Pottery fired to a low temperature employing a red-burning terra cotta clay covered with a soft opaque white glaze. Historically, majolica glazes (or tin glazed earthenware) were opacified using Tin Oxide, but now Zirconium silicate is most often used. Most majolica also has colored brushwork designs that are painted over the dried glaze (the painting process is tricky because you are painting on a very absorbent surface, you get one brush stroke!). Metallic colorants are brightest at low temperatures and the zircon-stiffened white glaze provides an ideal canvas for them. Colorant formulations (mixtures of stain powders, melters, hardeners, fillers) need to be tuned to melt enough so that they become one with the glaze below, but not so much that they bleed excessively at the edge of brush strokes. Different families of stains have different melting behaviors and chemistry requirements for the host glaze and medium (the color development and degree of melting in the final firing product depend on this). You can buy premixed majolica colors and often these work well, but do not assume that just because it comes in a jar it is perfect or that you cannot formulate something better for your application.
Test bars of different terra cotta clays fired at different temperatures
Bottom: cone 2, next up: cone 02, next up: cone 04. You can see varying levels of maturity (or vitrification). It is common for terra cotta clays to fire like this, from a light red at cone 06 and then darkening progressively as the temperature rises. Typical materials develop deep red color around cone 02 and then turn brown and begin to expand as the temperature continues to rise past that (the bottom bar appears stable but it has expanded alot, this is a precursor to looming rapid melting). The top disk is a cone 10R clay. It shares an attribute with the cone 02 terra cotta. Its variegated brown and red coloration actually depends on it not being mature, having a 4-5% porosity. If it were fired higher it would turn solid chocolate brown like the over-fired terra cotta at the bottom.
A dried terra cotta mug on the left, bisque fired to cone 06 on the right
These were fired to cone 06, about 1800F. Of course, there is normally some shrinkage so the bisque piece would be a little smaller. Even though the matrix is very porous and is under developed, the iron in the body is already beginning to impose its color.
How to test drying and firing compatibility between engobe and body
I have made bi-body strips for testing to tune a white slip for a terra cotta clay body (about 4 mm thick). They need to shrink a similar amount in drying and firing to be as compatible as possible. Here, the white body needs more plastic clay or a bentonite addition to shrink more. It also needs a little less frit or a coarser silica to shrink a little less on firing (pending porosity tests to match their fired density). Amazingly, the fired bars break much more easily one way that the other, because on one side the clay is being stretched (and ceramic does not do well under tension).
How can you test if an engobe fits your clay body?
This is part of a project to fit a fritted vitreous engobe (slip) onto a terra cotta at cone 02 (it fires harder there). Left: On drying the red body curls the bi-clay strip toward itself, but on firing it goes the other way! Right: Test bars of the white slip and red body compare their drying and firing shrinkages. Center back: A mug with the white slip and a transparent overglaze. Notice the slip is going translucent under the glaze. Why? It is too vitreous. That explains how it can curl the bi-clay bars toward itself (it has a higher fired shrinkage). So rather than add zircon to opacify the slip, it is better to reduce its frit content (thereby reducing its firing shrinkage). Reducing the frit in the slip will also make it more opaque (because it will melt less). Front: A different, more vitreous red body (having a frit addition) fits the slip better (the strips dry and fire straight).
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