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GLAZE DEFECTS

GLAZE DEFECTS. All the little surprises you can find when you open your kiln. PRESENTED BY:. COMMON DEFECTS. PINHOLES BLISTERS CRAWLING or CREEPING CRAZING SHIVERING or FLAKING OFF BURNING OUT (REDS) GLAZE RHEOLOGY Settling out Thickening up Brush-ability. PINHOLES.

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GLAZE DEFECTS

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  1. GLAZE DEFECTS All the little surprises you can find when you open your kiln. PRESENTED BY:

  2. COMMON DEFECTS • PINHOLES • BLISTERS • CRAWLING or CREEPING • CRAZING • SHIVERING or FLAKING OFF • BURNING OUT (REDS) • GLAZE RHEOLOGY • Settling out • Thickening up • Brush-ability

  3. PINHOLES • Caused by gases blowing through molten glaze • Due to the vaporization of tiny pieces of charcoal which are present in the clay body • Usually due to underfiring of the bisqueware in low fire ceramics • For low-fire ceramics always bisque fire at least 1 cone hotter than the glaze firing temperature • i.e. for cone 05 glaze firing, bisque fire to cone 04

  4. CRAWLING • Caused by an excess of surface tension in the glaze during firing • Usually due to the glaze being applied too thickly • Can be caused by the glaze not adhering well to the bisque such as in corners or over dirty finger marks • Sometimes it can be a result of using glaze materials that are too finely ground

  5. BLISTERS or CRATERS • Caused by gases erupting from the molten glaze • Usually due to overfiring of the glaze • Can be caused by insufficient drying of the piece • In high-fire ceramics it may be due to the presence of impurities such as iron or manganese in the clay body

  6. CRAZING • Caused by a mismatch in coefficients of expansion between the clay body and the glaze • Crazing is a result of using a glaze that is too high in expansion for the clay body or bisqueware in use • Usual solution is to use a lower expansion glaze or a higher expansion clay • It can sometimes be fixed by bisque firing hotter • You can also try lowering the expansion of the glaze by adding a small amount of silica (flint)

  7. SHIVERING or SPAWLING • Shivering is also a mismatch of expansions but the reverse of crazing • Caused by using a glaze that is too low in expansion for the clay body • Need to change to a higher expansion glaze or lower expansion clay • It is possible to raise the expansion of the glaze by adding a small amount of high expansion material, such as a clear crackle glaze

  8. BURNING OUT • The old lead based cadmium reds were very prone to burning out due to overfiring • This would frequently happen even at cone 06 • Lead reds are no longer manufactured in North America due to health concerns • They have been replaced over the last 5 or 10 years by much more stable non-toxic reds

  9. GLAZE RHEOLOGY • SETTLING OUT • The most common cause of glaze settling is adding too much water when trying to thin a glaze • THICKENING UP • Some glazes tend to thicken up over time due to soluble materials in the glaze that gradually dissolve into the water • BRUSH-ABILITY • Glazes can lose their brush-ability if bacteria grows in the glaze and consumes the cmc gum which is the only organic component of a glaze

  10. SETTLING OUT • Caused by the failure of the glaze suspension system • In a mild case, you will see water rising to the top of the container and thick gooey glaze sinking to the bottom • In a severe case, all the glaze material settles to the bottom in a rock hard mass • There are various ways to fix the problem depending on the circumstances • The most common solution is the addition of a suspension agent, such as bentonite, epsom salts or Spectrum Suspender (1071)

  11. THICKENING UP • Thickening up of a glaze over time is usually caused by the presence of a soluble material, such as zinc oxide • Do not mistake this for a glaze drying out because the lid was not replaced properly • Most brushing glazes are normally about the consistency of heavy cream • In a mild case of thickening up the glaze can still be stirred and can be thinned with water or with Spectrum Glaze Thinner (1072) • In severe cases the entire glaze can go semi-solid and is very difficult to rework

  12. BRUSHABILITY • Brush-ability of a glaze is due to the use of CMC gum as one of the suspension agents which slows down the drying time of the glaze • CMC (carboxymethylcellulose) is the only organic ingredient in most glazes and is therefore subject to bacterial or fungal attack • Bacteria problems are usually due to: • Glazes being stored in a warm place or in direct sunlight • Used, dirty brushes being dipped into the original glaze container • Left over glazes being pored back into the original glaze container • A short term solution to brush-ability problems is the addition of more CMC gum solution or Spectrum Brushing Media (1070)

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