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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.

Ware made using the Majolica process is not strong, the clays generally have 10% porosity or more (fired at cone 06-04). This low temperature means that the body-glaze interface is much less developed than in stoneware (so the glaze is not stuck on nearly as well). It is thus important that the thermal expansion of the glaze be matched to the body to prevent crazing and shivering. Ware is not as durable in use and any bare sections will absorb water and tend to water-log the clay matrix over time. It is thus best to make a foot ring so that the entire piece can be glazed, leaving only a narrow channel of exposed body to touch the kiln shelf.

Test bars of different terra cotta clays fired at different temperatures

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

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

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?

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).

Out Bound Links

  • (Articles) The Majolica Earthenware Process

    Understanding the advantages and disadvantages of low fire ware and the chemistry of physics of the glazes and bodies used is a key factor to exploiting this type of ceramics

  • (Articles) G1916M Cone 06-04 Base Glaze

    This is a frit based boron base glaze that is easily adjustable in thermal expansion, a good base for color and a starting point to go on to more specialized glazes.

  • (Glossary) Opacifier, Opacification

    A glaze additive that transforms an otherwise transparent glaze into an opaque one. Common opacifiers are tin oxide and zircon compounds. Opacifiers typically work by simply not dissolving into the melt, the white suspended particles thus reflect and scatter the light. Since they do not participate ...

  • (Glossary) Terra cotta

    'Terra Cotta' (Italian for 'cooked earth') is red burning earthenware. It has been made for thousands of years by indigenous cultures, most often unglazed. If glazed, high lead content mixtures have been traditional. It is fired at much lower temperatures than stoneware so, not surprisingly, it is n...

  • (Glossary) Crazing

    Crazing refers to small hairline cracks in glazed surfaces that usually appear after firing but can appear years later. It is caused by a mismatch in the thermal expansions of glaze and body. Most ceramics expand slightly on heating and contract on cooling. Even though the amount of change is very s...

  • (Glossary) Shivering

    A defect in glazed ware where the glaze is compressed too much by the body, the glaze actually peels off the ware on edges to relieve the stress. Shivering is thus the opposite of crazing and is also less common. This problem is potentially dangerous, since the tiny flakes of glaze having razor-shar...

In Bound Links

  • (Glossary) Earthenware

    A clay fired at low temperatures (cone 010-02) where it does not develop maturity (vitrify). The term earthenware almost alway refers to red burning terra cotta ware (although it is a somewhat more general term referring to a wider range of colors and more primitive forming and firing techniques). E...

  • (Videos) How to Apply a White Slip to Terracotta Ware

    The slip is L3685U. I specially formulated it to fire as a stoneware at cone 03 (it contains lots of frit). It works as a slip or as a body and I made a matching red (based on Redart) that also works ...

By Tony Hansen

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