Ceramicware Defects Handbook/Printable version

Ceramicware Defects Handbook

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Crack

Cracks found at mug

Cracks

Defects description

Cracks are usually caused by stress/strain in the clay material either due to the design of ware itself OR types of clay used. Cracks in clay are considered to be naturally occurring and its causes are often too broad and overtly complex in its nature. It is highly possible that the ceramic ware may be strained during the processing but are able to withhold the strain until changes in clay body causing the drastic failure. When searching for remedies, it should be understood that the objective is not to totally eliminate strain-free pots (due to the impossibility of this tasks as each ware have different shapes and sizes) but rather minimizes the magnitude of the problems and minimizes the effect due to natural variation in recipe ingredients. Solutions:

• Try to minimises the differences of section thickness to prevent uneven drying which causes the strain as thinner sections shrink earlier and faster to compared to thicker sections. The strain will be stored permanently once it is dried
• If the cracks are appearing at same places, it is advised to use paraffin. The paraffin will make the "invisible" crack line appearing for few seconds and the crack lines can be pegging to close the crack.
• Cut, trimmed and fettled areas should be wiped with sponges to close up the minute cracks as it will act as stress manipulators causing the cracks to deepen and open out during firing.

There are a few types of cracks namely:

• Edge cracks
• Surface cracks
• Handle cracks
• Biscuit cracks

Edge cracks

Causes:

• Too fast drying
• If appeared even before drying occurs, the raw materials have excess addition of filler, grog or sand or reclaimed clays.
• Uneven cross-section thickness
• Readsorption due to dried clays exposed to high humid temperature. If clays readsorp the water vapour, the slight expansion will take place. Usually, ceramic ware with low clay contents may not enough strength to withstand the moisture expansion causing fine cracks that are visible after biscuit firing.
• Clay is too dry when cut or fettled upon. The clay is much weaker when too dry and can have minute cracks in trimmed surfaces. Smoothing the surface by wet sponging will remove the problems
• AFter glost firing, the kiln is immediately open of its door without proper cooling down to more ambient temperature. This is called thermoshock as the just fired ceramicware is exposed to relatively cooler air. The kiln door should be opened between 80 C - 150 C to minimizes the crack.

Surface cracks

• If excess water formed at the base of the ceramic wares, it will form S crack on the bottom of the wares. Use the suitable sponge to remove excess water after jiggering may help
• If the straight crack appears at the base of the jiggering ware, perhaps it is caused by uneven drying

Biscuit cracks

• Rapid firing /Uneven firing will cause uneven shrinkage. If the top of ceramic wares reaches high temperature quicker than the bottom of the wares, it will cause the cracking due to uneven firing.
• Cracks can be repaired by using stopping without the need of re-firing. Stopping is a filler made from pre-fired grounded materials mixed with special gums and glazes.

Handle cracks

Cracks found in handles joint

Descriptions:
Cracks occurring at joint may be due to imperfect joining or using too much/little slips. It also occurs when the handle "spring" aways from its main body due to internal stress during drying and/or firing. It is usually caused by different "clay hardness" between handles and the main body. Therefore, during drying/firing, one part may shrink faster compared to others, giving it increased stress and eventually cracking.

Solutions:

• It is also generally found better to mix a bit of Epsom salt (magnesium sulfate) into the slips as Epsom salts will increase the plasticity of slips. It is also a flocculant, meaning the clay particles will attract each other.
• Ideally, the handle clay and main body should be of same type of clay and ideally in same conditions to reduce the risks of differences shrinkage causing cracking
• If handle section is too wet, it may cause the handle to become weak causing cracks

Dunting

Dunting

Dunting shown at inner mug

Dunting shown at outer mug

Descriptions:
It is the type of cracks that passes through the pottery and formed as the result of thermal stress. There are many underlying causes but the trigger point is usually caused by thermal shock. Dunting is also more usually confined to glost ceramic wares as the ware has cooled down after being heated up.

There are types of dunting which are more dangerous that are "hot water dunting" (See pics attached). It occurs when hot water are poured into the ware ,where cracks formed on side wall, it may also occur at junctions where wall and base are formed together.

Solutions:

• Cracks are usually formed from corners and sharp angles. Therefore ceramic wares should be designed with the rounded shape. If there are angular holes or crevices, it is highly advisable to wet sponge the area to remove minute cracks which will be served as stress multiplier. Here are some design considerations to follow to minimizes dunting.

