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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).

It seems logical that ceramic powdered materials, being ground up rock or glass (in the case of frits) would not be soluble. But this is not the case. Solubility is often a matter of chemistry. Certain frit chemistries, for example, are much more vulnerable to dissolution. This is because frits often need to push the boundaries of stability (therefore solubility) to deliver the desired chemistry (e.g. those lacking alumina). Solubility is often a simple product of increased surface area. For example, the solubility of feldspar or nepheline syenite rock is very low, but when ground into a powder its surface area is multiplied thousands or millions of times. This is enough to create noticeable solubility. Other materials are known to be contaminated by soluble salts. Gerstley borate, for example, gels glaze slurries, so obviously it is releasing ions into the suspension (its constituent minerals are also less stable than most other common ceramic materials). Ball clays likewise release scum as they dry (solubles are sourced from contaminants that get ground up with the clay itself, these are carried to the surface and precipitate there as brown scum as the water evaporates away). Manufactured materials like barium carbonate and lithium carbonate are partially soluble (this is just what they are).

Precipitation is not just a product of over saturation that occurs over time. It can also be a product of the interaction of various ions to be found in solution (which may well want to interact). Often it may appear that a particular material is guilty of precipitating in a specific glaze, however that same material may appear in another recipe where no precipitation occurs.

Typically the best way to deal with this issue is to simply sieve the glaze and remove the crystals.

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.

Out Bound Links

In Bound Links

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By Tony Hansen

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