1. Drying
As plastic clay dries, it shrinks 5%.
Very plastic clay shrinks 8% as it dries.
Shrinkage is slow and can create problems.
The average plastic clay is 35 parts water to 100 parts clay.
(a mass of clay is 25% water)
As humidity falls (in the atmosphere), below 100%, drying takes place. If an object is even, it will dry relatively evenly. If it is too thick, cracks may occur.
Drying clay is always accompanied by shrinkage. As water leaves, the particles move closer together. Fine particle clay shrinks more than large particle clay because of the presence of more water.
Drying shrinkage is always related to the grain structure of a clay and also to plasticity. When the water has evaporated from between the particles and they are all in contact, drying shrinkage is complete. This is called leather-hard. The object will still be damp and drying is not complete but the drying will not cause further shrinkage because the particles are in contact.
To avoid warping and cracking, plastic clay must be dried slowly and evenly. Drying is improved by the presence of any sort of non-plastic particles. They take up water and furnish pores for the water to escape in drying. Clays which contain a large percentage of large, nonclay particles are called "open bodies". Grog (having been fired) undergoes no more shrinkage. Additions of grog decrease overall shrinkage. Flint and Feldspar also promote drying and decrease shrinkage.
For throwing a plastic clay, non-plastic materials must be held to a minimum.
A piece of dried clay will contain some free water because atmosphere has humidity. Drying is actually completed in the kiln. At the boiling point (100oC) all uncombined water will be evaporated and clay will then be completely dry. Dry clay bodies vary in dry strength. The more plastic a clay is, the more strength it will develop in the dry state. Ball clay is 6 -7 times stronger than Georgia Kaolin.
2. Early Stage to Firing
Drying and water-smoking first change completion in drying. This must be done slowly of formation of 100oC steam will occur within the body and may cause it to burst.
The next change is at 350oC. Chemically combined water begins to be driven off. (This is part of the molecular structure of clay and is unaffected under 350oC.) This release of sudden steam must be done slowly or object will crack.
At 500oC, the clay is completely dehydrated, no longer slake or disintegrate in water, has lost all plasticity. It may not be reclaimed and used again. An irreversible change has taken place - dehydration - this is not accompanied by any shrinkage. At 500oC, the clay is more fragile now than it was when put into the kiln.
3. Oxidation
Oxidation is not complete until the temperature has reached about 900oC and the organic matter has been removed. (matter, such as carbon, inorganic carbonated and sulphates.) The object must have sufficient oxygen and not be fired too rapidly. If oxidation is not completed, it may cause bloating in glaze fire. As firing advances, the dissociation point of the compounds are reached and the carbon or sulphur is driven off.
4. Quartz Inversion
All clay contain quarts, either naturally or in the form of added: free silica. When temperature advances, the crystals of quartz rearrange themselves into a slightly different order. The rearrangements are accompanied by slight changes in volume. When 573oC us reached, quartz crystals undergo a change known as a change from alpha to beta quartz. This is marked by a 2% increase in volume. Reversible upon cooking, the quartz changes from beta to alppha and resumes its original crystalline form and size. This change of volume, though slight, must be done slowly to avoid damage to the ware. A large percentage of ware which comes from the kiln cracked is damaged by either too rapid heating or too rapid cooling at this critical temperature. In uneven firing kilns, one must by very careful with large pieces.
5. Vitrification
As temperature increases beyond red heat, vitrification occurs. Hardening, tightening and finally, partial glassification of clay. Vitrfication gives the fired clay its characteristic hard, dense, durable and rocklike properties. It is accompanied by shrinkage in the clay. Vitrification proceeds gradually as temperature increases and becomes increasingly hard, up to a point of melting and deformation. Hardening results from the melting of some of the components in the clay, in part, those components we consider impurities, iron oxide, etc.(clay has numerous oxides) it tends to fuse gradually, impurities (as temp) melt in small beads of glass, soaks into surrounding areas, binding particles like a glue and act as a solvent in promoting further fusion. If firing is carried on clay would reach a liquid state and when cooled, be glass. We don't do this.
Common red clays with high % iron and other impurities, have a relatively low melting point. The tendency is to melt at cone8 to cone11, we use them as slip glazes. (these glazes are made up of fusible clay) or can become a major part of a glaze.
The strength of fired clay is due, not only to glassification but also to the formation of new crystalline growths within the clay body.Particularly the growth of Mullite crystals. Mallite (alumina silicate) is characterized by a long needlelike crystal. Mulite crystals tend to grow at higher temperatures depending upon their composition. Common red clay (many impurities) fires hard and dense at 100oC; melts to liquid at 1250oC. Kaolin will still be porous at 1250oC and may not melt in excess of 1800oC.
Further shrinkage occurs during vitrification, due to the diminished size of particles as they approach fusion and the closer arrangement of particles in their glassy structures (Matrix). Firing shrinkage of clay is usuallly about the same as drying shrinkage. The total may be 10%, depending on the degree of vitrification.
When objects are overfired, they melt, bloat, expand and grow in size like a cake because of trapped gasses.
