|Monthly Tech-Tip |
Let's suppose you need strength and density for utilitarian ware.This is a terra cotta body, L4170B, it has a much wider firing range than most terra cottas. Still, it is not like white-burning bodies, its "practical firing window" is much narrower than what looking at these fired bars and graph suggest. On paper, cone 5 hits zero porosity. And, in-hand, the bar feels like a porcelain. But ware will warp during firing and transparent glazes will be completely clouded with bubbles (when pieces are glazed inside and out). What about cone 3? Its numbers put it in stoneware territory, water tight. But decomposition gases still bubble 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.
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.
These were cast by Anna Lisovskaya, they are fired at cone 03. They are supposed to fit into hexagonal welded frames, but during firing many of them warp enough to fit poorly. Why? The color differences are most obvious here. With that color associates a firing shrinkage difference, the darker ones shrink significantly more. Something less obvious: the sides against the elements receive direct radiant heat, so they shrink more, turning a perfect hexagon into an imperfect one. Terra cotta clays are volatile, that means their approach to maximum density during heat-up, accompanied by shrinkage, happens across a narrow temperature range. Accurate and even firing are paramount. In a radiant-heat electric kiln this can be very difficult. Two approaches could work here: Fire at a lower temperature, perhaps cone 04. Or, greatly slow rate-of-rise for the last 100F, perhaps over several hours.
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.