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

Related Pictures

Mexican terra cotta bisque tile is "complicted"

This picture has its own page with more detail, click here to see it.

Bisque tiles like these are made in large quantities in tile cities like Dolores, Quanajuato, Puebla and Mexico City (and in countless other smaller places). The name "Dolores", meaning "pains", may be appropriate for the workers who toil in the piece work sweat shops. They have little equipment and work long hours in difficult third-world conditions. Some even live in tents at the factory with no facilities. They are at the bottom rung of a multi-country industry that buys the bisque tile and glazes and decorates it (like those show here). Their lack of resources affects decorators downstream in an interesting way: They are unable to incorporate an essential additive into the clay to prevent soluble salts (effluoresence) buildup on the surface: barium carbonate. These keep increasing every time the tile gets damp and dries out. How is this dealt with? An engobe is applied to the bisque surface to create a white background for glazing. But that is not without issues (e.g. the flaking shown here).

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

This picture has its own page with more detail, click here to see it.

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).

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

This picture has its own page with more detail, click here to see it.

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	Barium Carbonate in Clay Bodies Considerations regarding the use of barium carbonate in pottery and structural clay bodies for precipitation of soluble salts.
Glossary	Sulfates Soluble sulfates in clay produce efflorescence, an unsightly scum that mars the fired surface of structural and functional ceramic products.
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	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	Terra Cotta A type of red firing pottery. Terra cotta clay is available almost everywhere, it is fired at low temperatures. But quality is deceptively difficult to achieve.

Got a Question?

Buy me a coffee and we can talk