Ceramic glazes are glasses that have been adjusted to work on and with the clay body they are applied to.
A glaze is a glass that, on its most basic level, has been tuned to melt to the desired degree at the target temperature, have a thermal expansion compatible with the body to which it is attached (a typical soda-lime bottle glass, for example, would craze badly on typical clay bodies). In ceramics, glazes contrast with bodies in that all particles melt, none of them retain the crystal identity they had. Glazes are made from finely ground mixtures of mineral and man-made powders. At higher temperatures, like cone 10, a wide range of these powders will melt on their own to form a glaze, others that do not will react with others to form a melt. As temperatures go down, fewer and fewer materials melt enough and more and more of the fluxing burden rests with boron containing materials and frits. Glaze contain additions that can be tolerated by the glass and which achieve the color, opacity and character desired visually.
Glaze recipes need to contain materials that contribute both to the desired fired chemistry of the glass as well as to the physical working properties in production. Glazes are normally suspended in water (specific materials in the recipe, like clays, keep the particle mix in suspension and also harden it during drying). Glazes are applied to ware by painting, dipping, or spraying.
Glazes are classified in many ways (e.g. unleaded, raw, fritted, slip). The chemistry of glazes is the aspect most related to the way they fire. People who learn to understand the relationship between the chemistry and the fired properties can exercise alot more control. Many physical factors also play in the way a specific glaze fires (thickness of application, body it is applied to, firing schedule and temperature and atmosphere, method of application, etc). The physical presence of the glaze slurry in production is related to the physics of the materials, those who understand the interplay of these as well as the chemistry can create glazes that have good working properties and fire the way they want.
Glass vs. Crystalline In ceramics, understanding the difference between what a glass and crystal are provides the basis for understanding the physical presence of glazes and clay bodies.