All Glossary

Efflorescence

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.

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Details

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 (e.g. 0.2-0.5%) 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).

Related Information

The magic of a small barium carbonate addition to a clay body

Two bisqued terracotta mugs demonstrate efflorescence. The clay on the right has 0.35% added barium carbonate (it precipitated the natural soluble salts dissolved in the clay and prevented them from 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).

Which clay contains more soluble salts?

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.

How bad can efflorescence of soluble salts on fired ceramic be?

Like this! This terra cotta clay matures to good strength around 1950F. Notice how the soluble salts have concentrated on the outer and most visible surface. The piece was dried upside down so of course, all the water had to escape through that route. A complicating factor is how handling of the piece at the leather hard stage has made it even more unsightly. 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. There is good news: Solubles salt deposition can actually be much worse than you see here.

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 of this DFAC test disk 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.

Ravenscrag Saskatchewan clays fired at cone 10R

Glazeless (top) and with glaze (bottom): A1 (bentonitic), A2 (ball clay), A3 (stoneware), 3B (porcelains), 3C (lignitic ball clay), 3D (silt). The bottom row has also shows soluble salts (SOLU test).

Soluble 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.

Plucking in a cone 10R stoneware body having soluble salts

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.

Silicone sealer for porous ceramic for outdoor use

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. It also has a slight sheen and better red color. This sealer even makes it possible to use a porous clay body for outdoors, terra cotta bricks have long be protected against freeze-thaw spalling using products like this.

Acid products are available to remove efflorescence from ceramic surfaces

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).

Inbound Photo Links

OM4 Ball clay fired from cone 10R (top), 10 down to 4 (downward)

Fired bars of a bentonite used in porcelains! Is this possible?

Brown soluble salts that appear after drying, but disappear on firing

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