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Words beginning with R
Raku
The raku process is an economical way of firing ware in reduction to achieve metallic and carbon effects. Normally ware is heated in a kiln until the glaze is melted to the desired degree then it is removed with tongs and put into a container of organic material (i.e. sawdust) where it is cooled. The organic material burns and uses the available oxygen in the container and the metallic effects develop. Because of the heat shock during heat-up most people bisque fire ware before rakuing.
Raku ware is usually crazed and very porous and lacking in strength. Thus it is only suitable for decorative ware. Metallic effects that looked great out of the firing can tarnish and disappear with time and people have developed ways to preserve these with various fixatives and surface treatments.
Raku firings often generate a lot of smoke, an people have developed different ways to try to contain this smoke. Unfortunately some have developed and even promoted quite casual methods that pose significant dangers. Raku kilns can emit harmful metal fumes (depending on how pots are decorated) and vapors of chlorine and sulfur (from salts, chlorides, and sulphates). Some people have been seriously injured in this regard.
Inhalation of Chlorine can cause chemical pneumonia, immune reactions, rashes, irritated mucous membranes, diarrhea, nausea, vomiting, cancer, brain damage, etc. Smoke from raku is hazardous simply because ALL smoke is hazardous and possibly carcinogenic. Incomplete combustion ("yellow" smoke) produces the most hazardous substances in smoke (i.e. wood tar and thousands of other compounds). While some people claim to wear respirators with "smoke blocking filters" there is no such thing. This is why firefighters wear tanks of compressed air.Refractory
The ability of a material to withstand heat without deforming or melting. Kiln shelves and firebrick are refractory. Many clays and minerals are also refractory. A fireclay with a PCE of 35 is said to be a super duty fireclay.
Reduction, Reducing Atmosphere
A kiln atmosphere which is deficient in free oxygen. This condition is accomplished in gas kilns by increasing back-pressure or reducing the amount of primary air available to each burner. The result is an increase in gases like carbon, hydrogen and CO. These are very aggressive in wanting to combine with oxygen. Hydrogen is small and particularly oxygen-hungry and can thus steal it from within bodies and glazes. Reduction firing produces different colors and visual effects because metallic oxides willing to give up oxygen convert to their reduced or more metallic form. Good examples are copper which burns red (it fires green in oxidation) and iron which becomes a powerful flux and produces earthtone browns (it is refractory in oxidation). Because almost all natural clays contain iron, reduction firing normally gives completely different clay surface effects than oxidation.
Many people do a period of oxidation at the end of a reduction firing to clean the atmosphere and soak the glaze to heal bubbles that result from the active volatilization (an accompanying bubble formation and surface disruption) that reduction induces. In many cases color breaks in glazes are a result of localized reoxidation of the melt surface. The effect depends on glaze thickness and evenness of coverage. Tenmoku glazes are an example of this, the brown thinner areas are oxidized.
It can be a challenge to reproduce the same effects in firing after firing using the reduction process. Many people have developed great skills in this area. However the oxygen probe is promising to revolutionize reduction firing, especially for small scale industry and hobby operations. It provides a direct measurement of the amount of reduction and enables one to more easily maintain the critical balance between oxidation and incomplete combustion. While these devices are quite expensive there are very few people employing this process that are not at least planning to get one.
Reduction firings are not without hazard. Any form of incomplete combustion can generate smoke and deadly gases. CO for example, is deadly and is colorless and odorless. It is important that gas kilns be vented well and if possible that a CO alarm be installed.
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