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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 1 mm-sized crystals were found precipitated in a couple of gallons of glaze containing 85% Ferro Frit 3195. They are cubical, hard and insoluble. Why and how to do they form? Many frits are slightly soluble, the degree to which they are is related to the length of time the glaze is in storage, the temperature, the electrolytes and solubles in the water, interactions with other material particles present and the diligence of the manufacturer in mixing, correctly achieving the target chemistry and firing. The solutes interact or saturate to form insoluble species that crystallize and precipitate out as you see here. These crystals can be a wide range of shapes and sizes and come from leaded and unleaded frits. In industry this issue is not generally a problem because glazes are used soon after being made.
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.
Glossary |
Precipitation
Crystals or crystalline particles will often form over time in ceramic glaze slurries that contain slightly soluble materials. |
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