If a glaze slurry contains soluble or partially soluble raw materials or is made using hard water, then solids can precipitate over time forming hard lumps, crystals or a scum. At a minimum the solubility of many materials is enough to stain the water in a slurry (seen on the top after the powder has settled).
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).
A glaze slurry precipitates at flakes
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.
Are frits partially soluble? Yes, many are.
These 1 mm-sized crystals were found precipitated in a couple of gallons of glaze containing 85% Ferro Frit 3195. They are hard and insoluble. Why and how to do they form? Many frits are slightly partially soluble and the degree to which they are are related to the length of time the glaze is in storage, the temperature, the electrolytes and solubles in the water and interactions with other material particles present. The solute then interacts with other materials particles to form insoluble species that crystallize and precipitate out as you see here. These crystals can be a wide range of shapes and size and come from leaded and unleaded frits.
Precipitate can forms in firtted glazes, remember to screen it
Potters often store glazes for long periods so tiny spherical precipitate particles can form. These were found in a months-old bucket of G2926B (M370 clear) cone 6 clear glaze (about 2 gallons). These can appear over time, depending on factors like temperature, electrolytes in your water or solubility in the materials (likely, the frit is 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 screen. Note the brush, using one of these gets the glaze through the screen much quicker than using a rubber spatula.
Add 5% caclium carbonate to a tenmoku. What happens?
In the glaze on the left (90% Ravenscrag Slip and 10% iron oxide) the iron is saturating the melt crystallizing out during cooling. GR10-K1, on the right, is the same glaze but with 5% added calcium carbonate. This addition is enough to keep most of the iron in solution through cooling, so it contributes to the super-gloss deep tenmoku effect instead of precipitating out.
Precipitated crystals from a glaze having 60% lead bisilicate frit
It also contains less than 10% borax frit and some Cornwall stone.
Here is what happens if you do not sieve your glazes when needed
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.
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