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 and terra cotta tile industries. 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).

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How bad can efflorescence on terra cotta be?

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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. A tiny addition of barium carbonate can precipitate the salts inside the clay matrix so they do not come to the surface on drying.

How bad can efflorescence be in natural clays? This bad!

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No amount of barium carbonate will precipitate this amount of soluble salts. Normally clays like this are detected and discarded early in the prospecting stages (when they react strongly to the application of acid). But what if one really wants to use the clay? The only viable option would be grinding the clay as much as possible (to increase surface area) and mixing it into a low specific gravity slurrying, letting it settle and removing the water. Multiple cycles might be needed to remove enough soluble material. Of course, such an endeavour would only be practical for a potter and would depend on the amount of water available and options to process and recycle it.

Videos

Links

Hazards	The Use of Barium in Clay Bodies Considerations regarding the use of barium carbonate in pottery and structural clay bodies for precipitation of soluble salts.
Materials	Barium Carbonate A pure source of BaO for ceramic glazes. This is 77% BaO and has an LOI of 23% (lost at CO2 on firing).
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.
Glossary	Soluble Salts In ceramics, certain compounds in clays and glazes can dissolve into the water, then on drying these are left on the surface.
Glossary	Brick Making Brick-making is surprisingly demanding. Materials blending and processing, forming, drying and firing heavy and thick objects as fast as possible are like no other ceramic manufacturing challenge.

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