Q: Are glass and glaze the same?
A: Sometimes, but not often. Glass itself dates back at least 50 centuries. Mix sand and some other dry minerals, heat in a fireplace, and you obtain a material often called a “network polymer” (a “mer” is a unit molecule and “poly” means many, joined together). Today, most glass is made in a special furnace from four elements: Calcium, Sodium, Silicon, and Oxygen (from air). Of these, a combination of Silicon and Oxygen serves as the “backbone” of the network polymer. The batch recipe usually lists silica sand (silicon oxide), soda ash (sodium carbonate), and lime (calcium oxide, but limestone or calcium carbonate may also be used). These three ingredients, each at high purity, are fed into the furnace in precise proportions to make a batch of container glass (or window glass or specialty glass). Also, if some borax (sodium borate) is added to the basic recipe, the result is hardened glass suitable for laboratory glassware.
Despite some limitations, a “container” glass, for example, has great versatility. A major virtue is its capability to be recycled many times; it can be made into a jar to hold say, pickles, then remelted and made into a wine bottle, remelted again, and again, before the glass gets degraded by contamination and becomes too costly to salvage. A glaze, however, differs markedly from the common glass jar. Once formed on a clay pot, the glaze is fused to the rock-hard ceramic and cannot be economically separated for recycle.
For most potters, a glaze starts off as a “slurry”, that is, a mix of fine powders suspended in water. Then, this mix is transferred to the surface of the clay pot (usually “bisqued-fired”) by one of three common methods, dipping, spraying or brushing. The clay-pot’s surface has an affinity for the glaze powders and holds them in place. After the wetted pot has dried again, it is placed in the kiln and heated (“fired”) to the proper temperature.
This two-stage process requires that the glaze will stay on the pot while it is drying and later, in the kiln, that the glaze ingredients will form a viscous glass on the clay surface as the pot itself also undergoes change. During the firing of the kiln, almost all glaze materials undergo physical change, going from individual particles to large clusters that can form liquid glass.
Some materials also undergo chemical change either by giving off gases or by re-arranging their molecular shape. If the mix of oxides falls within a certain range, the materials mix will form glass on the fired pot, that is, become a glaze.