1. Redesign the ceramic ware with rounded holes or rounded edges whenever it is possible. Avoid any of the sharp corners/angles
2. Made the sections equally thick to prevent thinner sections dry faster compared to thicker sections
3. Ensure the wall thickness (> 8mm and above) should be adequate to withstand compression and also the rapid expansion of glaze when exposed to boiling water

• The glaze should be as thin as possible as the glaze layer will exert compression force on clay as it cools down.
• Dunts spread readily in smooth dense clay body. Use lower firing temperatures and lower temperature glazes can help to reduce the dunting.
• Refiring may cause cristobalite in clay materials to form causing excessive shrinkage. It will fatigues the ceramic ware after each refiring. If the wares wouldnt be made good after 2 or 3 attempted re-firing, it is best to sell as second grade wares.
• Kiln are opened way much earlier during cooling down period. It is advisable to open up kiln only when the inside kiln reaches 150 C to reduce the thermal shock causing dunting
• If the ceramic wares are complex, it is advisable to "split up" to two piece from one piece to avoid accumulation of stress points.

Pinhole

Pinhole

Defects Description

Pinhole in glaze layers

Pinholes are usually a small holes that appears on the surface of ceramic wares and mainly caused by the materials that burnt out during firing leaving a minuscule crater on the clay bodies.

Causes & Solutions

There are many possible sources of defects such as:

• Airborne particles stuck onto the surface before firing

Solution: Ensure the firing kiln are cleaned at least once a week and generally ensure proper cleaning is done in the production floor where the unfired wares travel

• Contaminated sands used during slip mixing.

Solution: Need to sieve the slips before casting

• Foreign contaminants such as fibers from cleaning clothes, particles of sponge or foreign debris from water that are burnt off when firing leaving a hole on its place.

Solution: Try replacing the consumable materials (such as sponge) to materials which are less likely to leave foreign contaminants (such as silicone scrubber)

• Poor pugging due to faulty de-airing machine (vacuum parts). The vacuum should extract any of the air bubbles inside the clay cakes.

Solution: Check the vacuum functions of de-airing machine are still functioning according to the manual

• Rapid firing may cause trapped remnants air bubbles inside clay to burst through the surface

Solution: Try the slower firing allows the air to percolate to the surface without rupturing clays causing pinhole

• Direct uses of reclaimed slip cast. Slips drained from moulds may pick up dust and dirt from pouring holes, casting throughs and etc.

Solution: Only mix the reclaimed slips to main stocks if the slips are ensured in good condition; otherwise return to blunger

• Insufficiently blunged slips (especially if dried scrap clays are added)

Solution: Increase the blunging time and blunger speed and manual checking on slips to see if the slips are properly blunged.

• Thick glaze coating where the bubbles may form as bubbles may take times to travel from bottom of layers to surface

Solution: Thinning the layers to reduce the travels of bubbles

• Short heat soaking time causing the glaze is still bubbling at end of glost firing

Solution: Extend the soaking time at max temperature by half-hour or more to allow the pinholing to recover and glaze to stabilize

• Contamination in glaze tubs due to dried glaze falls into the glaze slips during next dipping sessions. The dried glaze has the air pocket which will cause air pockets

Solution: Cover the glaze tubs when not in use, the sides of tubs should be wiped with clothes or hands

Specking

Specking

Specking can caused by a great variety of sources, To narrow down the potential causes, it would be best to break up the wares to observe if the specking is observed inside the ceramic wares or merely on surface If observed inside and outside of the ceramic wares, it is highly caused during body preparation stages If observed at both top and bottom facing, it implies it occurs during the early stages of clay processing Brown or black specking may occurs due presence of iron contaminants Sources of the contaminants are as followed:

• Presence of iron / rusts insisde the slip. Use double/triple sieve with decreasing aperture sizes (or deck sifters). Sieves should be magnetted to removed iron particles and magnets should be regularly cleaned.
• Regulary checking on slip storage do not have contaminants
• Filtered pressed cakes may be contaminated by pallets rusted nails
• During rainy seasons, the rain drops may bring the rusts of rooftop trusses possible dripping on clay cakes
• Electromagnet are not functioning
• Pug mills rusted in the barrel screws (No proper maintainence)
• Airborne dust that maybe blown around due to poorly design ventilations/ blower fans
• Exhaust discharge from grinding wheels. Silicone carbide are notorious specking agent and if possible, use sintered alumina
• Contaminations in water supply due to rusted metal
• Particles that are logged in to the kiln shelf onto ware below. Placing the wares from topmost shelf slowly downard to bottom to minimses any of the dislodged particles fall to the shelf below
• Contamination of the glazes (dipping) Keeping glaze tub covered when not in use.
• Repair technician perform repairing which includes but not limited to metal cutting, welding metals, knocking metals by hammers/chisels which may cause debris of iron particles spreading especially if it is near or inside production area
• Inadequate cleaning of kiln may causes kiln's batt particle to be spread over

Crazing

Crazing

Fine crazing defects lines

Descriptions: It is a network of fine cracks spreading throughout the glost ceramic wares. If the pottery body is porous, crazing permits the moisture to enter into pottery articles (such as earthenware) making the ware unhygienic for dining purposes. Some of the craze lines is so fine that it is usually undetected with normal visual inspections.

