Potters are constantly creating, changing and improving the way that they decorate ceramics. This is generally done in the form of a glaze. The difference between one type of glaze and another can be as complex or as simple as the potter imagines and the effects as numerous as the number of potters themselves.
Nowadays a glaze is generally applied to the surface of a ceramic object after it has been through the kiln for a preliminary firing, when it is dipped into a glaze mixture. Glaze can also be sprayed or brushed onto the surface of the object to be decorated. A glaze takes the form of an opaque mixture – similar to a thin cream in consistency. After firing, the glaze becomes fused to the surface of the pottery and it is only then that it becomes transparent and the final colours are revealed.
Glazes have been discovered to have been used in the decoration of ceramics1 as far back as the early Egyptian period. To begin with, the glazing material was incorporated into the body of the ceramic object and this is known as 'frit paste'. Over the succeeding millennia, developments in glazing techniques slowly took place, and the practice of glazing has since become commonplace. Powdered glass was originally used as the base of historic glazes. Minerals and water-soluble salts were added which altered the colour and texture of the final product.
The introduction of lead into glazes was an important part of ceramic history. This practice is believed to have first occurred in Syria although it had quickly spread across to China by 500 BC. The 6th Century BC saw large improvements to the kiln which would now allow temperatures of up to 1,200°C (2,200°F) to be reached. The addition of lead allowed many glaze materials to melt at a far lower temperature than would normally be needed to get the materials to blend into the glaze. It was common practice for lead to be used in glazes for any purpose until the discovery, in the 19th Century, that this was a common cause of poisoning of the pottery workers in industry2.
On 28 July, 1898, the UK's Home Secretary announced that:
Her Majesty's Office of Works have issued a regulation that all articles for domestic use supplied under contract for the Public Department, the Houses of Parliament, and such of the Royal Palaces as are under their charge shall be made with leadless glaze.
However, the use of lead in a glaze is not a thing of the past, although you will be glad to know it is no longer used on dishes that are designed to hold food. Many earthenware glazes still use lead in their composition, due to the lower firing temperature of the earthenware body; this is especially true in transparent glazes.
There are many technical terms involved in the production of ceramics, some of which it may be helpful to explain here.
Oxidation Firing: Can be produced in all kiln types. Air is circulated freely through the kiln during firing; this is the basic type of firing.
Reduction Firing: Fuel-burning kilns only. The oxygen is restricted in the kiln, producing specific atmosphere conditions. Some glaze effects can only be produced in this type of firing. It is generally more expensive and takes longer than oxidation firing.
Biscuit Firing: After drying, the work is fired to between 980 and 1,100°C (1,800–2,000°F). This makes the work stronger for the glazing stage; if you plan to 'Raku' your work, it is recommended that you go to the higher end of the biscuit temperature scale.
Raw glazing: This is the process of glazing a pot without the primary biscuit firing. This can be a less expensive way of producing work but is much more hazardous for the pot as any remaining moisture can result in a steam explosion during the firing process.
Glaze Temperature Range: This can differ between glazes. A glaze that produces pastel colours, especially pinks, typically need a lower temperature than other glaze colours. The general rule of thumb is:
- Earthenware: 950–1,150°C (1,750–2,100°F)
- Stoneware/Porcelain: 1,200–1340°C (2,200–2,450°F)
Soak: Once the required temperature is reached, it is then maintained for a period of time. This process can eliminate any temperature differences between the top and bottom of the kiln. This is more important in fuel-burning kilns.
Raku is a type of glaze firing in a specially made kiln. Once the glaze has melted, the pot is removed quickly from the kiln and dropped into a bucket of sawdust or wood-chips. This produces a post-firing reduction atmosphere which produces somewhat random effects with lustres or crackle glazes. When Raku was originally brought over from Japan, by Bernard Leach, it was seen as nothing more than a novelty and a curiosity. Now, however, Raku is viewed by many as a form of pottery in its own right.
Salt and Soda Firing
Salt is introduced by throwing a handful of salt into the kiln at around 1,240°C (2,260°F). The salt fuses with the silica in the clay, producing a characteristic 'orange peel' effect. It should be noted that it is not advisable to use salt glazing without proper extraction of exhaust gases due to the release of poisonous chlorine vapour. Soda firing is a similar technique to salt glazing: sodium carbonate mixed with water is sprayed into the kiln rather than dry salt, and poisonous gases are not produced. The sodium vapour combines with the silica in the clay body to form a sodium-silicate glaze.
Wood is used as the fuel within the kiln. This can produce rich and warm surface effects as the wood ash fuses with the pot and forms the glaze. A constant vigil is needed to produce a regular temperature rise. Adding too much wood at any stage can raise the temperature too quickly which will result in the destruction of all the pots in the kiln. Wood firing is very popular among traditional potters but should not be tried unsupervised by the inexperienced.
The colorants used in pottery are derived from different metals.
Cobalt: Produces a rich blue at concentrations up to 1.5% in a glaze solution. Cobalt comes in oxide and carbonate forms although carbonate can produce somewhat milder results. Cobalt can turn green in the presence of titanium dioxide.
Copper: In oxidation, copper produces a turquoise in an alkaline solution or bottle green when mixed with lead. In reduction, copper will produce a deep red glaze known as Ox Blood.
Chromium: Also known as chrome, it produces a range of green glazes. When mixed with tin, it can produce a strong pink/red. Chrome may affect other pots nearby in the kiln.
Iron: This is the most commonly used colorant as colours vary from pale honey to near black in oxidation. Iron is also used to produce the grey/green and blue for a celadon - a pale, almost transparent blue/green glaze.
Manganese: Creates browns in a solution with lead. In alkaline glazes, it yields purples and violets.
Nickel: In oxidation, nickel produces cream and brown when mixed with magnesium. In reduction, nickel produces a green/grey.
Tin: Used in concentrations of between 5 and 10% in solution, it produces a very soft, opaque blue glaze.
Vanadium/Uranium: Produces yellow glazes when mixed with lead. It is very rare to find uranium oxide used in glazes.