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Pottery FAQs |
What is Raku?
Raku is a firing process that can produce a number of special effects such as metallic sheens (especially copper), crackled glazes and blackened (smoked) clay. The clay item is first bisque fired in a normal kiln and raku suitable glazes and slips are then applied. The raku kiln is designed to raise the temperature of the items very rapidly (in maybe only 15 or 20 minutes). Tongs are used to transfer the hot piece to a container (e.g. a metal dustbin) containing combustible carbon-based materials such as wood shavings, sawdust, shredded newspaper, or even dried leaves. More may be thrown on top. Once the material bursts into flames, an airtight lid is placed on top to create a carbon-rich reducing atmosphere around the pot. The raku process is only really suitable for decorative pieces since the clay body remains slightly porous making it unsuitable for functional items involving liquids or food.
More on this subject can be found on a very comprehensive raku FAQ on this Ceramics Today website: http://www.ceramicstoday.com/articles/branfman_raku.htm
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What is Earthenware?
Earthenware is a type of pottery clay that must be fired at a relatively low temperature. It supports a wide range of bright glazes but has less strength than other clay mixtures so it can crack or chip relatively easily. Earthenware items are usually fired at temperatures in the range of about 1000 to 1200 degrees Celsius.
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What is Stoneware?
Stoneware is a type of clay mixture that must be fired at a relatively high temperature. It has great strength and a hard finish. Stineware items are usually fired at temperatures in the range of about 1100 to 1300 degrees Celsius.
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What is Porcelain?
Porcelain is a very pure type of clay made mostly from kaolin with some added feldspar and flint. It can be used for very thin pieces. Once fired, it has a white somewhat translucent appearance. Porcelain is fired at a fairly high temperature in the range of about 1200 to 1400 degrees Celsius.
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What is a bisque (biscuit) firing?
Virtually all pottery is kiln fired at least twice. In fact the word ‘biscuit’ comes from the French meaning ‘twice cooked’ and the first firing is referred to as the bisque or biscuit firing. The purpose of the bisque firing is to fuse the clay into a hard durable form by raising it to a temperature that melts some of its ingredients whilst sufficient remains unmelted to retain the form of the piece. The melted constituents fuse the remainder together into a hard mass. Some chemical alteration may also take place, particularly at higher temperature firings. Second or subsequent firings are always to a lower temperature and serve to fix the decoration (glazes).
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What are cones? (I have seen them mentioned on websites)
Cones (or pyrometric cones) are not actually cone shaped but are small stretched, slightly sloping, square pyramids of different clay mixtures that serve to visibly indicate the temperature inside a kiln when viewed through a spyhole. The clay mixtures are chosen to melt at different temperatures and when that temperature is reached, the tip of the cone will lose its supportive strength and start to keel over. Cones are numbered from 022 (lowest temperature) to 42 (highest temperature). Think of the zero as being a minus sign and then you will see that the numbering goes 022, 021, 020, 019, . . . 03, 02, 01, 1, 2, 3, 4, etc. (there is no cone 0) A table of approximate equivalent temperatures can be found here:- http://www.miniworlddolls.com/evenheat/ConeInfo.htm
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What is slip-casting
Clay blended with sufficient water so that it can be poured, is called a slip. Slip-casting involves pouring this into a hollow mould in a large block of absorbent plaster-of-Paris. As the water is drawn into the plaster, the slip adjacent to the plaster loses its ability to flow and remains in the mould when the surplus is poured out. Once the remaining clay has dried out sufficiently to have some strength on its own, the plaster mould is disassembled to leave the clay in the desired form. This is used for anything from small detailed figurines to commercially produced sanitary ware.
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- More will be added shortly.
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