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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.
It also contains less than 10% borax frit and some Cornwall stone.
Wrong. These tiny spheres (actually they are not so tiny) form over time as a precipitate in a glaze having a high concentration of a boron frit and mixed in hard water. This may be an example of how interactions can affect the degree to which materials dissolve in water (in this case the electrolyte in the water could be a trigger). These are likely ooids.
Crystals or crystalline particles will often form over time in ceramic glaze slurries that contain slightly soluble materials.