Mexican terra cotta bisque tile is "complicted"

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. 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.

Related Pictures

White engobe flaking off terra cotta tiles

White engobe flaking of terra cotta tile

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Ideally, engobes are applied at leather hard stage. Their body-like makeup enables a plastic bond (#1) and their body-matching fired shrinkage (#2) and COE (#3) maintain that bond through the firing. That being said, many companies turn this logic on end and apply engobes at the bisque stage - where mechanism #1 is missing and they ignore fired fit #2 and #3. An example is terra cotta tile, companies buy it in bisque form, apply the engobe and then a glaze over that. And somehow it works! Or, at least, appears to work. But here is a time it does not. And how often are the tiles flaking time bombs?

How do this company, and others, get away with this most of the time? One secret is initial laydown - This mix is converted to paint by the addition of CMC gum. Second, it is sprayed on in an even and just thick enough layer. Third, 10% frit is added, enough to start melting (usually augmented by high zircon content), this can crowbar adhesion enough. These measures, most of the time, are enough it ignore #2 and #3 and so the item survives firing and light-duty use.

This company does not have the flexibility to do the ideal, they don't make the tile. What can they do? Discover the right balance between adhesion and fired fit. Do EBCT testing by cutting a tile into thin slices, applying the slip and noting the bend. If the fit is way off then start testing using the L3685Z5 recipe, tuning the amount of frit to get best compatibility. Then increasing it until too much opacity is lost or flaking occurs. Then decide on a compromise percentage between bond and fit.

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.

Links

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.

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