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Like this! This terra cotta clay matures to good strength around 1950F. Notice how the soluble salts have concentrated on the outer and most visible surface. The piece was dried upside down so of course, all the water had to escape through that route. A complicating factor is how handling of the piece at the leather hard stage has made it even more unsightly. This problem is common in many terra cotta materials but can also surface in others. Barium carbonate can be used to precipitate the salts inside the clay matrix so they do not come to the surface on drying. There is good news: Solubles salt deposition can actually be much worse than you see here.
Hazards |
The Use of Barium in Clay Bodies
Considerations regarding the use of barium carbonate in pottery and structural clay bodies for precipitation of soluble salts. |
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Glossary |
Sulfates
Soluble sulfates in clay produce efflorescence, an unsightly scum that mars the fired surface of structural and functional ceramic products. |
Glossary |
Soluble Salts
In ceramics, certain compounds in clays and glazes can dissolve into the water, then on drying these are left on the surface. |
Glossary |
Efflorescence
A common problem with dry and fired ceramic. It is evident by the presence of a light or dark colored scum on the dry or fired surface. |
Glossary |
Terra Cotta
A type of red firing pottery. Terra cotta clay is available almost everywhere, it is fired at low temperatures. But quality is deceptively difficult to achieve. |
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