Fillers - Alfred's Clay Store

Fillers - Alfred's Clay Store

Fillers - Alfred's Clay Store


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<strong>Fillers</strong>Lecture #4


<strong>Fillers</strong>• <strong>Fillers</strong> are non-plastic materials included in a claybody to adjust shrinkage (both pre and post fired)– <strong>Fillers</strong> displace some of the clay content in a body• <strong>Fillers</strong> don’t shrink during drying; the volume they take uptherefore reduces the total wet-to-dry shrinkage• Act as a ventilation system– Open up dense plastic bodies and allow water tomigrate to the surface for evaporation– Allow gases/carbon to escape from body during earlyfiring stages• Also affect workability, strength (both green andfired), texture, color, CTE, and even weight

<strong>Fillers</strong>• Most fillers tend to be refractory• Fluxes act as fillers in the dry stage– They cut down on the amount of clay, andtherefore the amount of shrinkage duringdrying• But their fluxing action during the firingexcludes them from being considered“fillers” in the broader scope of the word

<strong>Fillers</strong>Silica basedQuartz (flint/sand)GrogCalcined clayPyrophylliteWollastoniteFeldspar ChipsFiberglassVermiculite/PerliteAlumina basedMulliteKyaniteAlumina HydrateAlumina OxideNonorganic(don’t burn out)Nylon FiberSawdustPaperRiceEtc…Organic(burn out)

Non-organic <strong>Fillers</strong>• Along with the benefits through the drying stages, nonorganicfillers– Can add fired strength (depending on their composition / firingtemperature / clay body); especially Flint• Reduce some warping during firing by adding support to clay duringfiring• However they tend to create tension in dry ware– They do not contract upon drying• <strong>Clay</strong> shrinks around non-plastics, creating micro-cracks• Excessive use of fillers (especially when not properly distributed) canlead to cracking (more on this latter)• In general the higher the amount of filler, the lower thegreen strength

Keying / interlocking properties• By providing a network of friction, fillersensure that the clay particles do not slide tooeasily past each other during forming– This allows the clay to “stand up”– However too much filler allows water to moreeasily penetrate the body during throwing,leading to early slumping• Natural sources of fillers (I.e. Sands) tend tobe rounded = less friction• Man-made fillers (I.e. Grogs) are crushedand are therefore more angular = morefriction

Silicon/Silica/Silicates• Silicon: Si (Si 4+)– An atom; Unstable on its own– Looks for atoms or combinations of atoms which cangive it 4 electrons to complete its outer shell• Silica: SiO 2– silicon + 2 oxygen atoms– Occurs naturally as a mineral in the form of quartzrock, flint, sand or free silica in clay deposits• Silicate:– A family of materials which are mainly composed ofSilica• Kaolin: Al 2 O 3 • 2SiO 2 • 2H 2 O• Potash Feldspar: K 2 O • Al 2 O 3 • 6SiO 2

SiO 2Quartz• Quartz = crystalline silica (perfect crystals)• 2 main sources: Flint and Sand• Flint = a powdered form of sand– Inexpensive– Pure (less than .1% Fe and Ti levels)– Increases whiteness– Adds strength to the body if glass forms• Sand– chemically identical to flint but much larger grain size– A cheap general-purpose grog– Unlikely to melt in the body (due to large size)– More likely to remain crystalline (high CTE)• Poor shock resistance– Rounded shape (smoother than grog)

SilicaUnmelted silica remainscrystalline after firing & hasa high CTE (i.e. In manynon-glassy clay bodies)Sand / Flint(when un-melted)Melted silica forms glass,which has a very low CTE(i.e. Glaze, Porcelains andother dense bodies)Flint(when melted)

CTE• Coefficient of Thermal Expansion is reversible– Each material will expand when heated and contractwhen cooled by “x” amount every time it is fired• CTE of filler affects body’s CTE and thereforeglaze fit• Most man-made fillers have low CTE• Low CTE fillers = less strain on clay body duringfiring– Thermal shock resistance (ideal for raku, sculpture,thick-thin walls)• High CTE fillers (I.e. un-melted flint) in the bodycan help reduce crazing but overuse can lead toseparation of body and glaze (shivering)


