Majolica is pottery, almost always decorative and non-functional, fired to a low temperature, employing a red-burning terra cotta clay, covered with a soft opaque white glaze and decorated with colored overglazes. Historically, majolica glazes (or tin glazed earthenware) were opacified using Tin Oxide, but now Zirconium silicate is most often substituted for some or all of the tin. Metallic overglazes 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..
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.
Bi-Clay strips test compatibility between engobe and body
Slips and engobes are fool-proof, right? Just mix the recipe you found on the internet, or that someone else recommends, and you are good to go. Wrong! Low fire slips need to be compatible with the body in two principle ways: drying and firing. Terra cotta bodies have low shrinkage at cone 06-04 (but high at cone 02). The percentage of frit in the engobe determines its firing shrinkage at each of those temperatures. Too much and the engobe is stretched on, too little and it is under compression. The lower the frit the less the glass-bonding with the body and the more chance of flaking if they do fit well (either during the firing or after the customer stresses your product). The engobe also needs to shrink with the body during drying. How can you measure compatibility? Bi-body strips. First I prepare a plastic sample of the engobe. Then I roll 4 mm thick slabs of it and the body, lay them face-to-face and roll that down to 4 mm again. I cut 2.5x12 cm bars and dry and fire them. The curling indicates misfit. This engobe needs more plastic clay (so it dry-shrinks more) and less frit (to shrink less on firing).
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).
What does it take to opacify a low temperature terra cotta glaze
Tin oxide is a powerful opacifier, but the 5% in the glaze on the left is clearly not enough. 10% more zircon had to be added to produce the one on the right.
White majolica bases have very, very low melt fluidity
This is Linda Arbuckle's base recipe (66% frit 3124, 23% feldspar, 13 kaolin/bentonite, 9 zircon, 4 tin oxide mixed to 1.62 specific gravity). It is fired at cone 05 creating a super gloss. This is applied very, very thickly (double the thickness of what a stoneware glaze would be). Yet notice how the air-vent holes did not heal during firing, these would have filled in easily had this been on stoneware or porcelain. For this reason, majolica glazes must be applied to ware not having any abrupt concave contours (this also happened on the handle-joins and where foot-meets-wall inside). Bisque must be clean to assure good adherence. Any glaze coverage issues must be repaired carefully before firing. This piece was bisque fired at cone 06, that is the reason for the air vent holes. Had it been bisque fired at cone 04 (higher than the glaze firing) these would not have occurred. However that would have extended the dip-time, this one was held under 10 seconds whereas a cone 04 bisque would have required 20 seconds.
For even coverage white majolica glazes must be applied by dipping
The mug on the left has three coats of Spectrum Majolica base, painted on by brush. Drying was required after doing the inside coats, so the total glazing time was several hours. The glaze layer is way too thin and it is not even at all! The one on the right was dipped in a 5 gallon bucket-full of Arbuckle white (that was weighed out according to a recipe and slurried at 1.62 specific gravity). It took seconds to dip-apply, the thickness coverage are good. As is obvious, it makes sense to make your own base white. Then, you can decorate using the overglaze colors (e.g. the Spectrum Majolica series). Another advantage of making your own white is that you can splurge on the amount of opacifier (in this case 9% zircon and 4% tin oxide), to achieve maximum whiteness and opacity. And, you can proportion a mix of two frits (having higher and lower thermal expansion) to fine-tune the fit with the body (a big issue at low fire).
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