Solutions:

• The crazing line is usually found more on thicker glaze areas. Therefore, one of the most obvious solutions is to reduce glaze thickness.
• Underfiring in glost firing (Temperature must be over 1100C) as the glaze obtain body thermal expansion that will be sufficient to avoid crazing as with most cases of glaze. If craze appears, refiring the crazed ware should clear the problem provided the glaze can withstand high firing temperatures
• Insufficient heatwork as the firing time is reduced making the glost firing not giving enough heatwork to the ceramic ware. The usual standard firing cycles is around ramping of 100C-120C/hour should suffice to reduce crazing in ceramic ware
• A Combination of low expansion body/high expansion glaze may cause crazing at normal glazes. For body, consider adding cristobalite to induce body expansion. For the glaze, to reduce expansion, consider adding zircon (5% amount) into the glaze to reduce thermal expansion and increase hardness. Also consider replacing soda, potash and/or alumina with zirconia, magnesia, ,zinc oxide or boric oxide.
• Heat shock also causing crazing. Therefore to reduce crazing, it is ideal to open up the kiln only when the kiln is around 150C and below.

Crawling

Crawling defects on the underglaze decorations

Crawling defects (Beading) due to glaze

Crawling

Defects description

Crawling is described as the bare area(s) in which the molten glaze does not cover during the glaze firing. The severity of the defects may vary from case to case from the isolated area (pin hole) to more serious cases such as 'beading' which it is akin to forming beads of water on waxed surfaces. Generally, it is caused by surface tension forces (particular in molten glaze) overcoming the adhesion forces.

Causes and solutions

The common cause of the crawling such as:

• Underglazes decorations may cause crawling due excess of the medium (in underglaze decoration) or insufficient fluxes.

Excessive mediums cause the greasy surfaces causing glazes unable to stick to the underglaze decoration. Hardening (about 600 C) should burn away the medium and will cause colour to sinter and bonded firmly to the ceramic ware.

Insufficent flux can be remedied by adding flux (borax frit) in 5%-50% or SCMC binders in 1%-3% to colour pigment may help for the glazes to attach to underglaze decorations.

• Area may be contaminated with grease, specks of dust or similar materials which will cause lack of adhesion glaze which causing the glaze to roll back during glaze firing
• Thick glazes may aggravate the problems since minute cracks may be formed during firing which is then healed leading to the formation of large cracks in molten glaze producing crawling. It is usually seen in inside holloware and around handle joints. Thin the glaze by using water or adding a bit of binder (to increase adhesion)

Consider using spray glazing instead of manual dipping as this has several advantages

# The glaze is partially dried when travelling from nozzle guns to the ceramic surfaces (Reduced water contents will be helping in forming less crawling)

# The sprayed glaze is deposited in a more 'fluffy' arrangement which offers less resistance for water vapour to escape during glaze firing

• Overgrinding glazes may produce dusty glazes with poor adhesion. Also due to the chemical compositions, it will become more viscous increases crawling risks
• Zircon opacified glazes, zinc oxides, high alumina and leadless glazes are considered as highly viscous glaze. Highly viscous glaze may be corrected using extra additives such as flux (borax flit) or firing at higher temperatures.
• Glaze recipes formed partly of colemanite by Asian sources which generate voluminous gases which cause patches of glazes ejected from wares. Adding flux calcium borate frit to the glaze or slower rate firing (increase soaking time) may reduce the crawling to ensure the gasses had ample times to escape.

Colour flow

Colour Flow

Defects description

The design shows colour flow

Some colour pigments especially cobalt blue and nickel can be affected by covering glazes during firing which will produce blurred outline The problems are more noticeable if the decorations are at slanted and not flat-down surfaces

Causes & solutions

• Glaze is too thick. Thicker glaze may "flow" down and drag colour pigment with it. Thinning the glaze as realistically possible by applying lesser glaze.
• Lower the glaze firing temperature for the glaze to melt lesser molten glaze (To minimiises potential of molten glazes "dragging" colour pignments)
• Adding 5%-10% alumina in the glaze to increase molten glaze viscocity
• Shorten the soaking period as realsitically as possible (Also not too short until the glaze layer is not properly melted)

Colour variation

Colour variation

Defects Description

Colour variation between differing batches

The tonal of the underglaze decorations shown has shown significant differences where the colour reproductions are differs from batch to batch

Causes & Solutions

• Glazes

Solutions: Original colour batch are fired alongside with new colour batch to see if there are differences between these two batches. Repeat the test in glazed ware, if there are differences noted, the problem are highly possible resides with glazes used. For examples, low solubility glazes may give brighter colour response but take note that low solubility glazes differ from each other. Potclays 2204 glaze may give superior colour response compared to Potclays 2205 glaze.

• Underglazes coating are too thinly applied

Solutions: If too thinly applied the colour may changed for example from brown to gray

• Differences in firing temperatures

Solutions: Pigment colours that contain selenium, cadmium, copper will burn away at elevated temperatures.

• Sulphur trioxide chemicals may affect colours (browning of chroma green)