketing. The bucket or the pump inletshould not be dipped into the settling pondbecause hat would stir the clay. Instead thesurplus water should be conveyed to a smallthird pond f rom where it can be returned tothe first pond. If there is a sma!l streamnearby. the waste material can be used formaking a small dam to provide water for theclaywashing.Washmill

    which is fusibie compared to mo;e refrac-tory silica s?Ad.) From time to time the trayshould be turned upside down for cleaningand occasionally the washmii wil! have to beemptied of sand.

    1.3.2 GrogGrog is burned clay which has been crushedto grains of various sizes. t is used for mak-ing solid firebricks, saggars nd slabs,etc. Thegrog is mixed with a plastic clay which bindsit together. The additiona! firing and crush-ing make it more costly to use grog insteadof raw clay but the benefits soon becomeobvious.These are:1. The firebricks or saggarsare much less i-kely to crack with sudden changesof lempe-rature.2. They will better withstand loads withoutbending.3. The tendency of spalling is greatly re-duced.4. The drying shrinkage is reduced as lesswater is used n the clay and grog mixture.5. F iring shrinkage is reduced because thegrog has already been fired once. Generallythe !ligher the con ent of grog the better the

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    Fig. l-8: A manual hammer for grog crushing

    Hroken saggers

    Fig. 1-9: A pan grinder supplied with a double-deck screen grades the crushed grog in two sizes.

    the grog which is placed under the metal orstone hammer by a second person.

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    Fig. l-10: The inside of ;i hammer mill. The hammers swing out when they rotate. The grog is fed throughthe crntre hole to the right and Iewes through a screen (removed) on the left.

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    1.4 Kiln furniture

    1.4.1 Saggars nd slabsSaggarsare used for protecting the glazedware against the action of flue gases andashes rom combustion of solid fuels, whichotherwise might cause discolouration *mdgive the glaze a rough surface where ayh hassettled on the ware. At higher firing tempe-ratures firewood ash melts together with theFig.1-l I : Saggus stacked 3 m high

    glaze and if the co!txr effect of the ash isacceptable an open setting with kiln shelves,also called slabs, is preferable. Saggan areheavier and take up more spacecornpart d tothe same wei@ ot kiln shelves.However, fthe height c:f the setting is more thdn twometres a setting with kiln shelves ,:nds tobecome t;Jo unstable whiie sagpars can bestacked to above four metres. The sameclaymixture can be used for both saggarsandslabs though clay mixtures ibr kiln shelvesneed essplasticity.Fig. 1-l 2: Open setting on kiln slabs in ;1 kiln firedwith wood to 1280 C

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    Grog contentAs a rule of thumb the mixture should con-tain as much grog as the shaping techniqueallows. Normlly that is 40-60% but it de-pends on the plasticity of the bond clay. Itis unusual to find a clay being both highlyplastic ann refractory, and so usually thebond clay is made from a mixture of a stone-ware clay or a vitrifiab le plastic clay and arefractory clay such as kaolin. The bond clayshouid start to v itrify at a low temperaturebut should not soften before well above hefiring temperature. The kaolin crystals inthe clay start slowly to change nto mullitecrystals (fig. l-13) above 1000 C. Mullitegrows irlto long needle-shaped rystals whichform a lattice that will reinforce the firedclay in much the same way as iron bars inreinforced concrete. This lattice-work enab-les the kiln furniture to carry the load of thew3re at high temperatures. The partly melt-ed mass between the grog particles will en-able the mullite crystals to grow freely. Ifthe bond clay was too refractory the needlecrystals could not grow properly and theslabs would bend. This can be seen rom thefact that newly fired slabs tend to bend.However, after they are fired a few times

    and the lattice-work is allowed time to growthey no longer bend.As the right proportion of grog and bondclay depends on the quality of raw materia ls,firing temperature and shaping technique,the local potters will have to find their ownrecipe by trying a number of different mix-tures.R ecivesThe following recipes are practical examplesof saggarbodies: (parts in weight)recipes bsaggarclay 3: 25 3;kaolin 15 15 30grog: 55 60 30Bodies for making slabs ciLrrbe made with ahigher content of grog compared to bodiesfor saggarproduction.

    1.4.2 Thermal shockIn mcst cases he potter will be more troub-led with cracking of saggars or slabs thanwith fusion and softening of the ki!n fumi-ture. Due to their shape, saggsrs end tocrack more easily than slabs but the problemis often caused by the same problem, ther-

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    ing and cooling should be done slowly (seep. 115 f.). The following can be done to re-duce the problem of crxking:1. Reduce the amount of sand free silica) inthe clay. Sand is not the only source of freesilica. Clay produces free silica when heatedabove 1000 C. Kaolin-type clay releasesabout 36% whereas other clays as montmo-rillonites (bentonite) release up to 60 5%.Thus a changeof bond clay should be cord-ered too.2. Increase he amount of grog to make thefired body more porous. A porous body W8more easily accommodate tensions than adense body. Porosity of saggars hould be18-25% (seep. 42j.3. If the firing temperature s below 1250 Can addition of 5-12% talc will improve resis-tance to thermal shock. Tdc will reduce themelting point and lherefore can be usedOnlyat lower temperatures. Talc is used for mak-ing corderite bodies Hrhichhave a high resis-tance to thermal shock. The formation ofcorderite is difficult to achieve.4. Biscuit-fire the kiln furniture.5. Change he firing and cooling scheduie 6ensureslow change of temperature at 230 C

    and 573 C. One pottery found that saggarslasted 6-l 1 fir@, when cooling of the kilntook 24-72 hours. The samesaggarsasted

    Fig, -14: Verticalpugmill for claymixing, A simi-lx ptlgmill can e poweredy an ox.alternate layers on top of each other. Eachlayer is watered as required. After one daythe clay is soaked and the mixture is turned

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    Fie. I-l 5: Different shape.\ 111 agprs

    by the formi:lg method, i.e. throwing andjolleying will produce only round saggurswhereas with hand-moulding and sLp-castingmore shapesare possitle.Also, separate sides and bottoms will reducethe stress on the saggars due to thermalshock, but it demandsaccurate shaping.Saggarscan be made by five different me-thods: a) thrown on a wheel, 0) jigger-jolley-ing, c) hand-moulding, d) press-mouldingand e) slip-casting.a) Throwing

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    Fig. I-17: Jalley machine with rotatlq! pbstcrmould and template in this case for m;lking plates

    li) Jigger-jollcyingA saggarby this method is normally formedinside a rotating mould (jolleying) by thepressing of a template. This method is espe-cially suitable for shaping of smaller saggarsup to 20 cm in diameter. The saggars an bemade into shapes which allow a dense set-ting. A normal potters wheel can easily beequipped to work as a jigger-jolleying ma-chine. The moulds are usually made of plas-ter of Paris but can also be made of clayburned below 900 C to give the mouldsFig. I-18: Plaster moulds filled with drying saggar\

    high porosity. The saggar cla) should besofter than clay for throwing. The mouldshould be slightly wet before throwing therequired amount of clay into it. The clay ispressed nto shape by lowering the template.For bigger saggars t is necessary o press heclay out evenly inside the mould by hand be-fore lowering .the template. Excess clay iscut o ff at the rim and the mould is lifted offto be replaced by another.Depending on the clay and the weather eachmould can be used 2-4 times a day.c) Haa,td-mouldingThe saggar lay for hand-moulding should bestiff. An iron ring or frame slightly biggerthan the bottom of the saggar s placed on aboard. The board is dusted with fine grogand saggarclay is thrown into the frame andbeaten out until it f ills the frame. Excessclay is cut off by a wire and the frame is re-moved. The sides of the saggarare moulded

    Fig. l-1 9: \vooden mould for saaar-mak ing

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    joining slab and base

    t removing mould


    Fig. I-20: H and-moulding of a saggar

    into a long slab of clay paste between two

    Fig. 1-21: A saggar-moulding shop. On the table tothe right the skbs of bottom and sides are beatenout. On the revolving table to the left the bottomand sides are joined and made smooth.

