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I found seven secrets with recipe, process, glaze and firing to make durable terra cotta.
1. A lot of Zircopax, in this case 20% (for whiteness, opacity).
2. The whitest burning materials: New Zealand Halloysite as the kaolin and nepheline syenite as the feldspar.
3. 3% Veegum to gel the slurry (enabling low specific gravity for thin and even coating).
4. The recipe, L3685Z2, has 55% kaolin, that will certainly produce drying cracks. But 1% CMC gum stops that and makes it brushable. It even works on on bisque, I pour-applied it to the insides of these two slip-cast pieces, it drained to perfectly even coverage (in a very thin layer).
5. A terra cotta casting/throwing body to fit the engobe to (has the same fired shrinkage at a target temperature, e.g. cone 04): Initially I am using L4170B.
6. A clear glaze that fits and is transparent: Notice how much whiter the left one is, G3879. At the same thickness as the G1916Q on the right, it is more transparent, better transmitting the white of the engobe.
7. The right firing curve: The 04DSDH drop-and-hold schedule for defect free surfaces.
The fired shrinkage of a clay can be measured accurately - and compared with that an engobe. Engobe-fit on terra cotta is very important because the bond is sintered, not glassy. These test bars show how this terra cotta body, L4170B, performs from cone 8 down to 06. Using the SHAB test procedure, I measured the dry and fired lengths of each bar and recorded the data in my Insight-live group account (it displays like the chart shown here). I compiled the same data for a super-white engobe, L3685Z3. Its calculated firing shrinkage came in much lower at all practical temperatures. At the target temperature of cone 02 a 5% addition of Ferro Frit 3110 increased the firing shrinkage from 2.1% to 5.5% (while the red body is 4.3%). To better the match I adjusted to 3% frit. What about matching the thermal expansion? I don’t have a dilatometer so I slurried up the engobe and used it as a body, it fits the same glazes without crazing or shivering (demonstrating they are under some compression).
URLs |
https://insight-live.com/insight/share.php?z=No2m98AEhJ
Terra cotta casting slip, engobe and glaze recipes for cone 04 An Insight-live.com project to develop a body, engobe and glaze that you can adjust to your own needs. |
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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. |
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