A knowledge of ceramic chemistry is not required to understand how the properties of a stoneware clay body develop in a firing. What is required is proper firing to peak temperature in order to produce a vitrified stoneware.
Defining the Terms
Amorphous Solid: A solid material with no regular crystal structure.
Feldspar: Naturally occurring alkali and alkaline earth alumino-silicate minerals of variable composition.
Glass Phase: The portion of a fired ceramic that melts to a liquid during firing and then becomes an amorphous solid upon cooling.
Mullite: A crystalline compound of alumina (Al2O3) and silica (SiO 2) formed by firing the common clay mineral, kaolinite, to above 1922°F (1050°C).
Stoneware Clay: Primarily kaolinite clay contaminated with flux elements and/or feldspar minerals as well as iron and sometimes other metals. Stoneware clays are moderately refractory, maturing between cones 7 and 12, but due to their metal content do not fire white.
Stoneware Clay Body: Stoneware bodies vary widely. Their major constituent is plastic fire clay to which as much as 15% ball clay may be added. Similar fractions of quartz and feldspar may be added to control the body’s maturation temperature.
Vitrification: Formation of a glass phase during the firing of a ceramic object.
Properly fired stoneware is composed of an amorphous aluminum silicate glass that cements together crystals of mullite and silica. The glass phase solidifies from a liquid formed when all the feldspar and some of the quartz in the clay body melt.
The mullite is derived from the clay in the stoneware body. The silica is usually quartz sand that has been used as a grog in the body and melts as a direct function of the peak firing temperature. The portion of the silica that melts becomes part of the glass phase. Because mullite forms long, slender crystals within the stoneware glass phase, the mullite contributes to the strength of the fired ceramic.
The most abundant and inexpensive feldspars are rich in sodium and/or potassium. Those elements serve as fluxes and cause the feldspar minerals to melt at relatively low temperatures to produce the stoneware glass phase. Other flux elements found in alumino-silicate minerals, however, have the same effect and may be found in stoneware bodies. Lithium, as well as sodium and potassium, are alkali flux elements, which may be used to produce the glass phase in stoneware. Magnesium, calcium, strontium, and barium are alkaline earth fluxes, which may also be used for the same purpose.
Possibly the simplest, and fastest, way to determine whether a clay body is underfired is to take a sample test fresh from the kiln, cool it, and touch your tongue to an unglazed bit of it. If your tongue clings to the sample, the work is porous. If you have any doubt about that test, try it with an ordinary drinking glass. The glass is not a bit porous so your tongue slides on it, not clinging at all. A porous ceramic surface will not feel the same.
More formally, for tariff purposes, the United States Industrial Trade Commission (USITC) has defined stoneware as a fired ceramic that can absorb no more than 3% of its dry weight of water measured using a specific test procedure. That test, American Society of Testing Materials (ASTM) test C373, involves boiling 5 bone-dry samples of at least 10 grams each for 5 hours in deionized water, cooling the samples in a cool, deionized water bath for 24 hours, then blotting the samples dry and immediately weighing them on an accurate scale. The gain in weight is calculated as a percent of the original dry weight of the samples.
For reference, earthenware is defined by the USITC as having a water absorption of more than 3%. Porcelain, in comparison, is defined as absorbing no more than 0.5% water using the same test.
Before scientists developed an understanding of ceramic chemistry, potters still knew how to make durable stoneware. When they found a new clay, they dug some of it, fired it to determine its maturation temperature, then made glazes for the temperature they knew they had to fire to in order to have sound ware. Most potters now dig their clay from plastic bags that come in boxes from a commercial manufacturer. Unfortunately, this process of testing is often no longer followed. Potters should be able to trust manufacturer recommendations, but it’s risky.
Firing should always be to the maturing temperature of the body. For example, if a clay is listed as a cone 10 body, but absorbs 5% of its weight as water, it’s going to be relatively weak and it will most likely leak. That body will either require some addition of feldspar to lower its maturing temperature to cone 10 or it will need to be fired to cone 11+1 or so to be adequately dense. If that temperature yields over- or under-matured glazes, then of course those glazes either need to be adjusted or replaced with others. One simply won’t achieve desired results if the body itself is either overfired or underfired for the benefit of the firing requirements of the glazes used.
Vessels made of underfired clays will leak. While this is unfortunate for pitchers that hold liquids such as milk or water, it has proven dangerous in cases involving underfired oil lamps that leaked and the oil then ignited.
It is also possible, of course, to encounter a clay body that is over-fired at its listed maturing cone. That creates a risk of slumping or bloating of the body. Stoneware fired to significantly less than 1.5% water absorption is likely overfired.
Stoneware is not limited by the goals of translucency and whiteness that characterize porcelains. Thus, stoneware can be made almost entirely from clay. Any fairly high-firing clay will do, even if the clay contains titanium or iron, elements which make the fired work gray to brown and also opaque.
Stoneware bodies that are 85% or more clay are common. Additions of silica to control expansion and contraction behavior and feldspar to manage maturing temperature are relatively small. The clay-rich recipes make stoneware bodies easy to handle with most forming methods.
High shrinkage and warping can be problems with stoneware bodies. An addition of grog may be needed, especially with larger work. Grog helps speed drying and reduces total shrinkage. Both effects are helpful to reduce warping and cracking.
Stoneware Clay Bodies
the author Dave Finkelnburg is a studio potter and practicing engineer. He earned his masters degree in ceramic engineering from Alfred University.
This text was excerpted from the April 2019 issue of Ceramics Monthly.