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Sun Clay - Air Clay

Faults & Remedies

Walker Ceramics are proud of the technical support and after sales service we provide. Listed below are some of the most common faults and their remedies. For a more detailed reference we recommend :

"Ceramic Faults And Their Remedies" 

(Product Code SA110) by Harry Fraser.

Bloating
Fault - Bubble formation within body during firing.
Caused by expansion within body producing by pressure build up of gases trapped in a partially vitrified body. 
(1) Fine bubbling is due to over firing. Large bulges usually caused by trapped carbon are due to firing too rapidly. 
(a) Reduce top temperature or fire more slowly. 
(2) Carbon trapped within vitreous body. 
(a) Fire more slowly. 
(3) Over reduction. 
(a) Reduce reduction firing time. 
(4) Large bubbles caused by too rapid firing.
(a) Fire between 600C and 1000C at 100 / hour maximum.

Blow Out
Fault - Holes in biscuit body, but often days or weeks after firing.
1. Presence of impurities in clay or glaze. Re-hydration of lime bearing intrusions, such as plaster, concrete, cement.
Remedies
(a) Remove any plaster plucked away from mould surface.
(b) Use filter pressed bodies.

Cracking
Fault - Cracks in biscuit ware. 
(1) S cracks on inside base. 
(a) Remove excess water in base of pot  during throwing - also apply forming pressure to base during throwing before raising walls. Dry base more quickly to match wall drying speed. Remove pots from batts earlier.
(2) Handle cracking. 
(a) Mismatched water content of handle and body. Strain during attachment.
(3) Spiral cracking following lines of rotation of the wheel. 
(a) Over extended through rushed throwing. Thick and thin throwing, rings causing differential drying rates.
(4) Drying cracks - fine network or general.
(a) Too fine grained clays or thrown too wet. Dry more slowly. Fire more slowly to 300°C. Sharpen turning tools. reduce overworking of clay during making.

Crawling
Fault - Bare (unglazed) on surface of ware. Glaze forming small islands. More prevalent in once fired ware.
Cause 
(1) Excessive handling of dry clay ware before glazing. 
(2) Glaze layer too thick. 
(3) Oil, grease, dust on ware before glazing. 
(4) Bad handling of glazed ware. Add gum to harden glaze. 
(5) Excessive colloidal matter (clay or zinc) in glaze. 
(6) Over ground glaze. 
(7) Too viscous glaze - add flux or fire higher. 
(8) Add 20% of glaze to oxides if used as underglazes. 
(9) Over fast firing of wet ware to 600C for once fired ware or up to 100C for glazed biscuit ware. 
(10) Soluble salts in sponging water - add a little vinegar to the sponging water.

Crazing
Fault - Fine cracks in glaze surface.
Cause
(1) Mismatch of glaze and body thermal expansions. 
(2) Glaze applied too thickly.
Remedy
BODY: Fire earthenware body to higher temperature, soak longer, or add silica to body. Reduce porcelain firing temperature and/or soaking or add coarser silica to body. 
GLAZE: Add silica or china clay to glaze. In earthenware glaze, use Borax Frit of lower expansion.

Devitrification
Fault - Glossy glazes, usually leadless, appearing matt. Milky appearance of transparent glaze (often bluish pink over terra cotta bodies).
Cause 
(1) Precipitation taking place during cooling on glazes to about 800C. 
(a) Appearance of small crystals on surface of glaze. 
(b) Milky precipitate. 
Remedies
(a) Cool more quickly down to 700C. 
(b) Reduce lime content of glaze. 
(c) Add a mix of lead bi-silicate frit containing 10% china clay progressively to glaze. 
(d) Use low solubility glaze instead of leadless glaze.

Dimpled Surface
Fault - Orange peel glaze surface.
Cause - Gas bubbles in the glaze.
Remedies 
(a) Slower firing cycle or longer soaking period. 
(b) Check under firing.
(c) Biscuit firing temperature to minimum 1000C. 
(d) Excessive over firing - check with cones.

