A glaze, basically, is a type of glass that is especially made to stick onto pots and other ceramic surfaces. Vitrification is the process of melting that clay and glaze go through as they are fired to maturity. In a fully matured clay body, the spaces between refractory particles are completely filled up with glass, making the clay body impervious to water. Glazes provide strength, color, finish and impermeability.
Glazes are applied onto bisques pots by dipping, spraying or brush. Before doing that, a coat of wax is applied at the base so that the glaze doesn’t trickle down and stick the pot to the shelf of the kiln. Wax can also be applied for experimenting with wax-resist glazing effects.
A glaze has four key components:
• Silica, the Glass Former:
Silica is both the base material for glass and for ceramic glazes. It melts at about 1710 °C. It can be introduced into glazes as silica oxide, flint or silica sand.
• Fluxes, the Melting Agents:
Fluxes lower the melting point of silica, making it usable to create ceramic glazes.
• Alumina, the Refractory:
Alumina or aluminum oxide is used in nearly all glazes as stiffening agent, allowing glazes to stick to a pot’s vertical surface without running off when it has melted.
Eg. China clay.
• Colorants and Glaze Modifiers:
Silica when melted is transparent. Colorants like metal oxides and stains are added to glazes to produce a wide range of hues.
Eg. Manganese dioxide, copper oxide, cobalt oxide. Three main factors affecting glaze color are the composition of the glaze, the temperature to which the glaze is fired and the firing process (oxidation or reduction).
In addition to colorants, glazes can also have other modifiers. These may modify the glaze opacity, iridescence or working qualities when the glaze is still raw (unfired). Matting agents like barium carbonate and titanium dioxide are used sometimes.
During the glaze firing, clay goes from this soft, totally fragile substance to one which is rock-hard, impervious to water, wind, and time. The change is nearly mystical in its complete metamorphosis. The most common kiln designs utilized by contemporary potters are the natural gas updraft kilns and electric kilns.
A thermocouple (heat sensor) is fitted through the wall of the kiln to read the varying temperature on a pyrometer.
- 100 °C: Water boils.
- 100-200 °C: Clays lose ‘mechanical water’.
- 374 °C: ‘Critical’ temperature of water. Chemically combined water leaves clay.
- 500 °C: Red glow in kiln.
- 573 °C: ‘Quartz inversion’
(Quartz has a crystalline structure that changes at specific temperatures, actually causing the pot to increase in size by 2% while heating, and lose this 2% as it cools. Ware is fragile during this and the temperature must be raised slowly through the change. Careful firing is required until 600 °C. (approximately 100 °C per hour).
- 800 °C: All organic matter in clay burns out by this temperature (carbon and sulfur).
- 800-1000 °C: Orange color in kiln. The clay particles begin to fuse (sintering). Low-fire earthenware and lowfire lead glazes mature. Normal firing temperature for red bricks and flower pots.
- 1000-1160 °C: Yellow color in kiln. High-fire earthenware matures. Feldspars begin to melt.
- 1170-1190 °C: Bright yellow-white color in kiln. Midrange clays and low-fire stoneware mature.
- 1250-1285 °C: White color in kiln. Stoneware clays vitrify, feldspathic glazes mature.
- 1285-1350 °C: High-fire stoneware, porcelains vitrify.
- 1712 °C: Silica melts.
- 2050 °C: Alumina melts.
Soaking a kiln means to keep it at the same temperature for a period of time. This gives the ceramic materials time to absorb more heat, and the glazes time to smooth out. Generally, a twenty to forty minute soak at the end of the glaze firing is appropriate. The burners should then be put off and the kiln be left to cool for couple of days (till it reaches room temperature) before it is opened to see the final product.