White engobe flaking off terra cotta tiles

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.

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).

Here is why you don't want an engobe to melt

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

Too much frit in an engobe and it will lose opacity and whiteness. The white slip on the left is an adjustment to the popular "Fish Sauce" slip recipe (L3685A: 8% Frit 3110 replaces 8% Pyrax to make it harder and fire-bond to the body better). The one on the right, L3685C, has 15% frit. Although applied at the same thickness, it is becoming translucent, moving it into glaze territory. That means it will have a far higher firing shrinkage than the body (a common cause of shivering at lips and contour changes). This slip is basically a very plastic white body. Since white burning slips are made from refractory materials they are not nearly as vitreous as red ones, at low fire they need help to mature and a frit is the natural answer. With the right amount of frit the fired shrinkage of body and slip can be matched and the slip will be opaque. This underscores the need to tune the maturity of an engobe to the body and temperature. Although zircon could be added to the one on the right to opacify and whiten it, that would not fix the mismatch in fired shrinkage between it and the body. And it would increase the price.

Links

Tests	Engobe Body Compatibility Test A bi-clay test strip curls to demonstrate the drying and fired compatibility or fit of an engobe (or slip) on a ceramic body or of two bodies for marbling.
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