There are four principal kinds of glaze: alkaline (feldspathic), lead, tin and salt. Alkaline, lead and salt glazes are basically transparent, while tin glaze is an opaque white. It is more accurate to describe tin oxide as an opacifier as it is added to other glazes, particularly lead, to make them white and opaque, usually to hide the colour of the body and blemishes such as inclusions, iron specks or air bubble “blisters”. As glazes are predominantly quartz (silica)-based, a flux has to be added to reduce their melting temperature. Alkaline glazes were the earliest glazes, first used in the Middle East. They were initially associated with the production of faience, but started to be used on pottery around 2,000 BC. They are generally used at higher temperatures (over 1,150 degrees C) and have alkaline fluxes to reduce their melting temperatures including soda (Na2O), potash (K2O), lime (CaO) and/or lithia (LiO). One reference to an ancient alkaline glaze cited its composition as 50% silica, 40% soda and 10% lime, producing a melting point of 932 degrees C, and another gives the wide range of constituents in ancient alkaline glazes as: silica (50-80%), soda (2-17%), potash (1-4%), alumina (1%) and zinc oxide (trace), with copper (9-19%) as colourant. These early glaze constituents were often fritted before being applied. Feldspathic glazes were used on stoneware and porcelain in China. They were typically made of kaolin with a little lime and potash as fluxes and were similar to alkaline glazes, but had a higher melting temperature. Particularly fine pieces using feldspathic glazes in China were the green celadon wares.

Chinese Sung/Yuan Fluted Celadon Bowl -
courtesy Glade Antiques

Lead glaze was discovered a little later and is a lower temperature glaze that can readily be used on earthenware. It was also probably first discovered in the Middle East (Mesopotamia). Lead sulphide (galena) was probably used by the ancient Babylonians for glazing pottery by dusting or painting on to the biscuit body. This lead glaze had a low melting point and fused with the silica in the clay to produce the shiny surface. However it was a soft glaze that decayed with time and became dull. The Egyptians also used lead glazes. Many early glazes contained some lead, but at relatively low levels. Later high-lead glazes used by the Romans from the first century BC usually contained 50-60% lead. Examples are lead oxide 60%, silica 40% or lead oxide 95%, silica 5% - melting temperatures 661 and 526 degrees C respectively; and lead oxide 80%, silica 15%, boric oxide 5% - melting temperature 485 degrees C, showing the effect of boric oxide. The Chinese Eastern Han Dynasty (25-220 AD) used a green coloured lead glaze on earthenware very successfully. Lead glazes volatilise (evaporate) at 1,150 degrees C so they cannot withstand high firing, and cannot produce the fine colours and brilliance of alkali glazes.

Babylonian Glazed Bricks from Procession
Street 604-562 BC - source Archaeological
Museum Istanbul

Chinese Han Green Glazed Vase - courtesy
R&G McPherson Antiques

Tin opacified glaze was also a Mesopotamian discovery, and was used by the Assyrians to coat decorative brickwork. The tin glaze technology was then lost until it was revived in the 9th century AD in Mesopotamia when tin oxide was added to a hybrid lead/alkaline glaze. In the 10th century AD Islamic potters used it extensively.

The usual method of glazing earthenware was to first fire the item to the “biscuit” state – fully fired but unglazed. It would then be coated with the lead, alkaline or tin glaze and fired a second time at a lower temperature, which had to be low enough so the body of the item did not melt, but high enough to fuse the glaze onto its surface. “Eraclius” also describes methods used for glazes. One for lead describes how the surface of the pot is prepared by coating it with a paste of wheat flower and water, and then sprinkling it with powdered lead to give a yellow glaze. To obtain a green glaze the lead is melted in a pot and stirred until the oxide is formed, then copper or brass filings are mixed with the lead oxide and applied as for yellow. Not the safest process for the poor potter!