Glazes - for the Self-reliant Potter (GTZ, 1993, 179 p.) 2. The nature of glazes 2.1. Glass and glaze, benefit of glaze 2.2. Glaze making is difficult 2.3. History of glazes 2.4. Classification (earthenware, stoneware etc.)

Glazes - for the Self-reliant Potter (GTZ, 1993, 179 p.)

2. The nature of glazes

2.1. Glass and glaze, benefit of glaze

Glass is a useful material that has been known for thousands of years. It can be produced in many different shapes for many purposes, and it has many useful qualities: it is transparent, hard, resistant to chemicals, and can have many colors.

Glaze is a special type of glass, made for coating ceramic products. Whereas glass is suitable for forming into bottles or windows, glaze is different because it is applied on a ceramic surface and must form a hard, durable coating after being melted in the kiln. It must not run off the product and must stay on the product after firing without cracking.

2.2. Glaze making is difficult

Because glaze before firing looks nothing like the finished product and because we are not able to directly understand what happens when glaze melts at high temperatures, making glazes is very difficult. We must try to understand which materials melt at certain temperatures and what happens when materials are combined. It requires a lot of direct experience before you start to understand causes and effects. In this way it is like cooking: we are familiar with cooking because we know the raw materials, and by trial and error we have a good idea of what the finished meal will be like. However, imagine that you are in a foreign country with unfamiliar food in the market and you want to make a meal: How do you start? The best way is with a cookbook full of recipes and a local friend to tell you if the result is correct or not. This book is intended as a cookbook for the independent potter.

Although by just reading this book and experimenting with it you will probably be able to make glazes after some time, there is no substitute for learning about glazes from an experienced teacher, who can save you a lot of time by guiding you in proven directions.

2.3. History of glazes

Unglazed ceramics have been in existence for over 10,000 years. It has only been in the last 2000 years that there have been glazed ceramics and only in the last 100 years that a scientific approach to glaze making was developed. For that reason, glazes still occupy a mysterious area somewhere between science and magic.

The first glazes were probably invented in middle eastern countries, where there naturally exist deposits of sodium and potassium compounds (soda ash and pearl ash) that melt at low temperatures (800°-1000°C). By chance, early potters discovered that some clays when put in the fire developed a shiny surface. These self-glazing clays are known as "Egyptian paste". They are not very useful for making household items, being difficult to form.

The next step was to develop these substances so that they could be applied to the surface of pottery clay in order to give it the desirable qualities of a hard, shiny, easy-toclean and durable surface. Because early potters did not have the technology to reach high firing temperatures, they had to use materials with low melting points, mainly sodium, potassium and lead compounds. Glaze development had to be done by trial and error, since these early potters had no idea of chemistry. This took a lot of time and effort, and naturally successful glazes were closely guarded secrets. These early glazes were often soft and not durable, and had problems such as cracking and eventually falling off the pot. Additionally, glazes based on lead were poisonous both for the potters who worked with them and for users.

It was only when potters learned to reach high temperatures that truly permanent ceramics were developed. There are many more common chemicals and minerals that melt above 1100°C to form glazes, and clay that is fired to these high temperatures is also much stronger and resistant to water.

2.4. Classification (earthenware, stoneware etc.)

Although there are many different ways to classify glazes, the simplest way to understand them is according to the firing temperature. The useful range of temperatures for glaze melting is from 900°-1300°C. In this book, we talk about two different categories of glaze:

- low temperature from 900-1100°C, called earthenware;
- high temperature from 1100-1300°C, called stoneware.

These two categories are used because they require different raw materials as the main ingredients of the glaze.