    While the drum is still inside, the outer sur-face is made smooth with a steel blade orsponge. After removing the drum the insideis also made smooth, The saggar s left tostiffen a bit and is then turned over so thatits bottom can be levelled and made smooth.

    d) Press-mouldingSlabs and saggarscan be pressed n a steelmould. Pressure is applied by a fly-wheel

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    Fig. l-22: Slabs, saggars and firebricks can bemoulded in iI fly-wheel press. Fig. 1-23: Release of ~ggar from an electricaly-powered fly-wheel p ress

    e) Slip-casting absorb the water in the clay slip and the claywill harden. After some time the clay shapeSlip-casting is done by pouring a clay slip can be taken out. Saggarsare normally castinto plaster of Paris moulds. The moulds will in solid cast moulds (fig. l-24).Fig. 1-24: Plaster mould for solid slip-castings of saggars

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    Slip-casting has the advantage hat the shap-ing does not require plastic clay and so themixture can contain a much higher propor-tion of grog compared to the other methods.Although the ceramic industry uses this me-thod extensively, smaller potteries may ex-perience difficulties due to the cost andavailability of plaster. Furthermore, chemi-cals such as water glassor sodaare needed ormaking the clay slip fluid with a water con-tent equal to p lastic clay (20-30%). Withoutthese chemicals the water content Leeds tobe 40-50%.

    1.4.5 Kiln shelvesFor an open setting, square flat kiln shelves,also called bats or slabs, re used. These arenormally made by hand-moulding althoughthey can also be press-moulded and slip-eaL.i. Gay mixtures and clay preparationfor hand-moulding are similar to those ofsaggar-making though a higher content ofgrog is permissible and the clay paste shouldcontain lesswater (semi-dry).Fig. 1-X: Kiln slabs used in a shuttlc kiln

    FormingAn iron or wor&n frame having the shapeand thickness of the finished bats is placedon a s&d bench or on a concrete floor andis sprirtied with grog dust. The semi-drycl?y paste is gradually added by starting atone end of the frame while beating the clayconstantly with a wooden hammer. Thestroke of the hammer should always havethe same direction, opposite the direction offilling the frame. After ftiing the frame com-pletely the surface is levelled by running awooden stick on top of the frame. The sur-face is made smooth by a sponge and a steelblade used alternately. A plate fitting exact-ly inside the frame is placed on top of theslab while the frame is lifted off. The foursides of the clay slab are carefully madesmooth and the slab is left to stiffen forabout a day. It is then turned over .and tsbottom is made smooth.

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    making and the fling and cooling of the kilnis done carefully, a saggar ife of lo-20 fir-ings may be possible.Slabs will normally withstand many morefnings than saggars.

    I .4.8 Glazed ware supportVarious types of supports for the setting ofglazed ware make it possible to place thepots more tightly in the kiln, thereby im-proving the firing economy. In the chapter*Loading and setting of the kiln page 105 ff.,a number of d ifferent supports are described.

    Clay body

    Supports such as spurs, thimbles and stiltsare made by press-moulding. The body forthis should be made from a fine-grained fire-clay or by mixing kaolin, silica sand andplastic clay in the following proportions:kaolinplastic claysilica sand(by weight)


    The clay body should be screened with afine mesh sieve 80-100 mesh, seep. 127).

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    Fig. l-29: Thimble press mould with ejectorFig. I-30: Tile setters for stacking glazed roofing tiles

    Press-mouldingThe clay body should be press-moulded n asemi-dry state with a water content of lo-15 5%. he higher the pressure applied in themould the less water is needed. The mouldcould be made of mild steel or brass f a everpress as shown in fig. l-28 is used. Themould should be made with a simple ejec-tion device, which will push the finisheditem out of the mould. The mould should begreased with oil before each ftiing in orderto ease the release of the press-mouldeditem. Alternatively oil could be mixed withthe clay body. The mould could also bemade of plaster or clay, but then less pres-sure should be applied.

    CrankThimbles can be used for stacking flatwaresuch as plates and tiles on top of each other

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    Fig. l-31 : Flat plates stacked tightly in a crank

    as shown in fig. l-31 provided that the flat-ware is made exactly samesize.A bottom and top plate each with three frx-ed sockets for the thimble pillars hold thesetogether and the top plate also protects theware from kiln dust. This kind of arrange-ment is called a crank and may hold 10-l 5pieces. The top and bottom plates are madein a flat mould and formed in the samewayas kiln slabs. A template should be made for

    Fig. l-32: Setting of pan rings for stacking platesand bowls

    Fig. l-33: Two different waysof setting the wareon pan rings

    tra care is needed to ensure that the step ofthe pan ring is filled completely with clayand that the step does not break off whenthe pan ring is released rom the mould .The curve of pan ring is made with a radiusthat fits the size of plates or bowls to be

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    1.5 FirebricksFirebricks are used for the construction ofa potters kiln and are also used in manyother industries such as glassworks, found-ries and boilers. If the potters can successful-ly produce firebricks for their own kilnsthey may be able to earn extra income byselling firebricks to these other industries.Industry today uses a number of differenttypes of firebricks according to specializedrequirements, but this book will deal mainlywith solid and insulating firebricks madefrom clay.Solid firebricks are used for the fireboxes,chimney, bagwalls, floor and flue systems,while the kiln lining may be made of insulat-ing firebricks.

    15.1 Solid firebricksProduction o f firebricks is less critical w!lencompared to saggarsbecause irebricks arenot exposed to as sudden temperaturechanges and rough handling. Furthermore,the shaping of firebricks demands essplasti-city from the clay. Some fireclays and kaolinclays can be used as dug. which is an econo-mical method to produce solid firebricks,but grog may be a worthwhile addition toimprove their refractory quality. The pro-portion of clay to grog will vary accordingto the plasticity of the clay and the condi-tions to which the firebricks will be expos-ed.

    Fig. l-34: Construction of a test kiln in Bhnktapur, Nepal. The insulating; firebricks for the inner lining werefired in il pit firing.

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    Two gradesThe addition of grog increases he produc-tion cost and it may be preferable to pro.duce two grades. For example, firebricks forfireboxes, grates and bagwalls can be madewith the highest possible content of grog(60-80%) while the rest of the kiln can bemade with less grog (ZO-40%). The samegrading could also be used for the bond clayso that first-grade bond clay, which has hadits sand fraction removed, will allow for ahigher content of grog.Bond clayThe clay binding the grog together should beless refractory than the grog. Otherwise thebrick will become brittle after firing. But thebond clay should not vitrify excessively orfuse because f the firebrick becomes toodense it will tend to spa11 fter long usethough a high grog content will counterba-lance this tendency. Often the best solutionis mixing two different clays, e.g. a fusiblestoneware clay with a fireclay or kaolin clay.The proportion will depend on the qualityof the clays and the intended firing tempera-ture.

