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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.
Reduction Firing
Firing a kiln for part of its cycle with an atmosphere having no free oxygen. In traditional ceramics reduction firing requires a specially designed fuel fired kiln that restricts the flow of incoming air so there is enough to burn the fuel and no more (in some cases it is restricted so that is actually less than enough to introduce carbon into the atmosphere). Reduction is generally done to produce the visual effects associated with reducing metallic glaze and clay body components to their metallic state and for variegated effects in glazes. These effects include some colors and effects impossible or difficult to achieve in oxidation (e.g. copper reds, earthtone colors, dolomite mattes, iron speckling in clay bodies). Many people fire their gas kilns up in oxidation but at two places in the ramp (e.g. cone 06, 10) they reduce the kiln for a period (for body and glaze reductions). Others begin reduction firing at (e.g. at cone 06) and continue it to the end and then oxidize for a short period to clear the kiln.
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Pictures
Alberta slip fired in reduction (left) is much darker than in oxidation at cone 10.
An example of how the same dolomite cobalt blue glaze fires much darker in oxidation that reduction.
An example of the effects of reduction firing on two clays with the same glaze, one is a iron brown body, the other a grey stoneware.
The same glaze in reduction (left) and oxidation at cone 10.
Copper red reduction glaze cone 9 courtesy of Angela Walford
Two buff stoneware clays, cone 6 (left), cone 10 reduction (right).
Two brown stonewares, cone 10 reduction (left) and cone 6 oxidation.
Reduction Speckle
An effect created by firing a clay containing high iron mineral particles (e.g. ironstone concretions, etc.). The iron becomes a flux in reduction and the particles melt and blossom and can even run down vertical surfaces. Plainsman Clays in Alberta, Canada is particularly adept at making this type of body because they have raw clays that contain concretions and their grinding process can leave them large enough to blossom and consistent over time.
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Pictures
Cone 10R dolomite matte glaze with 5% manganese dioxide
Dolomite matte glazed cone 10R mug courtesy of Susan Clarke.
A cone 10 reduction stoneware clay containing iron stone concretions ground to 20 mesh
An example of how iron stone concretions contained within two clay bodies (a white and brown stoneware) blossom and produce speckle at cone 10 reduction.
Example of mugs with different outside glazes fired to cone 10 reduction (mug on right is brown iron stoneware, others are buff stoneware bodies).
Cone 10R firing of Plainsman FireRed (left), St. Rose Red 42 mesh (center) and St. Rose Red 10 mesh (right). The 10 mesh material produces a reduction speckle and deep red color that is very unique.
Ravenscrag Saskatchewan clays fired at cone 10R (top) and at cone 10R with glaze (bottom): A1, A2, A3, 3B, 3C, 3D. The bottom row has also shows solubles salts (SOLU test).
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. While many metallic coloring oxide melt very actively, chrome and rutile, for example, are very refractory (for example, even when mixed 50% with a high borax frit they do not melt at cone 6)
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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Pictures
Metallic oxides with 50% Ferro frit 3134 in crucibles at cone 6ox. Chrome and rutile have not melted, copper and cobalt are extremely active melters. Cobalt and copper have crystallized during cooling, manganese has formed an iridescent glass.
Rheology
Rheology refers to the array of characteristics that a ceramic slurry exhibits, its flow, thixotropy, etc. Technicians seek to understand and control the dynamics of the slurries they use to maintain consistency and optimize them for the product and process at hand. This is done by the control of water consistency and quality, selection of materials, temperature, mixing methods, water content (and therefore specific gravity), and the addition of electrolytes. For example, when it is desirable to have a low water content in slurries (because of the energy needed to remove water and performance and ease-of-working considerations) deflocculation is employed. If prevention of sedimentation or movement of slurries when applied to ware is important, flocculants are used. When thixotropic characteristics are needed, specific electrolyte blends are often employed.
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- (Materials)
Claytone - Bentonite-based rheological additive
- (Materials)
Garamite - Rheological additive
- (Glossary)
Casting, Slip Casting
Forming pottery by pouring deflocculated (water re... - (Glossary)
Water
There is a need to discuss water in ceramic produc... - (Project)
Ceramic Minerals Overview
The materials we use are powders and we assess the... - (Project)
Ceramic Properties
A property in this context is a created physical p...
Rutile Glaze
Normally a fluid glaze containing 3-5% rutile will run in rivulet patterns and seed the development of some crystallization during cooling. Common from cone 6 to 10 in oxidation or reduction. These glazes can often be problematic in some circumstances, especially with regard to blistering and crawling, whereas in others they perform flawlessly.
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Pictures
Example of variegation and phase separation with about 5% rutile added to a dolomite matte cone 10R glaze.
2,3,4,5% rutile added to a 80:20 mix of Alberta Slip and Frit 3134 at cone 6
Fluid rutile glaze
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