A glaze defect where tiny holes are present in the fired glaze surface. These holes normally go down to the body surface below. Pinholing is a plague in industry, the tiniest hole in the glaze surface of a tile or utilitarian item can make it a reject. Industry goes to great pains to get materials of very fine particle size for their bodies and glazes to reduce the occurrence of glaze defects. Glazes which melt and flow well often still have pinholes if gas producing particles are present in the body (these expel gases up through the glaze melt thereby disturbing its surface). Thicker layers of glaze pinhole more than thinner ones. Blisters, dimples and pinholes often occur together.
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.
Let me count the reasons this glossy white cone 6 glaze is pinholing
First, the layer is very thick. Second, the body was only bisque fired to cone 06 and it is a raw brown burning stoneware with lots of coarser particles that generate gases as they are heated. Third, the glaze contains zircopax, it stiffens the melt and makes it less able to heal disruptions in the surface. Fourth, the glaze is high in B2O3, so it starts melting early (around 1450F) and seals the surface so the gases must bubble up through. Fifth, the firing was soaked at the end rather than dropping the temperature a little first (e.g. 100F) and soaking there instead.
How can you best tell if a base glaze is prone to pinholing with your body?
Testing for pinholes and dimples is often best done using a transparent glaze over a large surface and looking at the surface in the light. In this case, an open bowl is used. Heavy pieces like this are difficult to fire evenly and encourage under fired areas where pinholes are more likely to appear.
Pinholes often happen on trimmed surfaces
Pinholing is often related to the smoothness of the underlying body surface. The lower half of this vase was tooled during the leather hard stage, all the pinholes occur there. Even though this glaze contains 10% grog, the pinholes are not appearing on the upper half because the slip generated during throwing has left a smooth surface.
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.
Pinholing with a boron fluxed cone 04 opacified and stained glaze
This problem was suffered by a potter moving from Europe to Canada. In Europe she used lead based glazes and got smooth defect free surfaces. But in Canada she had to use our boron based (from Frits and Gerstley Borate) glazes and had many problems adapting to them.
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