Engobes at Bisque Stage: Breaking the Rules (and getting away with it)

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 apply bright glazes over that. And somehow it works! Or, at least, appears to work. But here is a time it does not.

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 to ignore #2 and #3 and still get an item that survives firing and light-duty use.

This company does not have the flexibility to do #1, #2 and #3 because they don't make the tile. What can they do? Discover the right balance between adhesion and fired fit. Cut a tile into thin slices, apply engobe variations and do an EBCT test on each. If the fit is way off then start testing using the talc-free L3685Z5 recipe, tuning the amount of frit to get best compatibility. Then increase it until too much opacity is lost or flaking occurs. Then decide on a compromise percentage between bond and fit.

Related Pictures

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

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.

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