Unglazed pots have been made (to the best of our knowledge) for somewhere around 15,000 years. Historians and archaeologists have unearthed all sorts of colorful pots; but how does raw clay turn into these vibrant vessels? The answer is in glazing, a technique of coating ceramic pots in different mineral and chemical combinations to seal and protect their surfaces. Basically, a glaze is a glass coating on ceramics. When first applied glaze is in a liquid form, being composed of three main parts:
- silicon dioxide to provide the main body
- aluminium oxide to enhance the viscosity of the glaze
- fluxes, generally alkali or alkaline earth metal oxides, to lower the melting point of the mixture to the temperature of firing.
- metal oxides to provide color
These components are mixed with varying amounts of clay and water (known as slip) and the liquid mixture is applied to the surface of a pot before it is fired for the final time. It is during the firing that temperatures reach a point where they will melt the dried glaze and turn it into a hard, glassy surface which protects the pot.
Applying glazes can be as simple or complicated as you want it to be, and the same goes for studying and fine-tuning the chemistry of the glazes you use. Changing the ratio of ingrediants even slightly can sometimes have a significant effect on the way the glaze fires and how it looks. Combining glazes is also quite common (seen above) and can produce beautiful outcomes. In the example of the mug I’ve shown, the entire thing is dipped in a black glaze, then the top portion is dripped with a yellow salt (which after being fired becomes a tarnished yellow/brown) and finally the very rim is again dipped in an iron oxide-based glaze which will drip down and mix with the yellow salt.
It all sounds somewhat complicated but after only a few glaze firings you start to see the general behavior of the glazes you use and how they interact.