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Sulfates, Sulphates

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

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

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?

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.

Out Bound 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

    In ceramics, when we speak of deflocculation, we are almost always talking about making a casting slip. Glazes can also be deflocculated (to reduce water content and densify laydown). Deflocculation is the process of making a clay slurry that would otherwise be very thick and gooey into a thin po...

  • (Materials) Barium Carbonate - BaCO3

    Barium Carb, Witherite

  • (Hazards) The Use of Barium in Clay Bodies

    Hazards of barium carbonate, considerations regarding its use in clay bodies for precipitation of soluble salts

  • (Glossary) Efflorescence

    A term describing the whitish or brownish dry or glassy scum (depending on iron content and firing temperature) left on the surface of a fired clay body (most often red earthenware or raw stoneware and fireclays). Many clays contain soluble sulphates that migrate to the surface with the water and ar...

  • (Glossary) Plucking

    Plucking refers to the chipping away of small fragments of the base of a ceramic vessel because the piece sticks to the kiln shelf during firing. The stuck piece either pops off during kiln cooling (due to the difference in thermal expansion with the shelf) or it needs to be broken off. There are se...

By Tony Hansen

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