The original purpose of a glaze on pottery was to seal the pot, making it a more effective container, especially of liquid, and consequently making it much more useful in cooking. Pottery was being slip coated and painted as early as 3500BC but fired glazes of lead and tin were not being used until around 1200BC.
The largest technological influence on glazing has probably been the availability of increasing numbers of impurities to affect the appearance of the finished work. Contemporary glazing is primarily a decorative finish, although the commonest substance for construction of receptacles for liquid, particularly hot liquid is undoubtedly ceramic and will be glazed.
Glazing is an interesting art because the final stage of production transforms the bland into the beautiful, and the artist just has to know that is what is going to happen. Many times they don't, and the finished product has been strangely affected by unplanned environmental states in the kiln. Put it down to experience and start again - hopefully you know what the difference was and can repeat it if you like the effect. Commercial pottery naturally uses more controllable kilns, while artists often prefer outdoor ovens fired by wood or gas.
As well as being affected by the temperature and oxygen levels, the colour and texture of the finished glaze is affected by the combination of minerals included and the clay foundation. Glazes incorporating feldspar, lead and salt are typically clear, with tin resulting in an opaque white. More recently (13th century), copper might be induced to produce blue-green and red colours, or dolomite to give brown and blue hues. Modern artisans have a bewildering array of combinations in their arsenal.
The robustness of glazed pottery accounts for an enormous quantity of knowledge we have about ancient civilisations because it has frequently lasted for thousands of years, even without burial and archeological excavation.