How to Apply a White Slip to Terra Cotta 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 significant 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.

Links

Recipes	L3685U - Cone 03 White Engobe Recipe A white burning body with enough added frit to produce a cone 03 stoneware or white slip for use on the matching red Zero3 stoneware.
Recipes	L3724F - Cone 03 Terra Cotta Stoneware An experimental Zero3 using Plainsman 3D clay
Glossary	Engobe Engobes are high-clay slurries that are applied to leather hard or dry ceramics. They fire opaque and are used for functional or decorative purposes. They are formulated to match the firing shrinkage and thermal expansion of the body.
Glossary	Thixotropy Thixotropy is a property of ceramic slurries of high water content. Thixotropic suspensions flow when moving but gel after sitting (for a few moments more depending on application). This phenomenon is helpful in getting even, drip-free glaze coverage.
Glossary	Terra Cotta A type of red firing pottery. Terra cotta clay is available almost everywhere, it is fired at low temperatures. But quality is deceptively difficult to achieve.
Glossary	Majolica Majolica is white opaque glazed red earthenware clay having colored overglaze decoration. But if you know more about what it is technically you will have more control of your product.

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. They are L3685U engobe (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 03 (producing glaze micro-bubbles), but here the terra cotta is only a thin slip over the much cleaner burning white body.

By Tony Hansen
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