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What is a glaze?




What is a glaze?

A glaze is a specifically formulated glass that fits a ceramic surface. A good fit is only possible when the glaze and the ceramic have a close expansion and contraction rate. If the glaze shrinks more than the ceramic body, it will crack (in industry this defect is called crazing). If the glaze contracts much less than the ceramic body, it will simply shrink or peel away (in industry this is called shivering).

 

Glazes are made up of three fundamental components:

  • Glass former or Silica whose chemical formula is SiO2

  • Refractory material aka Stabiliser aka Alumina (a material that melts at very high temperature

  • Flux (a material that lowers the melting point of the glaze mix but may be refractory on its own)

 

Eutectic mixture:

A glaze is a homogeneous eutectic mixture which has a lower melting point than any of its constituents individually. (Silica melts at 1710°C while Alumina melts at 2050°C. When the two materials are mixed in a glaze in a specific ratio of 10% Silica to 90% Alumina, their melting temperature can drop as low as 1545°C. Most glaze recipes consist of 4-12 parts, which you may call ingredients for ease of understanding, whose interaction will have an impact on the overall melting point of the mixture.

 

Raw and Fritted Glazes:

Irrespective of their appearance, glazes can be described as raw or fritted. Raw glazes are formed by blends of natural and synthetic materials such as clays, feldspars, oxides and wood ash, etc, while fritted glazes contain a man-made glass specifically developed for low to medium range glazes. (Earthenware and Raku glazes can contain up to 90% frit while mid-range and high-fire glazes can contain 10-30% frit.)

 

Firing range:

Glazes as well as clay bodies are often categorised according to their firing range. For example the white clay body we use in the studio has a firing range between 1200-1300°C.

 

  • Raku approx. 900-1000°C

  • Low fire Earthenware approx. 900-1060°C

  • High fire Earthenware approx. 1060-1150°C

  • Low fire or Mid-range Stoneware approx. 1150-1220°C

  • High fire Stoneware approx. 1220-1350°C

  • Porcelain/Bone China approx. 1260-1400°C

 

Note: Ceramics fired above 1200°C have a much tighter bond and interaction between the clay body, slip and glaze compared to pieces fired at Earthenware temperatures.

 

What are some of the most powerful factors and conditions that influence a glaze result?

 

l Raw materials (eg. Origin, Purity, Processing, etc.)

l Clay body (eg. How much iron a clay body contains has an impact on the colours and effects that can be obtained from a glaze. White firing and porcelain clay bodies act as a white canvas while dark clay bodies mute or subdue colours.)

l Type and size of kiln (eg. Brick, ceramic fibre, top loader, front loader, etc.)

l Fuel (eg. Electricity, gas, wood, oil, coal, etc.)

l Firing temperature

l Kiln Atmosphere: Oxidising or Reduction

l Length and Schedule of Firing Cycle (some glazes require specific firing routines such as fast or slow cooling)

l Thickness and method of glaze application

l Quantity of work inside kiln

l Etc..

 

 

A few historical and contemporary approaches/practitioners to consider:

 

Jian ware

Celadon glazes

Crackle glazes

Snowflake glazes

Ash glazes

Crawling glazes

Crater glazes

 

Linda Bloomfield, UK

Emmanuel Cooper, UK

Akiko Hirai, Japan/UK

Lucie Rie, Austria/UK

Dick Lehman, USA

Svend Bayer, UK

Greg Daly, Australia

 

 

Books:

 

Emmanuel Cooper - The potter’s book of glaze recipes

Greg Daly - Glazes and glazing techniques

Greg Daly - Developing glazes

Linda Bloomfield - Special effect glazes

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