• A glaze that crazes is too small for the clay over which it isapplied (glaze contracts too much on cooling)– Make the body contract more by increasing the CTE (i.e. add enough Silicaso some of it remains crystalline… i.e. so it doesn't melt)• A glaze that shivers is too large for the clay over which it isapplied (glaze doesn't contract enough on cooling)– Make the body contract less by lowering the CTE (i.e. remove some excess,unmelted silica, increase the amount of glass, by adding flux, or add lowCTE fillers)CrazeGlaze in TensionGood FitShiverGlaze in CompressionBodydoesn'tcontractenough forthe glazeBodycontractstoo muchfor theglazeLow CTE bodyHigh CTE body

Quartz Particle Size/Dissolution• The smaller the particle size of quartz (or any other material)the less energy is required for melting• Large quartz grains (250 mesh or larger) will undergo littlemelting (sharp corners only start to soften, even at cone 9-10)– More likely to remain crystalline / high CTE• More Si is dissolved into glass with:– Higher temperatures / Longer firings– Increased fluxesIn the same body– Finer grain sizesQuartzMeshSizes2002703254006001000Cheap(this is what we stock)Expensive325 MeshCone 101000 MeshCone 6

Variable compositionsGrog• Usually made from new or used refractory rejects– Firebricks– Pottery– Other fired products• All the benefits common to most fillers (shrinkagecontrol; breath ability) plus:– Added wet and dry strength due to stable angular shape whichlocks together with clay particles and other non-plastics• Better load bearing than rounded sands– Wide selection of mesh sizes available for maximum packing• But, tend to be dark in color (not for white bodies)

Grog Sizes• Grogs are sorted by passing throughscreensTyler Mesh Mesh Opening# (mm)3 6.684 4.706 3.338 2.3610 1.6514 1.1720 0.8328 0.5935 0.4248 0.29100 0.15150 0.10200 0.07•Often designated by two numbers•First number is what the grogwill pass through•Second number is what it willnot pass throughi.e.20/48 mesh grog will pass througha 20 mesh, but not a 48 mesh-48 mesh grog (“48 and finer”) isone which passes through a 48mesh screen (no limit for finematerial)

GrogRelative scale12/20 mesh 20/48 mesh -48 mesh

Maximizing Particle Packing• Decreasing void space in the body decreases shrinkage• Void space (white space) is the same in the examplesabove, regardless of sphere size– All fine or all coarse grog gives roughly the same void space• So how can we minimize voids?...

Packing diagram of athree-component grogsystem• Densest packing isfound at– 50% Coarse– 10% Medium– 40% Fine• Regardless of howmuch grog is used,these proportions willminimize shrinkage inbodiesBestpackingFigures above represent porositybased on bulk volume

Example• To minimize shrinkage, a body with 30%total grog should have a distribution asfollows:50% coarse grog = 0.5 x 30 = 15 parts coarsegrog30 parts grog10% medium grog = 0.1 x 30 = 3 parts mediumgrog40% fine grog = 0.4 x 30 = 12 parts fine

•Excess coarse fillers may lead to cracking during drying and/or firing•Grade your filler sizes whenever possible and avoid excessively largefillers•The same principle of grading sizes applies to other materials in the claybody (i.e. especially clays)

Hard vs. Soft Grogs• Hard Grogs (what we sell)– Previously fired higher than clay body’s toptemperature– Hard Grog has done all its fired shrinking• Creates a refractory network throughout the body• Minimizes fired warpage• Soft Grogs– Previously fired lower than clay body’s top temperature(may even be lower than clay body’s bisquetemperature)– Increases fired shrinkage of body over hard grogs– Are more porous• Good for quick drying and tempering sticky clays

Pitchers• The best grogs are those you make yourself– “Pitchers”• Grog made from the same body they will be added to– Can be made hard or soft– All the advantages of hard/soft grogs + chemically identical tobody + no change in color– Same glaze fit as parent body/same firing schedule etc… (willnot affect glaze fit of the original body)• The possibilities are endless– Can be made to measure (color, size, shape etc.)• E.g. black grog for white body (or vice-versa)• Yellow stars, green clovers, blue moons, pink diamonds• We have the machines necessary to make grog– Jaw crusher / plate mill

