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.
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 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.
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