It depends on the clay. This is a terra cotta body, L4170B. It is not like white-burning bodies, its "practical firing window" is much narrower than what looking at these fired bars and graph suggest. Do you need maximum density? On paper, cone 5 hits zero porosity. And, in-hand, the bar feels like a porcelain. But in-use pieces will warp and transparent glazes will be completely clouded with bubbles (from gases of decomposition generated by the body). What about cone 3? Its numbers put it in stoneware territory, water tight. But it still bubbles glazes! Cone 2? Much better, it has below 4% porosity (any fitted glaze will make it water-tight), below 6% fired shrinkage, still very strong. But there are still issues: Accidental overfiring drastically darkens the color. Low fire commercial glazes may not work at cone 2. How about cone 02? This is a sweet-spot. This body has only 6% porosity (compared to the 11% of cone 04). Most low fire cone 06-04 glazes are still fine at cone 02. And glaze bubble-clouding is minimal. What if you must fire this at cone 04? Pieces will be "sponges" with 11% porosity, shrinking only 2% (for low density, poor strength). There is another advantage of firing as high as possible: Glazes and engobes bond better. As an example of a low fire transparent base that works fine on this up to cone 2: G1916Q. It will even tolerate a silica addition to make it more durable. Then just add stains and opacifers to that. An engobe? Try matching L3685Z3 or L3685Z2 to it.
Physical data. Data about the working, drying and firing properties that can measured. These test bars show how stable this unusual terra cotta body, L4170B, is across a wide temperature range (from cone 8 down to 06)! Using the SBAB test procedure, I measured the shrinkage and porosity of each bar (and recorded the data in my Insight-live group account, it displays it like the black and red chart shown). I also made test bars of a super-white engobe, L3685Z3, and compiled the same data. That enabled two approaches to fitting engobe-to-body. First, isolate a temperature at which both have the same fired shrinkage, and are therefore compatible. Unfortunately there isn't one, the white engobe has much lower fired shrinkage at all practical temperatures! Option 2 is to add frit to the white engobe to make it as vitreous as the body. I started with a 5% addition of Ferro Frit 3110. At my target temperature, cone 02, that increased the shrinkage from 2.1% to 5.5% (this red body is 4.3%). So 5% frit is too much. So, the next move is clear, I will try 3% frit. It will only be necessary to test it at the one temperature. Because both engobe and body and not volatile, I am confident in just testing at cone 02.
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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