    Fig. l-35: Slop-mou lding of bricks

    Slop-mouldingSlop-moulding is done with a very soft clay

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    prepare a lump of clay by bumping it severaltimes on the table, giving it a square formwhich is slightly bigger than the inside of themould. From above the head the clayshould be thrown with full force into thewell dusted mould. The clay should fill allcomers of the mouid and is then ievelled offat the top with a stick. The mould is thenlifted, with the brick inside, and an assis-tant can carry the mould to the dryingground where it is emptied by gently knock-ing the mould. Normally the bricks can beplaced on their edge immediately. For fire-bricks, dusting is done with fine grog andnot sand.Special shapesWedge and arch bricks are moulded iikesquare bricks but in specially made frames.Small numbers of special shapes, such as

    Fig. I-40: Cutting of special shapes with the helpof two templates

    bricks for skewbacks or rounded bricks forflue channels, can be made by cutting fresh-ly moulded square bricks. The square brickis placed between two templates of woodwhich have the desired profile, and a wire isFig. 14 1: Stacking bricks for firing in B clamp kiln. When al1 bricks are set the kiln is sealed with a plasterof clay and firing takes place in the two fireboxes.

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    then drawn along the templates, cutting thebrick (fig. 140).Firing solid firebricksThe problem in firing solid firebricks is thatthey should be fired at a higher temperaturethan the one at which they will be used la-ter. (The same applies for insulating fire-bricks and kiln furniture.) If the pottery isalready one that is in production then thebricks should be fired next to the bagwallsor in other hot spots in the kiln. In case nokiIn is at hand, the bricks will need to befired in a clamp kiln where the firing tempe-rature will seldom exceed 900 C.Kilns made of these ow-fired bricks will tendto crack more than usual. This is caused bythe extra shrinkage of the firebricks whenthey, as part of the brickwork in the newkiln, are exposed to a much higher tempera-ture.

    1S.2 Insulsting firebricksInsulating firebricks are made of a mixtureof fireclay and sawdust. Other combustiblematerials such as coal, lignite, peat, ricehusks, etc. can also be used as fillers and

    Fig. 142: Heat going through a sawdust insulatingbrick is stopped by all the pockets of air left by theburned out sawdust particles.

    Fig. l-43: If the insulating holes are too big the uirinside the holes can rotate and thereby heut istransferred through the brick.4. They are cheaper to make by using lessclay and needing no grog.For these reasons nsulating firebricks shouldbe used as much as possible. However, dueto their open structure they are sensitive toslag attack and cannot be used n salt glazingkilns.Insulating bricks will collapse at a lower tem-

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    Bond clay

    For a bond clay, the clay should be as refrac-tory as possible especially if the bricks arefor the inner lining. For a back-up insulationbehind an inner lining inferior clay may beused. The clay should have good bindingpower so that it can take a lot of sawdust.The binding power of the clay can be im-proved by the removal of its sand by wash-ing. A washed kaolin clay with the additionof lo-20% plastic clay often produces verygood bricks.

    Sawdust/clay mixtwesThe more refractoriness and binding powerthe bond clay possesses,he more sawdustcan be added to it. The potter will have totest a number of different mixtures and per-haps even different bond clays. Measuredbyvolume the sawdust content will be about40--60% with the remaining part bond clay.After adding water to the sawdust and clay,It should be mixed very thoroughly. Themixture is left a few days before moulding.


    Fig. l-44: Sawdust bricks are placed a fingersspace apart during firing.

    FiringThe bricks should be stacked in the kiln asshown in fig. 144. The burning out of thesawdust will raise the temperature rapidlyand it will be necessary o stop adding fuelwhile the sawdust bums. Otherwise, the ra-pid increaseof temperature will causedistor-tions in the bricks. The firing is easier tocontrol if the sawdust bricks are fired insmaller quantities along with other ware.

    1S.3 Ash bricksSilica in ash

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    1S-5 Mortars

    Mortars are used for joining firebricks in thekiln structure. They are also used for protec-tive coatings of brickwork such as the iningof fireboxes. The mortars should resemblethe bricks they join so that the joints andthe bricks expand and shrink at the samerate during a firing cycle. To enable an easylaying of the bricks the mortar should beplastic.

    GrogFine refractory grog (passing at least 24mesh) should be used for reducing shrinkagein the joints. If the joints shrink too muchthey will fall out after only a few firings.Sand can also be used instead of grog, buttoo much sand will cause spalling of thejoints. The amount of grog or sand dependson the bond clay, which may already con-tain sand. Usually grog makesup 50-65% ofthe mortar.Bond clayThe bond clay could be the same as thatused for making the bricks. The bond clay

    (a) fireclay 40grog 40 mesh 60(b) kaolin 25stoneware clay _ 8grog 24 mesh 67w sand 40WI3 40stoneware clay 20(6) grog (or sand) 1fneclay 2The mortar should be applied as thinly aspossible. In case arge gaps need to be filledby mortar it is better to add a lot of coarsefm?.Outer wallsThe outer walls are laid with common redbricks and a mortar made of a normal clayeysoil can be used for the outside. Where thewalls will be exposed to rain, the jointsshould be pointed with a sand/lime mortarin the proportion of five parts sand to twoparts lime.

    1.6 Testing refractoriesBagwalls, flue linings or saggars hat give in

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    1.6.1 High temperature testingCeramic instituteThe first thing we want to know is: can theclay withstand high temperatures? For this,a kiln which can withstand temperatures of1300-1400 C would be ideal for testing.Few potters will have access o such a kiln, but a sample of the clay could. be sentthrough local authorities to the nationalgeological department or ceramic institutewhich will normally be interested in gaininginformation about suitable clay sources.Production kilnQuicker results could be obtained by firingthe test piece n the flue, in front of the fire-box, or on top of the bagwall of a potterskiln. The temperature may not be 1300-1400 C but it is most likely thehighesttemperature the material will have to with-stand in practice.Fig. 146: Test kiln

    Test kilnin case no high temperature kiln is available,a small test kiln could be constructed. In ex-treme cases,where there are no proper re-fractories available for the construction of atest kiln, (this was once experienced by theauthor in Africa) the test kiln, built of theuntried refractories and fired to as high atemperature as possible, becomes the testitself. A small test kiln is also useful for fir-ing glaze and body tests and the one shownin fig. 146 is not expensive o construct. Bychanging the firebox arrangement it dan befired with firewood, oil or coal (p. 73-87).

