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The glaze defects are caused by precipitates that have formed in this glaze slurry within days of batching it. They are refractory and do not dissolve in the glaze melt - creating a defect that is unrepairable. In industry, glazes are batched and sieved as an ongoing process but in pottery and hobby ceramics they are stored for months or even years after batching. It is normal to have to sieve these slurries every few months but in recent years the precipitates form more quickly. Frits are theoretically insoluble, but in practice, they are not. Frit quality is determined not just by careful control of the chemistry but also of the smelting, mixing and water-quenching processes.
These flakes have been screened from a highly-fritted boron glaze mixed using hard water and stored for a year. They formed as a film across the top of the settled surface and on the walls of the bucket. Frits are stoichiometric, they should not dissollve, but they do. Perhaps this is evidence that frit makers are unable to precisely control production parameters.
These crystals were found in a months-old bucket (about 2 gallons). These can appear even quicker, depending on factors like temperature, electrolytes in your water or solubility in the materials (evens frits can be slightly soluble). The glaze slurry should be screened periodically (or immediately if you note the particles when glazing a piece). This is an 80 mesh sieve. Note the brush, using one of these gets the glaze through the screen much quicker than using a rubber spatula. The loss of material on the screen is tiny and inconsequential to the glaze. But it is crucial because these particles do not melt at cone 6, they will certainly mar the fired glaze surface if undetected.
This is a cone 6 transparent base glaze. It contains frit, silica, kaolin, wollastonite. Almost all glazes have materials that are slightly soluble and over time these can form scale on the sides of the bucket or even precipitate particles into the slurry. The defects here are those scales. Before dipping a production piece in any glaze that has been in storage it is a good idea to assess it first to see if it needs to be sieved.
Glossary |
Frit
Frits are used in ceramic glazes for a wide range of reasons. They are man-made glass powders of controlled chemistry with many advantages over raw materials. |
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