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.
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 are left there after it has 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). Efflorescence is an issue that has always plagued the brick and tile industries. The problem can be dealt with using additions of barium carbonate to precipitate the salts (barium carbonate and calcium sulphate react to yield barium sulphate and calcium carbonate, both of which are insoluble). Soluble salts can also be removed by slurrying a clay using excess water, allowing it to settle and pouring off the stained water (and repeating as necessary).
Example of sedimentation test to compare soluble salts water extracts from suspended clay. This simple test also reveals ultimate particle size distribution differences in clays that a sieve analysis cannot do.
Two bisqued terracotta mugs. The clay on the right has 0.35% added barium carbonate (it precipitated the natural soluble salts dissolved in the clay and prevented 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).
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.
The concentrations are not serious and are typical of what you might find on a commercial body.
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.
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.
The soluble 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.
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.
Goldart (left) compared to Plainsman Midstone (right). Goldart is a buff and vitreous stoneware at cone 10R. These are fired at cone 7, 8, 10 oxidation and 10 reduction (bottom to top). Soluble salts in the Goldart impart a darker coloration to the reduction fired bar). The Midstone has some coarser particles that make larger speckles in reduction.
Products like this are available at hardware stores. After you have removed the surface scum, be sure to seal it using a sealer (also available at hardware stores).
This is a common sealer available at a hardware store. I have dipped the terra cotta tile and it has dried. The surface of the dipped portion is smoother and has a slight sheen. That is the price paid for sealing the matrix against freeze-thaw spalling.
Perhaps you are shocked that a material this dark and dirty (the bars are fired from cone 1 to 7 oxidation, bottom to top), would be used in porcelains. Why? Bentonites are very difficult to process. This is just raw bentonite (HPM-20), dry ground to -325 mesh (to guarantee no fired specks). That grinding does not reduce the soluble salts (that melt by cone 4) or the iron (which accounts for the dark-burning color). These undesirable properties must be tolerated (as whiteness loss) to get the plasticity supercharge 3-5% of this can impart. Why not use super-white bentonites or smectites instead? They can cost ten or even twenty times as much!
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.
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.
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. These are very typical of what most ball clays look like.
Efflorescence at Wikipedia
|Tests||Drying Factor/Water Content/Solubles|
Water in Ceramics
Water is the most important ceramic material, it is present every body, glaze or engobe and either the enabler or a participant in almost every ceramic process and phenomena.
In ceramics, certain compound in clays and glazes can dissolve into the water, then on drying these are left on the surface.
Soluble sulfates in clay produce efflorescence, an unsightly scum that mars the fired surface of structural and functional ceramic products.
What is clay? How is it different that regular dirt? For ceramics, the answer lies on the microscopic level with the particle shape, size and how the surfaces interact with water.
Soluble Salts in Minerals: Detailed Overview
There are a wide range of soluble materials that can be in clay, this article enumerates them, provides procedures on identifying and measuring them and outlines what to do about the problem.
How to Find and Test Your Own Native Clays
Some of the key tests needed to really understand what a clay is and what it can be used for can be done with inexpensive equipment and simple procedures. These practical tests can give you a better picture than a data sheet full of numbers.
|Minerals||Gypsum, Calcium sulphate|
|Hazards||The Use of Barium in Clay Bodies|
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