A term referring to the degree to which a clay or glaze has vitrified or sintered during the firing. A 'mature' stoneware or porcelain clay is normally one that is dense and strong. Mature clays used for functional ware are dense enough to resist soaking up water. Firing commercial clays at too low a temperature will result in weaker pieces (than can become waterlogged over time, continually weakening). Every clay has an ideal range (often quite narrow) to which it should be fired to develop maximum maturity. Some manufacturers list wider-than-practical firing ranges for certain clay bodies, leaving customers to discover the best temperature. Others recommend a vitrification temperature that is too high.
Vitreous cone 6 stoneware. How?
Producing a zero-porosity cone 6 stoneware is not as easy as you might think. People expect stonewares to be plastic and fit glazes well. That means there needs to be lots of ball clay and silica in the recipe. These are refractory materials and they don't leave much room for the material that produces the vitrification: Feldspar. If the body does not need to be white there is another interesting approach: Use a red terra cotta material to supply plasticity and maturity. In this case I have made a 50:50 mix of a red-burning, super plastic, low fire clay (Plainsman BGP) with a refractory white-burning ball clay (Plainsman 3C). The result vitrifies to a zero-porosity, beautiful light tan body that even glistens in the light. One problem: Fired shrinkage. This body shrinks almost 17% from wet to fired.
Test bars of different terra cotta clays fired at different temperatures
Bottom: cone 2, next up: cone 02, next up: cone 04. You can see varying levels of maturity (or vitrification). It is common for terra cotta clays to fire like this, from a light red at cone 06 and then darkening progressively as the temperature rises. Typical materials develop deep red color around cone 02 and then turn brown and begin to expand as the temperature continues to rise past that (the bottom bar appears stable but it has expanded alot, this is a precursor to looming rapid melting). The top disk is a cone 10R clay. It shares an attribute with the cone 02 terra cotta. Its variegated brown and red coloration actually depends on it not being mature, having a 4-5% porosity. If it were fired higher it would turn solid chocolate brown like the over-fired terra cotta at the bottom.
Cone 2: Where we see the real difference between terra cottas and white bodies
The terra cotta (red earthenware) body on the upper left is melting, it is way past zero porosity, past vitrified. The red one below it and third one down on the right have 1% porosity (like a stoneware), they are still fairly stable at cone 2. The two at the bottom have higher iron contents and are also 1% porosity. By contrast the buff and white bodies have 10%+ porosities. Terra cotta bodies do not just have high iron content to fire them red, they also have high flux content (e.g. sodium and potassium bearing minerals) that vitrifies them at low temperatures. White burning bodies are white because they are more pure (not only lacking the iron but also the fluxes). The upper right? Barnard slip. It has really high iron but has less fluxes than the terra cottas (having about 3% porosity).
Underfiring a clay is OK if the glaze fits? No it is not.
Left: Plainsman M340 fired to cone 6 where it achieves about 1.5% porosity, good density and strength. Right: H550, a Plainsman body intended to mature at cone 10, but fired to cone 6 using the same glaze. Although the glaze melts well and the mug appears OK, it is not. It is porous and weak. In fact, it has cracked during use (the crack runs diagonally down from the rim). It was then dipped into water for a few moments and immediately the water penetrated the crack and began to soak into the body (you can see it spreading out from the crack). If this glaze were to craze the entire thing would be waterlogged in minutes.
These two pieces will not mature to the same degree in a firing
Soak the firing 30 minutes to mature the mug and the planter will not mature. Soak 2 hours for the planter and the glaze may melt too much and the clay be too vitreous. This is a troublesome issue with electric kilns. Furthermore, they employ radiant heat. That means that sections of ware on the shady side (or the under side) will never reach the temperature of those on the element side no matter how long you soak.
How much porcelain flux is too much?
A porcelain mug has pulled slightly oval because of the weight of the handle. This happens in highly vitrified porcelains (e.g. translucent ones). The amount of feldspar or frit in the body determines the degree of maturity, the correct percentage is a balance between enough to get the maximum translucency and hardness but not so much that ware is deforming excessively during firing. This is Plainsman Polar Ice at cone 6, this degree of warp is acceptable and can be compensated for.