Blistering
Fault 
(1) The glaze contains innumerable bubbles, pinholes and craters. 
(2) A vitrified body with minute bubbles on surface, sometimes only evident by roughened surface. 
(3) Blistering over underglaze decoration.
Cause -1) Over firing
Cause-2) In gas kilns, flame impingement will cause the fault.
Cause-3) Very rapid firing at maturity, does not allow the glaze to mature.
Cause-4) Low thermal mass - fibre kilns -can cool too rapidly.
Remedies
1(a) Slow down temperature rise to top temperature over last 50°C.
1(b) Increase soak time up to 30 minutes.
1(c) Alter glaze to a more fluid state e.g. use nepheline syenite to replace some feldspar in stoneware glazes.
1(d) Use wollastonite instead of whiting or dolomite in stoneware glazes.
1(e) Often only near elements or flame flashing in gas kilns ie move pottery away from edges of batts.
2. Lower firing temperature or alter body fluxes
3. Use coloured stains instead of metallic carbonates, especially manganese dioxide.

Dunting
Fault - Splitting of ceramic ware due to silica inversion. (When glaze has run into crack, dunting has occurred during heating cycle.  A crack with a sharp edge -dunting has occurred during cooling cycle.)
Cause
(1) Too rapid heating and/or cooling of body. 
(a) Fire more slowly, but more particularly cool more slowly. 
(2) Large variations in wall thickness of article giving rise to thermal gradients.
(a) Give careful consideration to design. 
(3) Over firing of body which results in a high cristobalite inversion. 
(a) Reduce glost firing temperature. 
(4) Design or production fault.
(a) Finish all edges, holes etc. with a damp sponge. Inspect carefully for minute drying or forming cracks and fill by wet smoothing. Round out corners, edges, holes. Glaze as thinly as possible - NO pooling of glazes. Reduce reduction time on iron rich bodies. Make walls thicker if too thin. Glaze all over instead of partial glazing.

Hot water dunting
Fault - Coffee mugs, tea pots, casseroles, etc. crack when boiling water is poured into the vessel. Usually around the base or spiraling up the side wall. They are usually reduction fired, iron rich bodies, which are partially glazed.
Cause 
(1) Grossly different coefficients of expansion between the over fired or over reduced sections of the pot. 
(a) Make pots thicker and even thickness for extra strength. 
(2) The reduction of iron and the formation of ferrous silicate glazes in localized areas, principally in the under glazed or thin areas near the base. 
(a) Reduce the time of reduction firing and start it later in the cycle.
(3) Poor geometry.

Engobe Faults
Fault -  (1) Flaking or cracking on drying. 
Cause - Shrinkage of slip too high. 
Remedy 
(a) Ware too dry when applying. Check settling of slip, stir well before applying. Too thick a coat, also try drying more slowly. Engobe has too low solid content, deflocculate and increase litre weight. Shrinkage of slip too low. Increase shrinkage of engobe, add 1-5% bentonite.
(b) Apply to drier ware. Apply thinner coat. Add gum Arabic to slip 1-2%
(2) Flaking or cracking after firing.
Cause - After firing is caused by shrinkage differences, usually on the convex faces and edges.
Remedy
Add 20% of glaze to engobe to help it bond to the body during firing.
Once fired ware faults - Glazing technique
(1) Body swelling and cracking - under the glaze - caused by moisture absorption into body which parts in thin layers parallel to the surface or bulges form due to the air entrapment in centre of thin walls.
Remedies.
(a) Completely dry ware to less than 0.5% water content.
(b) Deflocculate glaze to reduce water content but retain sufficient thixotropy to eliminate running. Use soda ash as the deflocculent.
(c) Hollow ware; thin dip inside and immediately dip outside with a quick drying time, if not dry well before glazing the outside.
(d) Watch drying time from dripping; it should be about 7 seconds. If longer than this body swelling and cracking can occur. To increase drying time, add a deflocculent such as Epsom salts or calcium chloride. To decrease drying time , add soda ash or Dispex or increase litre weight.
(2) Crawling - Appearance - Bare (unglazed) patches on surface of ware. Glaze forming small islands.
(a) Reduce handling  and blow dust off surface before glazing.
2) Rupturing glaze by steam generation from fast firing.
a) Slow firing up to 600°C  and ventilate kiln well.
3) The drying and firing shrinkage of the glaze is not matched.
(a) Glazes should contain about 30% clay or about 5-6% bentonite. watch for cracking during drying to guide shrinkage control.