The jaw crusherCreates coarse materialroughly ¼” in size

The platemillCreates fine materialroughly -40 mesh

Feldspar Chips• Crude, large pieces of feldspar (silica +feldspar)• Ours chips are Custer feldspar that has notbeen milled– Same chemical makeup as Custer but lessreactive because of its large size10 - will start to melt around the edges

Coarse chips(Feldspar)Fine chips(Feldspar)Grani-Grit(Granite)

Feldspar chips addedto a Barnard body

•Crushed granite rock•(silica + feldspar + mica)•Mixed with food and fed tochickens (helps digestion)•Retains mottled (black and white)colors at low temps•Melts at higher temps (createsglass beads on surface)Grani-Grit“It’s what’s in the gizzard that counts!”

Calcined <strong>Clay</strong>• Typically fired to 8 and above• Advantages– Physical and chemical effects of clay withminimal drying shrinkage– Fine particle size – unobtrusive (no texture)– Can be easily made (no need to crush)• Disadvantages– Super-fine particle size• clogs up drying passages (poor ventilation)• Less practical for the construction of thick work• E.g. Glomax, Calcined China <strong>Clay</strong>

3Al 2 O 3 •2SiO 2Mullite• Named after a natural deposit on the Isle of Mull (Scotland)• Very high alumina / very refractory• The end product of vitrification and crystal decomposition ofkaolinite (as silica is removed from kaolinite crystal during firingto form glass, kaolinite mineral decomposes)Al 2 O 3 •2SiO 2 •2H 2 O (kaolinite) 3Al 2 O 3 •2SiO 2 (mullite)• Needle-like square-sectioned crystals– Grow out from decomposed kaolinite (connect separate kaolinite crystals tocreate a stronger structure)• Hot load-bearing properties (anti slumping)– Stiffens the body during melting phase (reducing deformation during firing)

MulliteMullite increases mechanical strength during and after firing

Mullite• Forms readily at high temperatures (above 9)– Porcelain /stoneware is strong partly because of mullite– Earthenwares develop little if any mullite (lower temperature andless kaolinite minerals)• Earthenwares can’t be taken as close to vitrification as porcelains withoutslumping in part because they don’t form mullite• Grows more readily in oxidation than reduction– Oxidized bodies are typically stronger than reduced bodies– However, reduces translucency• Mullite can be added to a body in pure form (mullite comesin 35, 100, and 325 mesh grades)• Our sources that are less pure and may require particularfiring temperatures• Sources include Molochite, Kyanite, and Pyrax

Molochite• Trade name; made in England by ECC• Made from crushed vitreous white kaolin• General advantages of grog• Maintains whiteness (porcelain bodies)• Low CTE– Primary Kaolin is held at 1525°C for 60 hours• All silica is converted to glass (Low CTE)

MolochiteA network of needle-like mullite crystalsembedded in glassMineralogical Analysisby X-ray Diffraction:Mass%Mullite 55Amorphous Silica Glass 45•Molochite contains 96% of the possiblemullite that can be converted from kaolinite

Molochite• Rough surface texture of the particles givesexcellent keying properties• Available from 8 – 325 mesh•However, very expensive•$0.80 / lb.•High cost due to material purity, firing/processingcosts and transportation from England

3Al 2 O 3 •3SiO 2KaolinKyanite• A naturally occurring mineral• A 1:1 aluminum-silicate– High in alumina– Very high m.p.Al 2 O 3SiO 238%46%Kyanite62%38%

Kyanite• Below 19 it remains “raw”– Undergoes a large irreversible expansion (don’t confusethis with CTE)– Is used to compensate for body’s fired shrinkage (helpsequalize green and fired volumes in bodies)• Extensive use in large scale refractories• Try 25% in a body at 10– Supposedly good for filling cracks• If fired high/long enough it converts to mullite– Kyanite contains trace amounts of mullite– When fired to ca. 19, it fully converts to mullite

Al 2 O 3 •4SiO 2 •H 2 OPyrophyllite (Pyrax)• Almost identical chemical and physical properties to talc– Pyrophyllite - aluminum silicate Al 2 O 3 •4SiO 2 •H 2 O– Talc – magnesium silicate MgO•4SiO 2 •H 2 O• Expands when heated– Helps compensate for fired shrinkage in the body• Alumina content encourages mullite formation• Increases thermal shock resistance (if mullite forms)– Low CTE– High thermal conductivity (better thick to thin survival rate)

