A host of water soluble materials are available to source most of the important oxides needed in ceramic glazes. However such materials cannot normally be used in glazes. Why? Because glazes are suspensions of particulate materials, not solutions of soluble materials. Such suspensions have a far lower water content than a solution could ever have, this is necessary to create a slurry that will deposit an adequate thickness when applied to ware. Because most ceramic powders do not react much with water, a clay or glaze slurries can be stored and the rheological properties do not change.
Clay, feldspar, wollastonite, silica and frits are insoluble. Right?
Wrong! That is what the glaze was made of that was in this bucket. The scum on the inside is so hard that it is extremely difficult to remove, even using a scraper or a scrubber. Even lime-a-way does not remove it all. This is an example of how water-soluble materials can be. When this glaze settles out the water on top is brown (like this scum) yet all the material powders are white! So it is not surprising that glaze viscosity changes over time and things dissolve and impact rheology.
Frits do not dissolve in water, right? Wrong.
This is an example of two types of crystals that have formed on the surface of a fritted glaze after a long period of storage (Ferro Frit 3249 in this case). Frits are formulated to give chemistries that natural materials cannot supply. To do that they have to push the boundaries of stability (solubility). Any frit that has an inordinately high amount (compared to natural sources) of a specific oxide (in this case MgO) or lacks Al2O3 (like Frit 3134) are suspect.
Soluble ingredients in glazes always precipitate as angular crystals. Right?
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).
Should you throw out the brown water on top of settled glazes?
This is water from the top of a glaze that had been sitting for more than a year. Clearly, the solute contains iron. It is being dissolved out of one or more of the white powders making in the glaze recipe, so the iron at least is a contaminant. This should be thrown out and replaced with clean water. Why? We do not want anything dissolved in glaze slurries. It either migrates into the body with the water it absorbs during glazing or it migrates to the surface as the water evaporates. Both are bad. How much dissolved material would be lost? It would be measured in tenths or hundreds of a gram. Hypothetically then, if a bucket contains 1000 grams of the material, one ten-thousandth of it would be lost!
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