How to Apply a White Slip to Terracotta Ware

I will show you some secrets of making a base engobe (or slip) apply to leather hard terracotta ware in a thick, perfectly even layer.

B. Glazes

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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 as a vitreous slip or body at cone 03 (thus they can be used on each other). It has 45% kaolin and 3% bentonite, far more clay than any typical glaze, so it forms a slurry that responds well to Epsom Salts to make it gel (vinegar does not work as well). This slip also paints and trails well thus enabling stain additions for making brightly colored decoration. After bisque the slip decoration can be sealed on using the clear glaze (I am also actively developing a recipe for it).

Many people just use a white clay for an engobe at low fire, but this is not a good idea. Slips need to adhere well to the leather hard ware (very plastic slips work best). They need to dry hard and without cracks or flaking off. They need to fire onto the ware and adhere well to the fired ceramic below, that is not the norm at low fire so they need frit added. Engobes need to have a fired shrinkage very similar to the body (to minimize compression or tension). They need to have a thermal expansion compatible with that of the body (to minimize shivering/flaking). If you use this slip on a regular terra cotta (having no frit to make it vitrify) you will likely have issues.

Cone 03 stoneware. Red and white body and slips. Clear glaze.

Cone 03 stoneware. Red and white body and slips. Clear glaze.

Cone 03 white stoneware with red terra cotta ball-milled slip and transparent overglaze. These are eye-popping stunning. They are test L3685U (Ferro frit 3110, #6 tile kaolin, Silica), near the final mix for a white low fire stoneware. The G1916J glaze is super clear. Why? Two reasons. These were fired in a schedule designed to burn off the gases from the bentonite in the body before the glaze fuses (it soaks the kiln for 2 hours at 1400F). Terra cotta clays generate alot of gases at cone cone 03 (producing glaze micro-bubbles), but here the terra cotta is only a thin slip over the much cleaner burning white body.

Out Bound Links

  • (Glossary) Engobe

    A white or colored slip applied to clay as a coating. The term "slip" is often used interchangeably with this, but we think of slip more as a decorative, paint-on material/process. The tile industry uses the largest volumes of engobe by far, these are employed as opaque barriers between less-than-wh...

  • (Recipes) L3724F - Cone 03 Terra Cotta Stoneware

    An experimental Zero3 using Plainsman 3D clay

    2014-09-30 - This is an experimental version of the Zero3 red low fire stoneware recipe where we were attempting to make use of a Plainsman material. While it work...

  • (Recipes) L3685U - Cone 03 White Stoneware

    A white burning body with enough added frit to produce a cone 03 stoneware or white slip for use on a matching red stoneware.

    2013-08-01 - This is both a slip (engobe) and body recipe. It is part of a development project during 2014 (spanning mixes from L3685A to T) to create a low fire s...

  • (Glossary) Thixotropy

    Knowing about thixotropy enables you to mix non-gummed glazes that stay in suspension much better. They do not drip alot when a piece is draining. They go on evenly, of adequate thickness and do not run. They dry quickly (on porous bisque) and are just much nicer to use. The secret to all of this is...

  • (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) Majolica

    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 design...

By Tony Hansen

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