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Firing is the process of baking clay products in a high temperature oven.

Underglaze is a combination of clay and colorants mixed into a paste.  Underglazes can be thinned with water.  They can be used much like tempera paints because colors can be blended, tinted and toned.  Underglazes are used when the artist wants to use color in a detailed fashion to decorate pottery.  Underglazes may be applied to greenware or bisqueware.

Clay is formed from pulverized rock that has collected in low regions through the grinding action of erosion.  The particles of rock are extremely fine and become sticky as organic matter breaks down and mixes with the particles. 

Clay bodies are created by combining a different clays together in a specific formula.  Clay bodies are formulated for specific purposes such as hand-building, working on the potter's wheel, sculpting and even industrial purposes.  Artists will generally mix or purchase clay bodies based upon workability, texture and color.

When clay has been wedged to the right consistency it will be malleable and will maintain its shape.  It will not feel sticky or tacky.  It will not crack or crumble went bent, nor will it sag or slump when formed.  This perfect consistency is called plastic.

When bonding together clay pieces both mating edges should be roughly scratched with a sharp edged tool like a fettling knife or a fork.  This creates rough, grippy surfaces.  Then slip(sticky wet clay) is generously applied to the scored surfaces and they pushed together and smoothed.  Additional clay may be added to complete the bonding process.

When clay is fired to maturity it will be vitrified.  This means that all physical and chemical water has been burnt off, particles of clay are compacted and some of the minerals in the clay have melted.  Clay that has vitrified will be at it maximum strength and it has lost all porosity. Vitrification happens at different temperatures depending upon the clay body's formula.

Bisque firing is the first firing.  Generally to about 1600 degrees F.
After bisque clay products will no longer breakdown in water.  This process prepares the clay for glaze and stain application. The clay is porous and absorbs water in the glaze and stain causing the glaze and stain to cling to the clay.

Glaze firings reach the maturity temperature for the clay body being fired.  This varies depending upon the type of clay and can be anywhere from 1600 to 2400 degrees F.   The clay being fired does not have to be glazed to be fired in a glaze firing.

Glaze is a combination of chemicals and minerals formulated to melt and become  a glass coating when applied to clay products and fired to the proper temperature.
Glaze formulas must contain silica, alumina, and flux .  Colorants in the form of metal oxides are added to give the glaze color.