Firing Cycle From Clay to Mullite

Igneous rock Igneous rocks are formed from the solidification of molten rock material. There are two basic types: 1) intrusive igneous rocks such as diorite, gabbro, granite and pegmatite that solidify below Earth's surface; and 2) extrusive igneous rocks such as andesite, basalt, obsidian, pumice, rhyolite and scoria that solidify on or above Earth's surface. Pictures and brief descriptions of some common igneous rock types are shown below.

Granite People commonly think of lava and magma as a liquid, like molten metal, but geologists find that magma is usually a mush — a liquid carrying a load of mineral crystals. Magma crystallizes into a collection of minerals, and some crystallize sooner than others. Not just that, but when they crystallize, they leave the remaining liquid with a changed chemical composition. Thus a body of magma, as it cools, evolves, and as it moves through the crust, interacting with other rocks, it evolves further.

Feldspar Feldspars are the most important part of rocks like granite, syenite, gabbro, basalt, gneiss, and others that make up about 75% of the earth’s surface. These feldspars contain potassium, sodium, calcium, and barium, sometimes all together, sometimes one or two at a time. There are many, many types of feldspar. Each is mostly Al 2 O 3 2SiO 2 but each has a certain amount of other ingredients. The most common are potash (potassium) and soda (sodium).

Granite to Clay Granite is high in quartz (silica) and the mineral feldspar. This rock has spent years eroding, weathering into clay. After the rock is broken down, it picks up the chemical water (H 2 O) while just lying around in the bottom of swamps and lakes. The Al 2 O 3 2SiO 2 (feldspar) adds 2H 2 O Therefore, the basic chemical formula for clay is Al 2 O 3 2SiO 2 2H 2 O

Weathering Process During the weathering process the rock (granite) is mixed with other minerals (iron, magnesium and titanium are the most important). Each mineral makes the clay different, but each has the same basic formula. Al 2 O 3 2SiO 2 2H 2 O

Firing Cycle Once the clay has been shaped and is bone dry, (feels completely dry-room temperature)- the water of plasticity has evaporated. It is never completely dry because our atmosphere is not at 0% humidity. Therefore, the last 2% of the water of plasticity (mechanical water) is evaporated during the firing cycle.

Water Smoking When the temperature in the kiln reaches 212°F all remaining water of plasticity (mechanical water) is burned off. This is the first in a series of critical points. If the steam leaves the clay too fast, the pot explodes. Fire the kiln slow and easy with the door open a crack or the vent on to allow the steam a chance to escape. In large kilns, enough steam is often generated to be seen. For this reason the early part of the firing cycle is called water smoking.

Dehydration As the temperature increases to 660°̊F, the chemical water starts to burn off. This is the 2H 2 O part of the clay formula. By 950°F the clay is completely dehydrated-all the chemical water has been burned off. The clay is now a chemically different material than it was when put into the kiln. It is now an aluminum silicate known as mullite. (Al 2 O 3 2SiO 2 ) This change is not reversible. (notice the 2H 2 O is now gone)

Quartz Inversion The next critical change begins at about 1000°F. The quartz crystals in the clay undergo a change called the quartz inversion. The crystals grow and change in shape. This change must be done slowly as the crystals expand or the ware will crack.

Oxidation Oxidation (the burning off of organic materials) occurs at 1600°F to 1700°F. This has no chemical effect on the clay, but it does leave the clay more porous. This is an advantage when glazing bisque ware.

Vitrification Partial vitrification begins as the temperature rises. This will be determined by the type of clay body. Clay with a high alumina content (stoneware and porcelain) will vitrify more slowly and at a higher temperature than clay high in fluxes such as iron or talc (earthenware). Vitrification is the melting of the clay platelets. Complete vitrification results in a glasslike material: therefore, clay ware is never completely vitrified. (or it would be a puddle!)

Cooling The clay should cool slowly-especially as it drops to around 1063°F. The quartz inversion is reversed and the crystals shrink. Again, this is done slowly or the ware will crack. The ware is now and forever more mullite and will never again be able to be recycled and reused.