Ceramic materials, especially clays, often contain carbon and organic compounds. When they are fired in a kiln, these must burn out, often producing complications.
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Ceramic bodies and glazes contain materials that release carbon as they decompose on heating. Clays, gums, plasticizers are examples. These can still be burning out at higher temperatures than most people realize (cone 04 or higher). That burning generates gases, if glazes are beginning to melt before burnout is finished, those gases cause glaze imperfections and micro-bubbles (or tiny dimples that mar an otherwise glassy smooth surface). Carbon is also produced as carbonates decompose. Each carbonate has its own decomposition temperature range.
These are two 10-gram balls (formed by dewatering the glaze on plaster) of low temperature glazes (G1916J, G1916Q) containing only frit and kaolin fired to 1250F. The carbon is part of the LOI of the kaolin (that hardens and suspends the glaze). Yet these glazes have much lower carbon content than ones made from raw materials.
A broken test bar of ball clay fired to cone 10 reduction. Notice the black carbon core. Ball clays commonly contain carbon, many have a noticeable grey color in the raw state because of this. Notice it has not burned out despite the fact that the clay itself is still fairly porous, the firing was slow and the temperature reached was high. Ball clay typically does not comprise more than 30% of a body recipe so its opportunity to burn away is sufficient. However some specialized bodies have a much higher percentage.
Example of the lignite particles in a fireclay (Pine Lake) that have been exposed on the rim of a vessel after sponging. This is a coarse clay, but if it were incorporated into a recipe of a stoneware, glaze pinholing would be likey.
Example of the oversize particles from a 100 gram wet sieve analysis test of a powdered sample of a porcelain body made from North American refined materials. Although these materials are sold as 200 mesh, that designation does not mean that there are no particles coarser than 200 mesh. Here there are significant numbers of particles on the 100 and even 70 mesh screens. These contain some darker particles that could produce fired specks (if they are iron and not lignite); that goodness in this case they do not. Oversize particle is a fact of life in bodies made from refined materials and used by potters and hobbyists. Industrial manufacturers (e.g. tile, tableware, sanitaryware) commonly process the materials further, slurrying them and screening or ball milling; this is done to guarantee defect-free glazed surfaces.
These contaminating particles are exposed on the rim of a bisque-fired mug. The liqnite ones have burned away but the iron particle is still there (and will produce a speck in the glaze). Remnants of the lignite remain inside the matrix and can pinhole glazes. Since ball clays are air floated (a stream of air takes away the lighter particles and the heavier ones recycle for regrinding) it seems that contamination like this would be impossible. But the equipment requires vigilance for correct operation, especially when there is pressure to maximize production. Ores in Tennessee are higher in coal than those in Kentucky. North American clay body manufacturers who confront ball clay suppliers with this contamination find that ceramic applications have become a very small part of the total ball clay market - complaints are not taken with the same seriousness as in the past.
More carbon needs to burn out than you might think!
What if you just cannot solve a pinhole problem?
Loss on Ignition is a number that appears on the data sheets of ceramic materials. It refers to the amount of weight the material loses as it decomposes to release water vapor and various gases during firing.
Traditional Japanese high feldspar glazes having cream to orange color flashing or blushing. Potters today seek to emulate the Shino appearance using a wide range of recipes.
|Copper carbonate basic decomposes (300-330)
|Copper carbonate basic breakdown (1050-)
|Copper Carbonate decomposes to CuO (290-)
|Calcium carbonate decomposition (750-1000)
|Manganese Carbonate decomposes to MnO (200-)
|Decomposition of Barium Carbonate (1025-)
|Calcium carbonate, talc finished gassing (815-)
|By Tony Hansen
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