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Soluble Salts


In ceramics, certain compound in clays and glazes can dissolve into the water, then on drying these are left on the surface. For more information, see the topic Efflorescence.

Various cone 10R clays with soluble salts on the surface

Various cone 10R clays with soluble salts on the surface

These disks concentrate the solubles on the outer edge (because of the way they are dried). Soluble salts can enhance the visual appeal of a fired clay but they can also do the opposite.

Particle size drastically affects drying performance

Particle size drastically affects drying performance

These are DFAC drying performance tests of Plainsman A2 ball clay at 10 mesh (left) and ball milled (right). This test dries a flat disk that has the center section covered to delay its progress in comparison to the outer section (thus setting up stresses). Finer particle sizes greatly increase shrinkage and this increases the number of cracks and the cracking pattern of this specimen. Notice it has also increased the amount of soluble salts that have concentrated between the two zones, more is dissolving because of the increased particle surface area.

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.

Plucking in a cone 10R stoneware body having soluble salts

Plucking in a cone 10R stoneware body having soluble salts

The solubles salts have formed the brown coloration on the bare clay foot ring. While the actual salts layer is very thin, it is glassy and enough to glue parts of the base to the kiln shelf (the latter did not have adequate kiln wash or sand). The glaze line is close to the foot and this complicates the problem. There are a couple of solutions. Sand the foot ring at the dry stage to remove the soluble salt layer. Use a more refractory kiln wash that offers a powdery, non-stick surface.

Solubles salts on a porcelain mug are causing plucking

Solubles salts on a porcelain mug are causing plucking

Fired to cone 10R. The porcelain contains bentonite and a plastic kaolin, both are contributing iron-stained solubles that come to the surface during drying. They tend to concentrate on this foot ring. The solution is to employ a little barium carbonate in the porcelain recipe to precipitate the salts. These could also be sponged or sanded off in the dry state.

Out Bound Links

  • (Glossary) Efflorescence

    A term describing the whitish or brownish scum (depending on iron content) left on the surface of a fired clay body (most often red earthenware or raw stoneware and fireclays). Many clays contain soluble sulphates that are left on the surface after having been left there by water that has subsequent...

In Bound Links

  • (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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