    1.6.2 Refractory materials and bodiesIn most cases t will already have been estab-lished whether or not the type of clay inquestion is suitable for high temperaturesandthe individual potter or local pottery devel-opment centre will only need to check the

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    quality of clay supply and refractory bodymixtures. The following tests should be car-ried out with new batches of clay.SamplingThe clay to be tested should be collectedfrom at least four diffe:ent places at the claydeposit or from where the clay has beendumped. The four samples of about equalsize are mixed well on a swept concretefloor. The sampled clay is then divided intofour equal portions. Two portions oppositeeach other are set aside and the other twoare mixed thoroughly. This process of di-viding and mixing should be repeated at leastfour times. This method is called quartering(fig. 147) and ensures hat the final sampleis representativeof the bulk of the clay.Moisture con tentA sample of about 100 g is weighed on ascale. The weight Wm is recorded and thesample is heated to 1 O-200 C for anhour so that all water evaporates. t is thenput on the scale again mmediately and thedry weight Wd recorded.Moisture content in per cent

    = Wm-Wd )(1()(-j

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    warping. When the test bars feel dry the dis-tance between the two cross-linesare meas-ured in mm on all bars and the amount ofdr$ng shrinkage s found:Drying shrinkage n per cent

    = lOO--Drylength100 x 100As the distance was 100 mm the shrinkagein mm is equal to shrinkage in per cent. Af-ter firing the test bar to the highest tempera-ture possible additronal shrinkage is meas-ured m mm and recorded as:Firing shrinkage n per cent=: Dry length - Fired length x 1ooDry lengthTotal shrinkage in per cent = 100 - Firedlength in mm.The drying shrinkage indicates to some de-gree the plasticity of the clay. A large dryingshrinkage means that the plastic clay couldabsorb much water, which in turn indicatesfine clay particles. The figure for dryingshrinkage should be compared with figuresof former supplies o see f the present batchis of the samequality.The firing shrinkage indicates how fusiblethe clay is. A high shrinkage normally meansa lower melting point. The total shrinkage ofrefractory bodies tells us how much bigger

    Fig. 149 : Setting of test bars for fiiing. The bend-ing of the bars is compared with the bending ofcones.

    with the cones and results from former tests.When testing a new clay the test bar shouldbe placed so that it can be viewed througha spyhole and the approximate temperatureat which bending starts is noted.

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    2. Kilns

    2.1 Development of kilnsA kiln may be described as an enclosure tocont& heat. Potters use i? to fire their potsand they have developeda countless numberof different kiln types, each one reflectingthe demands of local markets, tradition,skills and materials.Even so the basicsof all ceramic kilns are thesame; heat is introduced into the enclosuresurrounding the pots. Some heat is lostthrough the walls or is carried away with thecombustion gases,but as more heat is intro-duced than escapes, he temperature risesand the pots will mature.

    2.1.1 Bonfire kilnsThe oldest type of kiln, dating back morethan 10,000 years, is the bonfire kiln. These

    tional unglazed red ware (terracctta) be-cause they are still the most suitable forsmall-scaleproduction of low-fired pot!: J-.This is due to the fact that no investment sneeded for a permanent kiln, that the firingat most takes a few hours and that cheapand readily available fuels such as straw,grassand cowdung can be used.

    Sukuma pottersThe Sukuma women in Western Tanzaniaoften use split roots of sisal as a fuel (fig.2-2). The roots produce intense heat and thefiring takes no more than half an hour. Thepots are fired no higher than 700 C. This isan advantage or pots made for cooking overan open fire because he clay has not startedto sinter and its open structure can moreeasily adjust to the thermal shock of being

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    Fig. 2-2: Sukuma potters firing their pots with sisalroots in a bonfire kiln. Bujora, Tanzania.Fig. 2-3: Bark glaze is applied to the still hotpots. D

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    Fig. l-4: Straw-fired kiln in Thimi, Nepal

    Nepalese po;terIn fig. L-4 a potter in Nepal is preparing his

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    Fig. 2-6: This kiln is constructed with broken potsforming walls, fireboxes and flues. The first layerof green pots is stacked on top of the flue pots.Sinde. Burma.

    2.1.2 Sinde updrought kilnThe kiln of the Sinde potters (fig. 2-6) hasno permanent structure. Four fireboxes, oneon each side, are constructed by the settingof pots. A bottom layer of once-fired, partlybroken pots works as flues through whichheat from the fireboxes spreads to all cor-ners. The green pots are stacked on top andother cracked pots are built into 3 kiln wail.

    Straw, pieces of broken pots and clay formthe outer layer. Vent holes are left in thecrown of the setting. Firing is carried out bystoking firewood in the four fireboxes. Thecombustion gases nd heat go up through thesetting and leave through the vent holes atthe top. Kilns of this kind are called updraught kilns. The use of fireboxes and flues,though simple, allows much better controlof the firing. In the beginning a very smallfire allows the pots to dry out completelyand at the end of the firing heavy stokingwill ensure 3 high temperature. The hot gasesand flames from the fire circulate all overthe kiln creating 3 more even temperatureand utilizing the heat better.

    2.1.3 Bangladesh updraught kilnIn fig. 2-8 3 simple up-draught kiln is nearlyready for firing. Once-fired pots are serving3s a kiln wall as with the Sinde kiln, but thisone has 3 permanent firebox dug out underthe kiln. Fuel is cowdung stuck on bamboosticks as this area, the western part o f Ban-gladesh,has hardly any firewood to offer.

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    Fig. 2-9: Ancient up-draughi kiln from Greece

    2.1.4 Permanent up-draught kilnsIn the Near East up-draught kilns with per-manent outer walls were developed fig. 2-9)and this type of kiln spread with migratingpotters from Persia o India. It is still widelyused and fig. 2-10 shows an improved typeof up-draught kiln which was constructed byIndian advisers n Tanzania. Stoking is donethrough firemouths at two sidesan! the hot

    gases enter the kiln chamber through theperforated floor and leaT{e hrough holesin the crown. Great skill is neededwhen set-ting the ware so that space s left for thegases o pass n a way that ensureseven tem-peratures. At cold spots more space s left sothat more hot gaseswill pass there while thespots tending to overheat are stacked moredensely. This kiln is fired to 900-1000 C.

    firemouthFig. 2-11: Setting of pots in an up-draught kiln hasto be done so that the hot gases rise evenly throughthe pots.

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    2.1.5 European up-draught kilnsThe up-draught kiln originating in the NearEast spread to Europe where it was furtherdeveloped and reached its perfection withthe bottle kilns (fig. 2-12). These kilns werewidely used untii the beginning of this cen-tury, when they were replaced by down-draught kilns. The bottle kilns could be firedup to 1300 C. Dampers on top of the domecould be opened and closed for directing thedraught. That enabled the skilled fireman toachieve fairly even temperatures. The warewas placed in saggars o protect it from theFig. 2-l 2: Bottle kiln with it s innovations: chinbnay. fircbrirks and iron grates for burning coal

    combustion gases.Often a biscuit chamberover the main chamber was added so thatthe otherwise wasted heat was used for bis-cuiting.Rejkactories, grates, coal, chimneyThese up-draught kilns were originally devel-oped in Germany, by the beginning of the17th century, in an attempt to produce por-celain which was then only produced inChina. The 1300 C needed for porcelainwas reached by constructing the kiln withfirebricks and by firing coal on cast-irongrates. The grates made it possible to speedup combustion of the fuel and reduce the in-take of excessair. A chimney placed on topof the chamber creates the extra draughtneeded to draw combustion air through thegrates.Fig. 2-13: Cross-section of a hovel kiln