Pyrophyllite• Can substitute for some flint in the body• Can also replace some feldspar in high firebodies– Unlike flint, it brings in alkali which help fluxbody• Fires to an off-white color– Darker than flint– Not suitable for white porcelains

Al 2 O 3Alumina Oxide• Very low CTE• Very refractory (uninvolved in the maturation of bodies)– Almost 100% alumina (.1% Na impurity)– Used in kiln wash and wadding• Commonly used in whiteware bodies (increases whiteness ofbodies)– However, reduces the translucency of vitreous bodies (alumina crystalis opaque)• Can substitute for some quartz in the recipe as a filler– Available in particle sizes down to 1 micron– We have it in fine and coarse grades• Alumina Hydrate is similar but contains some water– I.e Al 2 O 3 •3H 2 O• Both forms are limited in use by their high $$$

BauxiteBayer ProcessLiquorAlumina Hydrateis producedPregnantLiquorRedMudAluminumTri-HydrateAl 2O3*3H 2OGrinding:Bauxite isground toabout 100mesh by drygrinding9/14/2010Digestion:Bauxite isdissolved in hotNaOH (sodiumhydroxide),forming a sodiumaluminatesolutionDecanter:Insolubles (Iron,Silica, Titania)are allowed tosettle from thesodium aluminatesolutionFilter:Liquid fromdecantation isfiltered toremove anyremainingsolidsPrecipitator:Hot sodiumaluminate liquidis cooled to forma supersaturatedsolution which isseeded toprecipitatealuminatrihydratePart 1

Calcined Alumina ManufactureFiltration:ATH isdelivered fromthe ore plantas a slurry,which isfiltered to 14%moisture priorto being fedinto the kilnCalcination:ATH is calcined ina rotary kiln atbetween 1200°and 1400° C.Alumina OxideStorage:Material fromkilns is putinto astorage siloGrinding: Aluminacan be ground byeither a majac or dryball milling.Unground:Material for manyapplications issupplied in anunground form.9/14/2010is producedPart 2

Vermiculite & Perlite• Various industrial/commercial uses• Lightweight, fireproof, non-toxic/nonhazardous• Insulation, packaging, fillers (cement,plastics), filter media, abrasives (soaps)• Additions of either can significantly reduce dryand fired weight• Commercially prepared versions are expandedby heating to increase volume (always useexpanded forms of Vermiculite or Perlite)


Vermiculite• Hydrated form of Biotite mica (Fe-Mg rich)– Plate-like formation– Similar structure to montmorillonite (2:1 sheetsilicate) but do not swell in water– Naturally found in most secondary clays in very fineform• Compared to granular fillers, negligible effect onplasticity when used in finer sizes• Excellent thermal shock resistance• Very light-weight

VermiculiteElementPercent by Weight• Several commercial uses– Used for packaging (Orton cones)– Abrasives– <strong>Fillers</strong>– Fire-proofing– Etc.–Is compressible (less stress as clayshrinks around it)–Soft on handsColorExpanded Bulk DensityMesh SizesFusion PointGold-Brown4-10 lb/ft 32-40 mesh and finer2200-2400FSiO 2 38-46AL 2 O 3 10-16MgO 16-35CaO 1-5K 2 O 1-6Fe 2 O 3 6-13TiO 2 1-3H 2 O 8-16Other 0.2-1.2

Perlite• Glassy, small round pellets (pearls) of volcanic origin• Fused sodium potassium aluminum silicate (no crystallinesilica)• Commercially available Perlite is heated to 800°C toeliminate organic matter and increase volume– Water escapes, creating tiny bubbles• Expanded Perlite is extremely light weight– 2 lbs./cu.ft.• Ranges in color from snowy white to grayish white• Floats in casting bodies (very interesting!!!)

