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| H440 - Dark red-brown speckled iron reduction stoneware | |
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M350 - Tan smooth middle stoneware |
| H443 - Brown speckled iron reduction stoneware | |
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M390 - Red smooth middle stoneware |
| H440G - Dark red-brown iron reduction speckled sculpture body | |
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H441G - Buff-white grogged sculpture body |
This is a separate list of bodies selected from other categories. These bodies are made entirely from Plainsman balanced native clays and perhaps more than any other, they showcase unique advantages of the Plainsman suite of materials. Our iron brown bodies fire to a very unique surface and offer great workability and consistency compared with refined mineral alternatives.
These bodies deserve special firing consideration because of the response of iron oxide to reduction atmospheres. Fe2O3, the form of iron in the raw clay, is not a flux, it does not melt readily. Bisque firings in gas or electric must be slow enough and have enough air movement to supply oxygen to processes of decomposition and take away their byproducts, especially during the 700-900C range. If not, carbon in the body will take oxygen from the iron and convert it to FeO (thus 'reducing' it). Then, during the final firing the FeO inside the body, a powerful flux, will begin melting as low as 900C. It will break down the clay, giving off gases that may blister the body or glaze, it will alter the thermal expansion of the fired matrix, and it may cause the ware to warp and become more brittle. So remember to bisque these clays properly, that usually means slowly.
High iron stoneware bodies can greatly affect the color of overlying glazes. Generally they will subdue bright colors and darken the color in areas where the glaze layer is thinner. Many glazes just do not have a pleasant appearance over dark bodies, while others are enhanced. Red bodies work well where glazes have colors that vary with thickness and are applied over incised decoration in the body. They are also ideal when bare clay patches are used to add visual character.
Iron bodies are not as strong as buff or white firing materials. In the middle fire range body maturity (degree to which it vitrifies) must be cut to produce warm red or brown coloration. The result is a more porous and weaker fired body. In high temperature reduction this is even more true. Reduced iron melts vigorously and turns the clay dark brown. However the iron is usually present in relatively large particles (compared to the clay), and the color-change radiates from each particle of iron. The trick is to stop the firing midway through this process before the darker brown color has invaded the entire surface. The variegated 'patching' of light and dark areas is the result. Such bodies are therefore non-homogeneous by nature and thus more porous and weaker. In addition, iron bodies are more prone to bloating if overfired. Thus there is incentive, in fact necessity, to fire them to a semi-vitreous state.
Iron particles in the clay convert to FeO (from Fe2O3) and this new form is a very active melter. When the iron is present as relatively large particles the result is the characteristic reduction 'speckle'. Each of our stoneware raw materials has a typical amount and size of iron speck. We control the amount of speckle in each body by blending appropriately. However there is some variation in our grinding process and the amount and size of iron particles in the raw materials. We test for this and adjust if necessary but you will see some variation, especially in the more speckled materials. We have greater control over some bodies than others (for example, H550 is more stable than H435 or H431, H475 is easier for us to control than H443). Any changes in your firing will introduce further variations. Thus, if you make functional ware where color and speckle matching are important, you should carefully appraise which body you use.
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Plainsman Clays Ltd. 702 Wood Street, Medicine Hat, Alberta T1A 1E9 Canada |
Phone: 403-527-8535 FAX: 403-527-7508 |