|Monthly Tech-Tip |
In ceramics, bodies of different colours can be kneaded together to produce a marble-like result. But caution is needed.
Key phrases linking here: marbling - Learn more
The practice of partially kneading together clay bodies of contrasting colors (often porcelains colored using metal oxides or stains). Most articles completely overlook the importance of choosing or developing bodies of similar firing shrinkage - otherwise stresses accumulate within and seek opportunity for release (making pieces into time-bombs). Similar-as-possible thermal expansions is also ideal if rapid temperature changes will occur (to avoid thermal shock failure).
If making your own colored porcelains it is important to consult the stain manufacturer's website to see which stains they recommend for bodies. Experimentation is needed with each color to determine optimal percentages (these vary widely, usually 5-20%). Vitreous porcelains produce brighter colours.
A transparent glazed. It is a made from Plainsman Polar Ice in 2014 (a New Zealand kaolin based porcelain) and fired to cone 6 with G2926B clear glaze. 5% Mason 6306 teal blue stain was added to the clay, then this was wedged only a few times. The piece was thrown, then trimmed on the outside at the leather hard stage and sanded on the inside when dry.
Stains can and do influence the degree of vitrification of a porcelain. Some stains will make a porcelain more refractory (decreasing fired shrinkage), others will make it more vitreous (increasing the firing shrinkage). Obviously, the greater the percentage of stain the greater the effect. Stained porcelains having differing fired shrinkages will stress at boundaries in accordance with the degree of difference in their fired shrinkages. In this piece, you can see how the boundary between the red (more vitreous) and green (less vitreous) porcelains is the point-of-failure. The only solution is to adjust the porcelain recipe to move the fired maturity in a direction that counterbalances the effect of the stain. For example, you could employ three recipes (regular, more vitreous, less vitreous) and use the indicated one for each stain added.
Engobe Body Compatibility Test
A bi-clay test strip curls to demonstrate the drying and fired compatibility or fit of an engobe (or slip) on a ceramic body or of two bodies for marbling.
|By Tony Hansen|
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