Modified: 2020-04-20 14:44:07
An experimental Zero3 using Plainsman 3D clay
|National Standard Bentonite||5.00|
|Ferro Frit 3110||10.00|
|Yellow Iron Oxide||1.00|
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 worked version well we eventually decided to go back to the original concept of using pure Redart clay to make it useful in all of North America.
The original goal was a terra cotta clay with added frit to make it vitrify at cone 03. The previous version to this one, 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. Although the development effort spanned a year, it ended in only partial success. It was found that this body (and many variations of it) was 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. At the time we were working on a white fritted engobe, the L3685 series.
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 powdered Epsom salts until it gels enough to stay in motion for less than 2 seconds.
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 IWCT test without crazing!
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.
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.
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.
G1916Q - Low Fire Frit 3195 Glossy Transparent
An expansion-adjustable cone 04-02 transparent glaze made using three common Ferro frits (low and high expansion) and a suspension strategy that produces an easy-to-use slurry.
L3924C - Zero3 Porcelain Experimental
L3685U - Cone 03 White Stoneware/Engobe
A white burning body with enough added frit to produce a cone 03 stoneware or white slip for use on a matching red stoneware.
Plainsman Zero3 red body, engobe, glaze and firing schedule
|Media||How to Apply a White Slip to Terra Cotta Ware|
The term Terra Cotta can refer to a process or a kind of clay. Terra cotta clays are high in iron and available almost everywhere. While they vitrify at low temperatures, they are typically fired much lower than that and covered with colorful glazes.
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