Sulfates
Soluble sulfates in clay produce efflorescence, an unsightly scum that mars the fired surface of structural and functional ceramic products.
Details
Sodium, potassium, magnesium sulfates can be found in many clays. These are soluble and migrate to the surface during drying. Soluble salts can be iron stained, but often are white. Depending on the temperature of the firing and the nature of the salt, they can leave scum on the surface (often called 'efflorescence') that ranges from dry white to melted, glossy brown. Heavy clay industries can tolerate clays with higher sulphate contents, but other industries, such as tile, need lower contents). When the clay is fired to vitrification, the salt can act as a micro-thin glaze and actually harden and give a sheen to the surface. On foot rings it can also result in plucking problems.
Related Information
Soluble salts on a range of different cone 6 fired clay brown/tan bodies
The concentrations are not serious and are typical of what you might find on a commercial body.
A DFAC drying test disk of a terra cotta pottery clay from St. Ignacio, Sinaloa, Mexico
This clay is used by traditional potters in the Mazatlan, Sinaloa, Mexico area. This DFAC test shows a very wide main crack and number of edge cracks. These indicate very high shrinkage and plasticity. Although the clay has some coarser grains that help channel water out, this is a very poor showing for this test, no large scale manufacturer could tolerate this. Yet they use it with success, having learned how to adapt. Note alsohttps://digitalfire.com/4sight/admin1/area.php?area=9&clearfromrecent=793 that soluble salts are fairly low.
How bad can efflorescence of soluble salts of fired ceramic surfaces be?
Like this! This terra cotta clay vitrifies here at 1957F (cone 03). 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.
Links
| Articles |
Organic Matter in Clays: Detailed Overview
A detailed look at what materials contain organics, what its effects are in firing (e.g. black core), what to do to deal with the problem and how to measure the amount of organics in a clay material. |
|---|---|
| Glossary |
Deflocculation
The deflocculation process is the magic behind the ceramic casting process. It enables you to make a slurry of far lower water content and thus lower shrinkage. |
| 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 |
Plucking
A firing issue in ceramics where the foot rings of vitreous ware stick to the kiln shelf. Removing them leaves sharp fragments glued to the shelf. |
| Materials | Barium Carbonate |
| Hazards | The Use of Barium in Clay Bodies |
By Tony Hansen 
Monthly Tech-Tip from Tony Hansen
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