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    Hovel kilnA variety of the bottle kiln is shown in fig.2-13. It works in the same manner but ahovel encloses he kiln and protect: it andthe workers from the weather. The kiln itselfwas cheaper to construct as it did not needto carry the weight of the chimney and thehovel could be constructed entirely fromcommon red bricks. The potteries of NorthStaffordshire, England, were famous forthese kilns which literally dominated theskyline around Stoke-on-Trent. The hovelscould be up to 21 m high.Limitations o~tttcp-druugilt ilnsBy the turn of the century the up-draughtkiln was considered outdated. A ceramic ex-pert Mr. E. Bourry wrote: Intermittentkilns with up-draught ought to be condem-Fig. 2-14: Hovel kilns at Gladstone Pottery Mu-seum, Stoke-on-Trent

    Fig. 2-15: Chimney effect creates hot spots in ;1nup-draught kiln

    ned. They have the double effect of beingwasteful and giving an unequal distributionof heat . . . and only deserve o be forgot-ten.The up-draught k iln is wasteful because hehot combustion gases rush too quicklythrough the kiln setting, so that the heat ofthe gaseshas little time to be transferred tothe ware.The bottom of an up-draught kiln tends to

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    2.1.6 Down-draught kilnsIn a down-draught kiln the hot gases romthe fireboxes circulate to the top of the kilnchamber, are then pulled down through thesetting and leave through flue holes in thefloor. Under the floor flue channels lead tothe chimney (fig. 2- 16).Ever1 emperaturesHot air rises so the downward draught ofthe hot combustion gases tends to avoidthe hot spdts and seeks out the cold spotsFig.Z-16: lhwn-druught kiln wth tluechawwls undcr itic !luor I.

    where the downward pull is stronger. In thisway the draught will by itself evenout tem-perature differences.

    Bug wallsA wall, named a bagwall, at the inlet fromthe fireboxes directs the hot gasesupward.In case he top of the setting tends to be toohot the height of the bagwalls is loweredand vice versa. Sometimes holes in the bag-wall help but the holes weaken the wall andit may collapse during firing.


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    draught kiln, simply because they havefurther to go. So more heat is transferred tothe ware and consequently fuel is saved. Asthe hot gases eave the kiln chamber atground level it is easier to let them passthrough another chamber or several cham-bers before entering the chimney.

    A chimney for down-draught has to be tallto create a strong pull, which is required toforce the heat downward especially if morechambers are added. The Bujora kiln (p. 53)has an up-draught biscuit chamber which atthe same ime servesas a ch imney.Fig. 2-17: The downward draught avoids the hotspots and seeks out the cold spots in the kiln set-ting.Heat ecormnyThe combustion gasesspend a longer timeinside the chamber, compared to the up-

    2.1.7 Khuja kilnThe Khurja kiln is a typical example of acoal-fired down-draught kiln of Europeandesign fig. 2-18).

    Fig. 2-18: Circular coal-fired down-draught kiCn. Khuja. India.

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    insulating firebricks

    N- grou;evel

    flue channel to chimneyI IIm

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    Fig. Z-20: 30 m3 down-drx@t kiln .xndcr recan-struction. Muyangonc. Burms

    2.1.9 Bujora downdraughtThis kiln was constructed with a secondchamber which works as an up-draught kilnand chimney. When the first chamberreaches 1240 C the second chamber wouldbe 800-900 C which is sufficient for firingbiscuit ware and common red bricks. Thetop third chamber can be used for calciningfeldspar and quartz for glazes and clay bo-dies.Cllitnnty chamber.4s firebricks, of which a normal chimneywould be built. were made of kaolin which

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    %q e %u!+e)suarpv, ICauuqy~ aql JO Ilnd aql aseam! 01~apro u! Iauuey3 afy ayl ur I![ serriml lleuxsv *luaprJJns lou seM %u!q alewuols aq*JO li2ay alsells ayl am u! larl)mJ pasy aqpp103 laqureq3 puo3as ayl JO arn)eladuralayl jeya OS raqurey:,o&l alp uaaM$aq p:-;

    *Sg) ~auuey~anu ayl v paaeld aq pp103 au-mq aleId-d!Jp v */Cellnoawes aql IOJ amn3Jout Supy iuearu raqmy3 puosas e 'asenAue u] *sy3pq uounuo3 Jo saqu.my3Aauwly:,ayl ppnq 01 ajes $1 apew laiaump apy.yap!M ayL *~aqumy:, uo3ase oiu! paurn pmpapuedxa SeMAauw!y3 atp ahyuadxa set

    S[pM aq1 II! qauuntp arQ q,!M apnlu S!u[!y auom2A:cly al[L : [Z-Z %!.J

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    Fig. 2-26: Reconstruct ion of old cross-draught kilnin Thailand

    Stoneware temperaturesSuch simple kilns were capable of firingstoneware. The cross-draught through theware transferred more heat to the warecompared to up-draught and the fully en-closed kiln chamber retlined the heat well.The kiln developed into a variety of cross-Fig. Z-27: Two wood-fired kilns seen from firehox side

    draught kilns. Fig. 2-26 shows a reconstruc-tion of a kiln type which was used n CentralThailand 700 years ago for firing glazedstoneware, and similar kilns, though bigger,are still used throughout South-East .Asia.Fig. 2-37 shows a wood-fired kiln used forfiring celadon stoneware. It has no separatefirebox, but the front part of the kiln is0.5 m lower and serves as a fireplace. Thefloor for the setting of ware slants upwardand the kiln chamber narrows towards theexit flue. That helps to create a more evenfiring temperature.The faster the flow of hot air the more heatwill be transferred to the pots. Close to thefireplace the air is hot but moves slowly,whereas towards the back the air is coolerbut is moving faster due to the narrowingkiln chamber.Some kilns of this type have stoking holesat the sides so that stoking is done here to-wards the end of the firing.

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    2.1 .I 1 Tube kilnsThe cave kiln, supposedly, was made everlonger until it developed nto the long slop-ing tube kiln about one thousand years ago.Tube kilns tie up to 50 m long and are usedfor both earthenwareand stoneware.On p. 92a tube kiln is seen rom the firebox end. Thekiln chamber is a long uninterrupted tubewith an exit on top. The tube is ftiied withpots, traditionally in an open setting, butnow also with saggars fig. 2-28). The fire isstarted in the firebox and the combustiongasesgo through the whole kiln to the topexit and transfer all of their heat to the wareon the way. When the lower section of thekiln has reached maturing temperature stok-ing into the tube is begun through side holesjust above the matured section (see p. 7 1).The combustion air enters through the fire-box and is very hot when it reaches he fir-ing zone. In this way the firing zone slow-

    Fig. 2-28: Snggars set in a tube kiln under repairly moves upward until the whole kiln isfired. When the upper section is fired thelower section has already been cooled consi-derably by the intake of combustion air.The difference in height of the exit flue andthe inlet at the firebox is often enough tocreate sufficient draught through the kiln.However, some tube kilns have a low chim-ney as seen n fig. 2-29.

    Fig. 2-29: Upper section df a tube kiln. An entrance is made for every 5 m. Stoking holes are seen at theside of the arch.

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    Fig. Z-30: Cross-section of a chamber kiln

    2.1.12 Chinese chamber kilnIn China the tube kiln was further developedby breaking up the long tube into separatebut connected charrbers (fig. 2-30). The fireis started in the firebox and the first cham-ber is fired as other kilns. When the desiredtemperature is reached n the first chamber,say 1280 C, the second may be around1100 C and the third around 700 C. Firing

    kiln is about 20. The Chinesechamber kilnscould have up to eight chambers and wouldbe stacked with ware produced by many in-dividual potters. The largest kilns could beup to 400 m3 in total kiln space.

