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 subsequently evaporated. This can happen during initial drying after forming (during manufacture) or it can happen later after the clay is fired and subjected to repeated wet-dry cycles in use (e.g. brick). This problem can be dealt with using additions of barium carbonate or by slurrying a clay using excess water, allowing it to settle and pouring off the stained water (and repeating as necessary).
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.
A typical DFAC drying disk of an iron stoneware clay
The center portion was covered and so it lagged behind during drying, setting up stresses that caused the disk to crack. This test is such that most pottery clays will exhibit a crack. The severity of the crack becomes a way to compare drying performances. Notice the test also shows soluble salts concentrating around the outer perimeter, they migrated there from the center section because it was not exposed to the air.
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.
Soluble salts on cone 04 terra cotta clay bodies
Low temperature clays are far more likely to have this issue. And if present, it is more likely to be unsightly. The salt-free specimens have 0.35% added barium carbonate.
A sculpture bodies gets a lot more interesting surface
This is an example of how soluble salts can enhance the appearance of the fired surface of a cone 10R clay. This sculpture body is a vitreous dark brown burning base having lighter colored 20 mesh grog particles. The one on the left uses native stoneware clays that contain natural flux-containing solubles that migrate to the surface during drying. When fired they act like an extremely thin layer of glaze, producing a darker sheen on the surface. The thickness (and thus color) varies with contour and exposure of the surface during drying. The inside of the cone has no solubles at all.
OM4 Ball clay fired from cone 10R (top), 10 down to 4 (downward)
Ball clays are normally refractory, none of these are vitrified to any extent. The cone 10R bar is yellow because it is stained by the soluble salts present in the material.
The magic of a small barium carbonate addition to a clay body
Two bisqued terracotta mugs. The clay on the right has 0.35% added barium carbonate (it precipitates salts dissolved in the clay to prevent them coming to the surface with the water and being left there during drying). The process is called efflorescence and is the bane of the brick industry. The one on the left is the natural clay. The unsightly appearance is fingerprints from handling the piece in the leather-hard state, the salts have concentrated in these areas (the other piece was also handled, but has very little marking).
This is an expensive, quality bentonite! It is used in porcelains!
HPM-20 micro-fine bentonite fired from cone 1 to 7 in oxidation. This bentonite is expensive compared to others and it used for the guarantee that there are no speck producing particles. However it is still high in soluble salts (that melt by cone 4) and is very dark burning in color. It is not unusual to put 3-5% of this (and other dirtier bentonites) into Grolleg porcelain bodies (where whiteness is supposedly important).
Brown soluble salts that appear after drying, but disappear on firing
The soluble salts dissolved in the water of plasticity of this red body have migrated through the white engobe during drying of these earthenware cups. The cups were upside down so all the solubles have been left on the outside surface. The red body is made using a high percentage of Redart clay (a widely available commercial low-fire low-plastic clay in North America). It is plasticized using added ball clay. The brownish material is organic, because after bisque firing it has disappeared.
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