PerliteMajorElementsTypical Range in%SiO 272Al 2O 214Fe 2O 30.7Na 2O 4K 2O 8.8CaO 0.3MgO 0.1TiO 0.1• Readily availablethrough horticulture /construction suppliersMesh Size AvailableSoftening PointFusion PointAs desired, 4-8 mesh and finer1600-2000 °F (871-1093 °C)2300-2450 °F (1260-1343 °C)

10% addition ofperlite to a whitevitreous 04 bodyCourtesy of ElizabethEmery (2004 Cookbook)

Organic <strong>Fillers</strong>• Burn-out during firing, leaving voids in the body• Just about anything– Rice– Beans– Cereals– Pasta– Sawdust– Nut shells(avoid explosives, plastics or toxic materials)• May suck moisture from body– Soak in water before adding to clay• Fire slowly and provide adequate ventilation (I.e. may damageelectric kilns; USE ONLY IN GAS KILNS!!!)• Like any non-plastic, most will reduce green shrinkage• But can also reduce strength of fired body (due to higher porosityafter burnout)

Fibers• Improve green strength and can reduce weight• Anything fibrous will create an interlacing network– Paper/Pulp– Flax– Nylon Fibers– String– Straw– Hair• Affect workability of the body– Increase tensile strength in green stages• Leather hard clay slabs don’t snap when you bend them-theyslowly pull apart– More difficult to cut/trim cleanly (cut with scissors)

Paper• Cellulose fibers that make up paper can be brokendown in water• Cellulose is tube-like– Transmits water (acts like a wick)– Compressible like Vermiculite (less stress than hard fillers)• Paper works better at stopping cracks than other fibersbecause it is hairy– Hairy edges act as tiny anchors within the clay• Smoother fibers like nylon or fiberglass are more likely to be pulledout of position by the spreading crack• However– Preparation of paper is more laborious– Cellulose will break down over time (short shelf life)– It stinks!

Paper (cellulose) fibers

Flax• Derived from the stem of a plant and usedin the textile industry• Easily dispersed in water (unlike papercellulose)• Typically longer fibers than those of paper– Better cohesion, working strength, and dryingcrack resistance than paper cellulose• Not good for casting (unlike papercellulose)

Other Fibers• Nylon– Very user friendly• Easy to disperse• Soft / harmless• Burns out• Fiberglass– It is a glass• Will melt (temp. varies by product)• Much more rigid than nylon• Hard on the hands (may irritate skin)• Avoid using without gloves

Additions of fiberglassfibers (6, 9 and 15%) to#444 stoneware boxed clayCourtesy of Nick & Mindy(2009 Cookbook)

For Increased Green Strength• Plaster - CaSO 4 ·½H 2 O– Will set up just like plaster (short working time)– Should be dry mixed with clay first– Refractory up to about cone 6 (depending on the clay body)• Cement– Sets over time– Contains calcium, silica, and alumina– Portland cement is a product of calcined raw materials(dolomite, limestone, and clay)• Try using Luminte (a high alumina cement) instead of Portland• Setting times for both can be retarded with additives– I.e. Milk

ReferencesCeramics: Industrial Processing and Testingby J.T. Jones M.F. BerardIowa State University Press, Ames, IowaSecond Edition 1993Ceramic Industry(Periodical)Business News Publishing Co, Troy, MIJanuary 2002Ceramic Science for the PotterW. G. LawrenceChilton Book Company, Philadelphia, New York, LondonFirst Edition, 1972<strong>Clay</strong>s and Ceramic Raw Materialsby W. E. WorrallElsevier Applied Science Publishers, London and New YorkSecond Edition, 1986

ReferencesCushing’s Handbookby Val CushingThird Edition, 1994Paper <strong>Clay</strong> for Ceramic Sculptors: studio companionby Rosette GaultClear Light Books, Seattle, WASecond Edition, 1995Science of Whitewares IIedited by William M. Carty and Christopher W. SintonThe American Ceramic Society, Westerville, OH2000The Natural History of <strong>Clay</strong>by Alfred B. SearleCambridge: at the University Press1912

ReferencesThe Potter’s AlternativeBy Harry DavisMethuen Australia Pty Ltd, North Ryde, NSWFirst Edition, 1987The Potter’s Dictionary of Materials and Techniquesby Frank and Janet HamerA&C Black Publishers Ltd., London, EnglandThird Edition, 1993

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