    2.1.13 Champaknagar chamber kilnFig. 2-31 shows a three-chambered wood-

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    Fig. 2-3 1: Three-chambered wood-fired kiln underconstruction. The !irebox to the right and thechimney ;lt the other end are not yet re:ldy.Ch;lmp~knagar, Bangladesh.FiringThe firing is started at midnight so that thelast part of firing takes place during the day

    FirewoodThe first two chambers consume 2200 kgfirewood and each additional chamber about500 kg. Unfortunately the firewood is notproperly seasonedso heat is wasted dryingout the extra water in the firewood (p. 64 f.).

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    Fig. 2-33: Two-chambered wood-fired kiln. Champaknagar, Bangladesh.

    view fromfirebox-side l--lIi?:

    I# \\\

    side view

    flue holes

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    Extra chamberThe small extra cost of firing addirionalchambers makes it tempting to add severalmore. The additional chambers would alsoreduce the size of the chimney which is4.4 m for the twochamber version. How-ever, a huge kiln capacity would also meanlonger periods between figs and wouldmean that more space for storing potsawaiting firing would be needed. It may alsobe difficult to set aside enough money forthe production costs in the longer time be-tween making pots, firing and selling thefinished ware.


    It is better to start with a few chamberswhile the pottery workshop is starting up;later as production and confidence growadditional chambers can be added withoutmuch interruption to production. It is betterto plan for future expansion when designingand constructing the kiln, so that sufficient

    space s left to build on. In case he kiln isbuilt on a slope it is easier to add extrachambers at the firebox end as the chimneyis a larger structure to dismantle and recon-struct.FireboxThe firebox shown in fig. 2-34 is made verywide because the unseasoned irewood hasto spend longer time drying in the flreboxcompared to properly dried firewood. Othertypes of fireboxes as described under f ire-boxes (p. 73-87) can be used as well.A more rational solution of course would beto season he firewood properly. However,small village potteries have no money to in-vest n a stock of firewood sufficient for dry-ing 4-6 months. It is costly to be poor.SeljkupportingThe chamber kilns are constructed withoutany iron frame supports. The structure supports itself as the chambers lean onto eachother.

    Fig. 2-34: Champuknagarfirebox

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    2.1.14 Sumve cross-draught kilnThe cross-draught principle of the chamberkiln is used in a small waste-oil-fired kilnconstructed in a small viilage pottery inSumve,Tanzania. The kiln is fired to 1250 Cand uses an open setting. It is constructedwith self-made nsulating firebricks with anouter wall of common bricks.The chamber is constructed as a catenaryarch (see p. 102) which makes the structureself-supporting. The capacity of the kiln israther small but for newly started workshopsit is fine. This kiln could be expanded byadding more chambers as is done in Cham-paknagar. Fig. 2-35: Waste-oil-fired kiln in Sumve, Tanzania.To the right an oil heater.Fig. 2-36: Stoneware kiln with about 1 m3 capacity (cross-draught kiln)

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    2.2 Choice of fuelNearly everywhere the cost of fuel for firingkilns is the single biggest cost of ceramic pro-duction. In some areas he cost of fuel simp-ly rules out the production of modem pot-tery and only traditional pottery fired withagricultural waste materials is economicallypossible.Kilns heated by gas or ciectricity will not bedescribed here because these fuels are sel-dom available or their cost is prohibitive.(This might change in the future when bighydroelectric or natural gas projects willmake these ypes of energy more easily avail-able and cheaper.)That leaves us with three main sources offuel: firewood (and agricultural waste), coaland oil.Cost and supplyIn many areas only one type of fuel is avail-able for potters. However, those fortunateenough to be able to choose from severaltypes of fuel should consider which fuel willserve them the best by comparing (1) thecost of the fuels and (2) how reliable thesupply is.

    from the pottery the cost of hiring a lorrymay make this fuel very expensive. Or incase coal can be bought from a governmentstore the cost of employing a person to dothe necessary paperwork, etc. will also addto the fuel cost. The different fuels have dif-ferent heating values and this should also betaken into account, e.g. 1 kg of firewoodmay only produce half the heat of 1 kg ofcoal (see appendix).Table 2-1 is an illustration of how to com-pare fuel costs. This comparison is based onthe estimated fuel consumption for thefiring of a two-chamber kiln of Champakna-gar type. In this example firewood turnedout to be the cheapest uel, but if the sourceof coal had been closer it would have beenless costly to transport and could in thatcase become the chosen fuel. The cost ofcoal in this example also includes $ 12 foremploying a person to acquire the necessarylicence to purchase coal from a governmentstore. In areas with a higher cost of labour,firewood Gould become more costly due tothe heavy work involved with felling thetrees and cutting the firewood.SUPPlY

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    suffer the results of an insecure supply. Ifthe supply situation is difficult it is better toarrange fireboxes which can burn two OFmore different fuels. For example it is easyto place drip-plate burners in a tirebox forcoal or firewood. The additional cost ofmaking two firing systemsmay be recoveredin one or two firings.

    2.2.1 FirewoodFormerly fuewood was the main fuel allover the world but today in the industrializ-ed part of the world firewood accounts foronly 0.4% of fuel energyused,while in the de-veloping world firewood still accounts for25% of the energy used. In the poorestcountries about 40% of the energy is fromwood. That figure reflects the fact that oilproducts and coal are too expensive andoften not available o the majority of peoplein the developing world. Therefore, manypotters, especially those in rural areas, willcontinue to rely upon firewood for firingtheir kilns.

    Ash coburs

    Fig. 2-37: These saggers are provided with holesthat will allow the effect of ashes to reach theglazed wure.

    Heat valueThe softwoods such as fir and pine haveslightly more heat value per kg compared tohardwoods such as teak and oak. The weight

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    Fig 2-38: Special axe for splitting firewood. Theaxe does not cut the wood but splits it by impact.It is not suitable for splitting wood with longfibres.only bums badly but a lot of the woodsenergy will be used to turn the water intosteam. The wood should be stored until it iscompletely dry on the surface as well aswithin. In temperate climates this will takea year while in tropical countries the fire-wood should be allowed to dry throughout a

    dry season.Properly dried firewood still con-tains lo-15% water.StoringThe potter will need to keep a stock of fire-wood big enough to last for six months orone year depending on the prevailingclimate.The three-chamberedkiln from Champakna-gar (p. 58 f .) uses about 2.3 tons to fire to1100 C. It is loaded with 1800 mugs and isfired nearly every third week and so a stockof 20 tons of firewood will be needed n thiscase. Some potters may be able to buy woodwhich is partly dried but one can never besure of this and so it is better to buy freshwood which is easier to split. When settlingthe price for the purchase of firewood byweight, take into account that the freshwood weighs about 30% more than drywood.Normally it is much cheaper to buy largeamounts of wood by the lorry-load. This alsoFig. 2-40: A solid chump of wood half-buried inthe ground is the proper base for splitting firewood.

    Fig. 2-39: Firewood stocked so that air can pass

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    allows the potters to cut the wood into suit-able sizes.The cutting and splitting of woodis much easier while it is still fresh. Sticksabout 60 cm long and 3-5 cm thick areneeded for the last hours of stoking, whilethicker ones will be fine until then. Split-ting the wood makes it dry faster as doesstacking the long sticks so that air can passthrough the piie. Bamboo is an excellent fuelalthough it is usually reserved or construc-tion purposes.