Pin holed Glaze
Pinholes in glaze after firing.
Caused by gas evolution from body and/or glaze during firing.
(1) Under firing of body.
(a) Fire body to recommended temperature.
(2) Over application or over firing of underglaze colours.
(a) Reduce application thickness of underglaze colours or reduce temperature.
(3) Soluble salts (sulphates) in body.
(a) Add 0.01 - 0.25% Barium Carbonate to the body.
(4) Excessive whiting / calcite in glaze.
(a) Reduce amount of whiting / calcite in glaze.
(5) Under firing of glaze or too short a soaking time.
(a) Fire glaze to recommended firing temperature and soak sufficiently to fully mature glaze.
(6) Over firing of glaze giving rise to volatilization.
(a) Reduce firing temperature or soak time.
(7) Maybe the glaze binder is causing the problem.
(a) Try other glaze binders eg Bentonite , gum arabic , gum tragacanth, Tylose CMC.
(8) Glaze is too thick.
(a) Try applying thinner coat of glaze.
(9) Underglaze surface is too dry.
Add 10% of frit or compatible clear glaze to the colour.
(10) Over oxidized underglazes.
(a) Change to industrially prepared underglaze colours.

Peeling
Glaze lifting away from body. Mainly occurs on edges of pots such as cup rims and handles.
BODY
Caused by glaze being under excessive compression.
Reduce body expansion by
a) Reducing the firing temperature to reduce cristobalite conversion.
b) Reduce body soaking period.
c) Reduce the amount of silica in the body.
GLAZE
increase the glaze expansion by;
a) adding high expansion alkaline frit to the glaze.
b) Reduce silica and/or alumina content of the glaze.
(2) Migration of soluble salts to surface of the body on drying and firing giving rise to poor adhesion of glaze.
(a) add Barium Carbonate (.01-.25%) to body to precipitate soluble salts.
(b) lightly sponge rims and handles of piece before bisque firing.
(3) Over sponged rims.
(a) Use care not to over sponge, which will promote peeling.

Scumming
Usually grey white film on the surface of fired ware, most obvious on Terra Cotta.
Usually soluble Calcium Sulphate deposited directly on the dried clay surface and when fired becomes a coating, often vitreous. Sometimes it is ivory yellow due to soluble iron salts.
(a) Use filter pressed bodies which reduces the soluble salts substantially.
(b) Slow the drying time to reduce edge concentrations of scum.
(c) Add up to 0.2% Barium Carbonate to clay to convert the salts to insoluble sulphates.
(d) Sponging the dried ware with water, plus some vinegar, is sometimes beneficial.
(e) Wash the finished product with diluted spirits of salts, as sold for brick cleaning.
(f) Dry hollow ware inverted, to preferentially deposit salts on the under base.

Warping
Mis-shaped ware after drying and/or firing.
(a) Uneven drying or uneven firing.
(b) thick and thin sections and "bad" geometry of piece.
(c) "Clay Memory" i.e. the reverting of shape if strains are applied during forming.
examples - 
(i) clay spiral in thrown ware
(ii) strains during handling.
(d) Warped kiln shelves is a common cause.
(e) Firing too fast, 150°C per hour is sufficient
(f) Inadequate or ill positioned supports during drying or firing.
(g) Warping tiles can be cured by drying evenly from both sides.

Faults & Remedies - Casting Slips

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