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.
What is that black stuff on these two glazes?
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.
Carbon burnout in a ball clay
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.
More carbon needs to burn out than you might think!
Hard to believe, but this carbon is on ten-gram balls of low fire glazes having 85% frit. Yes, this is an extreme test because glazes are applied in thin layers, but glazes sit atop bodies much higher in carbon bearing materials. And the carbon is sticking around at temperatures much higher than it is supposed to (not yet burned away at 1500F)! The lower row is G1916J, the upper is G1916Q. These balls were fired to determine the point at which the glazes densify enough that they will not pass gases being burned from the body below (around 1450F). Our firings of these glazes now soak at 1400F (on the way up). Not surpisingly, industrial manufacturers seek low carbon content materials.
What if you just cannot solve a pinhole problem?
Pinholing on the inside of a cone 6 whiteware bowl. This is glaze G2926B. The cause is likely a combination of thick glaze layer and gas-producing particles in the body. Bodies containing ball clays and bentonites often have particles in the +150 and even +100 mesh sizes. The presence of such particles is often sporadic, thus it is possible to produce defect-free ware for a time. But at some point problems will be encountered. Companies in production either have to filter press or wet process these bodies to remove the particles. Or, they need to switch to more expensive bodies containing only kaolins and highly processed plasticizers.
Lignite can be big trouble
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.
Oversize particles in a typical manufactured porcelain body
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.
Lignite contamination in manufactured porcelain bodies
These particles 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.
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