    Fire hazardPotters often place the firewood for the nextfiring on top of, or on shelves,above the kilnduring a firing (fig. 24 1). The heat from thekiln dries out the wood completely therebyfurther reducing the cooling effect of themoisture in the wood. However, great caremust be taken to prevent the wood fromcatching fire.

    Plan tiq treesIn some areas there are large forests withplenty of trees and it may seem hat there isno need to worry about :i lack of wood forfuel. However, evenpotters in theseareasmayafter only a short time find there is not enoughfirewood because he demand for wood is sogreat. The price of wood goesup as firewoodis cut further and further away. A familyuses about 4000 kg of firewood each yearfor cooking alone. Therefore, one single vii-lage may soon use all the trees n the nearbyforest if no new trees are planted to replacethe ones which were cut and burned for fuel.Even though the potter may face no troublein getting firewood for the time being, thesefortunate conditions are unlikely to last for-ever. If at all possible, potters should try tosecure their source of firewood by plantingtheir own trees.In heavily populated regions land is scarceand expensive and so potters will be unable

    Fig. 2-4 1: Split bamboo sticks are dried on top of u sloping tube kiln. -

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    This stack represents a years supplyFi_e. 2-42: The big stack represents B familys annual use of firewood for cooking (drawing from: Apro-vecho-Institute. Fuel-Saving Cookstoves. GATE/Vieweg, 1984. p. 6).

    to buy or lease land for growing trees. Inother places, however, potters may throughlocal authorities be able to lease fallow landwhere trees can be planted. Often local de-velopment authorities offer seddlings ree ofcharge o villagers.It may seem an overwhelming task to start

    which should be able to advise on whichtrees would be most suitable in a speciticarea.2.2.2 Agricultural wasteAll agricultural waste products are bulky

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    SawdustSawdust has the sameheat value as the woodfrom which it was cut. It is an efficient fuelwhen used with the right firing system (p.76). The main problem is to keep it dry be-cause t soacksup moisture like a sponge.Rice husksMilling of 66 kg of paddy produces 10 kgof rice husks. The heat value is nearly asgood as for sawdust but this fuel has a highash content. Some rice mills bum the ricehusks for steam-powering the mill and dry-ing the rice hulls and so they may only havea little left over to sell.Other vegetable wuste,The following materials can be used duringthe initial firing of the kiln as an additionalfuel:- Peanut hulls; very bulky but have thesameheat value per kg as wood.- Bagasse; he crushed sugar cane after thesugar juice is extracted. 130 kg sugar pro-duces 100 kg wet bagasse. he bagasse as todry for a couple of months. With 15% mois-ture content it has a heat value close to fire-

    rests. Peat can be described as the first stepin natures production of coal. It is normallyonly covered by a thin layer of soil.WinningThe peat contains about 90% water when itis dug or rather cut. The soft mass s cut intoblocks (20x5~5 cm) which are first laid outon the ground for drying. As soon as theblocks can be handled they are stacked ntopiles in order to accelerate heir drying. Dry-ing may take several months anti the air-dried peat blocks will contain 20-30% mois-ture. Winning of peat requires only manualtabour and a few tools.Heat value

    The properties of peat are still close to thoseof firewood and peat can be burned on fire-wood grates although it needsslightly lessse-condary air compared to firewood. The heatvalue of air-dried peat is close to that of fire-wood.

    2.2.4 LigniteLignite is the step between peat and real

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    cause he coal to ignite by itself if it is piledin large heaps. The preventive rule is not topile the coal higher than 2 m, smaller pilesbeing ess ikely to ignite than big ones.2.2.6 Oil productsSome countries have many different types ofoil products while others only a few; there-fore, only the most common are mentionedhere. Petrol cannot be jrsed for firing kilnsbecause t burns explosLvely.Kerosene is anexcellent fuel but is normally more expen-sive than other o il products though in somecountries it is subsidizedby the government.Diesei oil and light fuel oil are rather similarwhen used for firing. Fuel pil is a verypowerful fuel with a heat value of about30% above good coal. However, it Is also themost expensive fuel and will in many casesnot be economical for potters.WasteoilWaste oil is as powerful as fuel oil yetcheaper. Waste oil can be obtained fromgarages, bus and transport companies andrailways which are left with a lot of lubrica-

    garage n Nevada was found to contain thefollowing contaminants listed as parts permillion (ppmj:ironcopperchromealuminiumleadtinsilica

    50187750055Some people regard burning of waste oil as ahealth hazard especially because of its leadcontent. However, the quoted amount oflead equals that in petrol while earthenwareclay may contain more than 320 ppm andpaper for candy-wrapping may contain asmuch as 7125 ppm. A potter firing withwaste oil will be no less safe than if he hadspent the day at the road side. At BujoraPottery in Tanzania the waste oil gave apleasan% hine to the unglazed clay. This wasprobably due to the contaminants of thewaste oil.PollutionOil and especially waste oil are dirty to work

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    -- - -Fig. 2114: Firing of a tube kiln in Thailand. An attendant stokes bamboo from each side.

    2.3 Combustion and fireboxes Carbon and oxygenIt is not possible to learn how to fire a kilnsuccessfully from a bcok. That has to bedone by participating in many. many firings

    When watching a small fire we notice that thefirewood slowly disappears eaving a littleash and that the fire needs plenty of air. But

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    fuels are mainly made of a material calledcarbon and the burning process akes placewhen the carbon combines with the oxygenin the air and forms a new material calledcarbon d ioxide. The process produces a lotof heat. The carbon dioxide escapes andonly ash is left. Ash is the part of the fuelwhich cannot burn.Fhh point

    When a piece of wood is heated, initiallywater and carbon dioxide are given off.Above 280 C volatile gases n the wood areFig. 2-46: Tempcraturcs of flnsh point, ignitionand flame of wood




    ky lionimpercliwe

    given off. These gaseswill bum if they comeinto contact with open flames and this tem-perature is therefore named the flash pointof wood. f:s;;:;;;, UL,c;jea;sesA$!! cpu\13;;topen flames only bum at temperaturesabove 600 C. This is called the ignition tem-perature. The temperature of wood flamesare 1100 C while fuel oil has a flame tempe-rature of 2080 C.Solid, liquid, gasFirewood and coal are solid matter while oilis liquid. However, the burning will onlytake place when carbon is in the form ofgas. All materials exist in three differentforms depending on the temperature. Thethree forms of water are well known (fig.

    Fig. 247: Water in its three forms


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    247). So first we will have to turn our fuelinto a gaseous form and then mix it withair. This is the job done in the fireboxes ofpottery kilns and it is done differently ac-cording to the type of fuel. For the potter,mainly three types of fuel are of interest:firewoocl, coal and oil.

