The earliest glazes were discovered in Egypt around 5,000 B.C. They mixed water-soluble soda and copper with clay. Glazes contain materials designed to form glass. Since they do not melt at temperatures ideal for firing pottery, they have flux added, such as potassium and calcium. This allows the glaze to melt at lower temperatures.
Chinese potters operated in 5,000 B.C. They used wood as fuel and noticed that the ash settled on the pottery and fused into it, changing the properties of the finished pottery. The Chinese experimented with other substances and found new ways to glaze pottery. Original glazes in China were designed to make the pottery water-tight. Over time, the Chinese developed two forms of glaze, one made of feldspar and the other made from the fusing of silica of quartz or sand with a lead oxide. The Chinese spent much time improving their glaze and used lead more frequently, adding wood ash to give the pottery a dull brown or greenish-gray color. Later, the Chinese developed colored glazes in the Tang and Song dynasties. They used a clear glaze throughout the Ming dynasty. Persian wares inspired them and they began painting their pottery using blue cobalt under-glaze.
Potters in Iran and Mesopotamia used an alkaline glaze made from quartz, with sodium and potassium mixed in. They later added lead to the glaze. Persians fired pottery at low temperatures, using lead and tin.
In the 16th century, Italian potters working in Bologna and Padura coated their pottery with lead glaze, which had a brown, copper, green or yellow tone. The potter would fire the pottery and then add details before glazing and firing it a second time. The English used salt glaze in the early 1700s.
Pottery made during the Colonial era in the United States was almost completely salt glazed. However, amateur potters use whatever techniques they like. Modern glazes are commercially prepared for many artists.