Glazes are a type of glass that are especially made to stick onto pots and other ceramic surfaces. When molten, this specialized glass is stiffer than glass that is poured or blown is. This is important, as otherwise the glaze would run off the vertical surfaces of the pots when brought up to temperature in the kiln.
Glazes come in a huge array of colors, the result of minerals and inorganic compounds. The most commonly used colorants are the iron oxides, cobalt oxide, chromium oxide, copper oxide and copper carbonate.
A glaze's color may also be effected by the firing process. If the atmosphere in the kiln has plenty of oxygen, it is called an oxidation firing. If the atmosphere has very little oxygen, it is called a reduction firing. The amount of oxygen present in the kiln can drastically change a glaze's color. For example, a basic glaze using copper carbonate to color it will be turquoise if fired in an oxidation atmosphere, or bright red if fired in a reduction atmosphere
Glazes range from being completely transparent to being completely opaque. Most opaque or partially opaque glazes derive their effect due to either tiny particles or trapped air bubbles held in suspension within the glaze. Many white glazes are white due to opacity rather than an actual colorant.
Different minerals can cause opacity in an otherwise clear glaze, but other characteristics such as surface and the development of crystals can also be achieved. These minerals are all called glaze modifiers.
Most of the time when we think of glazes we probably think of very glossy, shiny surfaces. This is not always the case, however. Glazes can also have a satiny finish or a dry-looking matt surface. Some specialty glazes can even be quite rough in texture.
Glazes don't all melt at the same temperature. Most glazes have fairly specific melting ranges. Too little heat, and the glaze won't fully melt or mature. Too much heat and the glaze will become too fluid, and may even run off the pot onto the kiln shelf.
In addition, some glazes flux at a lower temperature if the kiln atmosphere is in reduction. Temperature and atmosphere must both be taken into account.