    2.3.2 Firewood fireboxFirewood bums in two stages.When a newpiece of wood is added to the fire , the woodwill first give off volatile gaseswhich willburn. (In wood the volatile gasesamount toabout 8070 of the total mass, he remainderbeing in the form of fixed carbon (charcoal).)The flames of a fire are these burning gasesand they will often not even touch the fire-wood. After the volatile gaseshave escapedonly charcoal is left and it will bum withgentle blue flames.In the ceramic kiln the two-stage burningtakes place in the firebox which enablesus tocontrol the fire. The main problem is to

    Fig. 248: Wood burns in two stages. The fist, seento the left, is the burning of the volatile gases. Thesecond, seen to the right, is the burning of thecharcoal.ensure a good strong fire with just the rightamount of air needed to combine with thecarbon of the fuel. If we let in too little air

    Fig. 249: Function of a wood fuebox. The height of ash pit should equal the height above the grate.

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    some of the volatile carbon gas will go out ofthe chimney unburned which can be seenasblack smoke. That means wasted firewood.If we let in too much air this excessair willcool the kiln. That too is a waste.

    fiimary/secondazy airIn fig. 249 air enters at the bottom of thefirebox, passes over the embers and goesthrough the grate. Reaching the firewoodit helps to burn the carbon gas being releas-ed rapidly due to the high temperature in-side the firebox. Most of this air, primar?czir. is used to bum the charcoal and oftenthere will be too little air left for the volatilegases eleased rom the tirewood. Secondaryair entering above he grate ensurescompletecombustion for these volatile gases.By thusdividing the air inlet less air is needed andthereby lesscooling of the kiln takes place.The vola tile gases epresent up to 30% of theheat value of the firewood. In casesufficientprimary air passes he fuel the combustionwill be complete, but it would mean an ex-cessof air being 50%100% of the air used forcombustion. This excess s reduced to 30-50% when secondary air completes the com-bustion.

    Fig. Z-50: Fireclay bars made as long solid fire-bricks

    A mousehole letting air into the bottom ofthe ash pit can regulate the thickness of theembers. The grate for firewood should beabout 15-,215% of the floor area of the kilnchamber, the 15% sufficient for firings up to1100 C and the 25% for 1300 C and above.

    Firirzg techtziqueFiring is started in the ash pit with big piecesof firewood so that firing begins slowly andit also helps in building up a good layer ofembers. Later the firewood is placed on thegrate and the secondary air inlet is opened.A properly designed kiln should be easy totake up to about 1000 C but in order tosave fuel care should still be taken to con-

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    Fia. 2-51: Iron arates will last lonrter if they can be removed after firing has been finished (F. Olsen TheKin Book f&ire wood kiln, p. l?8).

    is not rushing through the open spaceon thefire grate. This would cool the kiln; only tenminutes neglect might cost an hours extraway round (fig. 2-52). The firewood is fedfrom the top and the box over the hob canbe filled up so that the firebox is kind of

    steps -where the fuel is burning ( fig. 2-54).

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    Fig. 2-53: S toking a bob firebos

    2.3.3 Sawdust fireboxSawdust, rice husks and other agriculturalwaste materials need special firing systems.The problem is that this type of fuel is verybulky and if used in a conventional woodfirebox it would block the grate and onlybum slowly on the surface. One solution isto let the fuel fall onto the top of a steepcast-iron grate provided with a lot o f smallFig. 2 -54: Grate for sawdust. The grate is set at aninclination of 50. The sawdust is fed at top andshould trickle down the grate.

    This system needs constant attention to en-sure proper flow of the fuel evenly over thewhole grate area. Otherwise areas withoutenough fuel will bum through and let in coldair. The system requires a rather big gratearea compared to kiln size but can be usedfor smaller kilns fired up to 1000 C. (Agrate measuring 40 x 100 cm fired a 1 mkiln to 1100 C with rice husks in eighthours consuming 650 kg husks.)Sawdust injectionIn this system sawdust is sucked into a cen-trifugal blower via a pipe system and saw-dust mixed with air is then blown into a con-ventional firebox. The firing is started withordinary firewood in order to slowly heatthe kiln. After smoking is over and whenthere is plenty of coal in the firebox theblower is started. In the beginning the firinguses only a little sawdust and firewood willstill be needed to ignite the sawdust. Whenthe firebox bricks are glowing red firewoodstoking is sropped and the flow of sawdustis gradually increased.A 1.7 m3 kiln is quot-ed to use 3 m3 sawdust to fire to 1300 Cin 11 hours. For this kiln a 23 cm straightblade blower powered by a 0.3 HP electro-

    Fig. 2-55: Sawdust injection system

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    The sawdust bums very fast almost likeliquid fuels. The asheswill not remain in thefirebox but will be blown throughout thekiln. Some will settle on the walls and sag-gars and form a glaze and the rest will leaveby the chimney. Firings to above 1250 Cmay be done in open settings because he

    ash will melt together with the glaze. Below1250 C the ash would produce a roughlayer on the glazed ware.A hole in the bottom of the chimney willhelp to provide air for burning up unburnedsawdust. The sawdust burning produces a lotof sparkswhich may start a fire.Fig. 3 -56: Sawdust is fed to the suction pipe by a hopper. The speed of the auger can be used to regulate

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    Fig. 2-59: Coal-fired down-draught kiln having itsfireboxes inside the wall. Kanagur Village PotteryInstitute, India.

    and wear out quickly. The cost of renewingthe grate bars is a big drawback of coalfiring.A water container placed in the ash pit willcool the bars. The resulting steamwill disso-ciate into ions when passing the burningcoal. This action cools the temperature ofthe burning coal without a corresponding ossof energy. At the same time the flames will

    IL. _ . .__ ~..F. -.-. -1Fig. 240: Typical grate bars for coal. The thicken-cd parts adjust the open space between the bars.

    become longer and the coal is less ikely toclinker.The life of the iron bars will be much pro-longed if they are designed o be removed assoon as the firing is stopped. Cast-iron barsare superior to mild steel bars.Flu tf inclined ,gmlesHat grates are mostly used for slow firingsup to 1250 C mainly in an oxidizing atmo-sphere. Inclined grates are used for porcelainand faster firings. (Inclination is usually 15-25 but there seems o be no fixed rule, e.g.lignite should be fired on inclined grates n-

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    Fig. 2-62: Coal firebox similar to the ones seen in fig. 2-59. The fuebox is within the outer wall of thekiln. Secondary air is regulated by opening of the stoking shutter and by placement of bricks just abovethe grate bars.

    stead of flat ones.) Types with steep nclina- Reheated air

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    tion are called semi-gasproducers becausethey are fmd with a thick bed of coal whichproduces a lot of half-burned carbon gas.This gas is fully burned by an inlet of se-condary air above the grate as in a firewoodfiring or inside the kiln in case ong flamesare desired.Semigcrs producer

    A semi-gas producer is shown in fig. 2-63.The inclination of the grate is SO and stok-ing is simply done by filling coal until thegrate is covered with a coal bed of the desir-ed depth. At full fling the whole stokingchannel can be kept full of coal which willslide down by itself. The problem with thistype of grate is to fire at a slow rate. Thatcan be overcome by covering the upperpart of the grate with a clay slab during theinitial slow rising of the temperature. Asmore intense fue is needed the slab can bedrawn out.

    Channels for secondary air are built intoboth sides of the firebox so that when thesecondary air enters above the coal bed it ispreheated (fig. 264). This will ensure a bet-ter combustion of the volatile gas, therebyadding to a better fuel economy.Stoking

    Primary air enters under the grate and if thecoal bed is thicker than 10 cm, secondary airis needed too for complete combustion.There are two basic stoking techniques:(1) Frequent stokings are made to e