Water-logging happens when a clay is underfired
The cone 6 glaze is well developed, it is not crazed. But the clay underneath is not developed, not vitreous. This crack happened when the mug was bumped (because of poor strength). It is barely visible. When the mug is filled with water, this happens. How fast? This picture was taken about 5 seconds later. If this was crazing, and this piece was in actual use, the clay would gradually become completely water logged. Then one day someone would put it in the microwave! Boom.
A red fireclay from cone 7 (bottom) to cone 10 and 10R
Notice how much the color changes as the clay fires to greater maturity. This is Plainsman FireRed.
A terra cotta body fired from cone 06 (bottom) to 4
Terra cotta bodies are more volatile, maturing more rapidly over a narrower range than others. These bars are fired (bottom to top) at cone 06, 04, 03, 02, 2 and 4. This is Plainsman BGP.
A sculpture body fired from cone 1 (bottom) to 11 and 10R (top)
Color, density, size and hardness all change as the firing temperature progresses. The color, for example, persists in zones, then changes suddenly. Notice that the colour of the grog particles contrasts more as the temperature increases. This body is completely vitrified at cone 10 and the grog is important for fired stability.
Cone 6 kaolin porcelain verses ball clay porcelain.
Typical porcelains are made using clay (for workability), feldspar (for fired maturity) and silica (for structural integrity and glaze fit). These cone 6 test bars demonstrate the fired color difference between using kaolin (top) and ball clay (bottom). The top one employs #6 Tile super plastic kaolin, but even with this it still needs a 3% bentonite addition for plasticity. The bottom one uses Old Hickory #5 and M23, these are very clean ball clays but still nowhere near the whiteness of kaolins. Plus, 1% bentonite was still needed to get adequate plasticity for throwing. Which is better? For workability and drying, the bottom one is much better. For fired appearance, the top one.
Particle size and LOI determine behaviour of over-fired bodies
These are four terra cotta body disks that have been fired to cone 10 reduction. The fluxing action of the iron has assisted to take them well along in melting. Notice that one is hardly bubbling at all, it is Redart clay that has been ground to 200 mesh (the lower right one is a body mix of 200 mesh materials also containing it). The upper left one is bubbling alot more. Why? Not just because it is melted more (in fact, the one on the lower left is the most melted). It is a body made from clays that have been ground to 42 mesh. Among the particles are larger ones that generate gases as they decompose. Yes, the particles in the others do the same, but their smaller size enables earlier decomposition and expulsion of smaller gas amounts distributed at many more vents. Some bodies cannot be fired to a point of zero porosity, they will bubble before they get there.
What happens when a terra cotta body is fired to cone 6
It may not melt, but will certainly warp and blister/bloat. If there is inadequate kiln wash it will stick to the kiln shelf.
Gradient bars show the degree of vitrification of a clay
These are the fronts and backs of dust-pressed bars. After final drying the width at each line is carefully recorded. They are fired horizontally in a furnace able to reproduce linear thermal gradients along the length of the bar. Thermocouples monitor the temperatures along the bar, so the temperature reached at each line is known. After firing the widths are re-measured, this produces a graph of fired shrinkage vs. temperature. Clays can be visually inspected side-by-side and differences or changes in maturity are immediately obvious.
Vitrification can be obvious by simple visual inspection
The unglazed surface of the left piece has a sheen, it is a product of glass development during firing to cone 6. That body is a 50:50 mix of a cone 8 stoneware and a low fire earthenware red (a material that would normally be melted by this temperature). Together they produce this dense, almost zero-porosity ceramic. The unglazed surface on the right looks more like plaster, and it is absorbent, about 5% porosity. It is a mix of the same stoneware but with 50% ball clay. The refractory ball clay assures that the stoneware, which was already inadequately vitreous, is even more so. As you can imagine, the left piece is far stronger.
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