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Cone 03 Terra Cotta Stoneware

Code: L3724F
Modification Date: 2015-12-07 14:43:25

MaterialAmountPercent
Plainsman 3D45.040.4%
Redart50.044.9%
National Standard Bentonite5.04.5%
Frit 311010.09.0%
Yellow Iron Oxide1.00.9%
Barium Carbonate0.30.2%
 111.25  

Notes

This is a terra cotta clay with added frit to make it vitrify at cone 03. L3724E has 5% frit, this one has 10%. The former has better red color as a slip, this one is better as a body itself. The intent was to use L3685T as a white and colored slip on this, then finish with a transparent glaze to produce stoneware. Although the development effort spanned a year, it ended in only partial success. It was found that this body (and many variations of it) is too volatile, there is too narrow a temperature window between too mature and too porous. When too mature, the fired shrinkage was very high, the color went from red to brown and the glaze was filled with bubbles from decomposition occurring within the over-fired body. When too open, the color was good but it was too susceptible to absorption of water into the matrix, this could be seen at sites of glaze imperfections where the water could get through. However, if you carefully watch glaze fit and make sure there are no imperfections in the glaze surface, this body could be successfully used at cone 04 to make quite strong ware.

However, as an engobe this appears to be a success. When used over the L3685T white burning body it produces a red color that varies in shade and intensity according to slight temperature variations in the kiln. This imparts a flashing effect reminiscent of wood and salt firings. The L3685T white engobe to be used on this turned out to be an excellent body as well, a low fire white stoneware with alot of potential.

The Plainsman 3D in the recipe is a vitreous light burning silty material having very good drying and slurry properties (Ravenscrag Slip is based on it). We do not know of another similar material. You could try using a ball clay instead, that will mean you can reduce or eliminate the bentonite. It will also mean that you will have to increase the frit to get the same level of vitrification. Whether it will still work well as a slip will be a matter of testing. Perhaps using all RedArt (95 parts) would work, you could eliminate the iron if you do that. You would also likely have to tune the bentonite to get the right degree of plasticity.

A vitreous terra cotta slip over a white low fire stoneware

A vitreous terra cotta slip over a white low fire stoneware

L3724F fluxed terra cotta slip applied over a white burning stoneware (L3685R) fired at cone 03. Most slips in use are not adequately fluxed and do not adhere well to the body below. The frit in this one attaches much better and even enables it to develop a sheen. Also, because of its volatility of color in the cone 03 range, variations in the shade and degree of sheen will impart an appearance like flashing.

One small pinhole in a terra cotta mug and we have a problem

One small pinhole in a terra cotta mug and we have a problem

This is L3724E terra cotta stoneware. The inside slip is L3685S, a frit-fluxed engobe that is hard like the body and attaches well to it (engobes are often insufficiently fluxed). The glaze (G1916Q) is Frit 3195, Frit 3110 and 15% ball clay. The body has about 3% porosity, enough to make very strong pots. However that porosity is still enough to absorb water (and coffee). Although not too visible here, the pinhole in the inner surface has enabled absorption and there is a quarter-sized area of discoloration below the glaze. The piece could possibly be fired a cone higher, but testing would be required to see if the slip is still firing-shrinkage and thermal-expansion compatible with the body and that the body would not be over-fired. A better solution is adjust the firing curve to heal the glaze better. High temperature stoneware can easily have a 3% porosity also, so this is not just a low fire issue.

Double-slip layer incised decoration: A challenge in slip-body fitting

Double-slip layer incised decoration: A challenge in slip-body fitting

An example of a white engobe (L3685T) applied over a red clay body (L3724F), then a red engobe (also L3724F) applied over the white. The incised design reveals the white inter-layer. This is a tricky procedure, you have to make sure the two slips are well fitted to the body (and each other), having a compatible drying shrinkage, firing shrinkage, thermal expansion and quartz inversion behavior. This is much more complex that for glazes, they have no firing shrinkage and drying shrinkage only needs to be low enough for bisque application. Glazes also do not have quartz inversion issues.

What does it take to get a crystal-clear low fire transparent? A lot!

What does it take to get a crystal-clear low fire transparent? A lot!

These three cups are glazed with G1916S at cone 03. The glaze is the most crystal clear achieved so far because it contains almost no gas producing materials (not even raw kaolin). It contains Ferro frits 3195 and 3110 plus 11 calcined kaolin and 3 VeeGum. Left is a low fire stoneware (L3685T), center is Plainsman L212 and right a vitreous terra cotta (L3724F). It is almost crystal clear, it has few bubbles compared to the kaolin-suspended version. These all survived a 300F/icewater test without crazing!

What does it take to get a crystal-clear low fire transparent? A lot!

L3724E ball milled flocculated slip has been applied to L3685U low fire white stoneware. Notice how silky smooth it is. What is the secret of getting a perfectly even layer that does not drip: Thin the slip until it is fairly watery (stays in motion for 10 seconds or more after stirring) and then flocculating it using Epsom salts until it gels enough to stay in motion for less than 2 seconds.

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By Tony Hansen

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