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Glaze Sinking to Bottom or Cracking as Drying on a Pot

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Here are some thoughts about the settling problem with your glaze:

Epsom salts can be added to the glaze to keep the materials in suspension and slow down the settling process. You can add 0.1% to 0.5% of the dry weight of the glaze (10 to 50 grams for every 10,000 gram batch). Do not add the Epsom salts directly to the glaze. First, dissolve them in hot water (about 1.5 times the amount of Epsom salts). Then, pour them into the glaze.

CMC powder can also be used to flocculate a glaze. It is also soluble and should be dissolved in water before you add it. You could try adding from 0.1% up to 3%. CMC powder will also make the glaze have a harder surface as it dries on a pot.

Many glazes that contain a high percentage of nepheline syenite have a tendency to settle into a hard brick at the bottom of the glaze bucket. As a last resort, the glaze could be recalculated with a lower percentage of nepheline syenite.

High percentages of nepheline syenite can also cause the cracking problem as the glaze dries on a pot

Other helpful notes:
1) 5 gallons of glaze is between 10,000 - 14,000 grams of dry glaze depending on the glaze
2) Do a small test first to verify the ratio and effectiveness
3) Epsom salt is organic and after a number of days or weeks will start to discompose, loose it's effectiveness and make the glaze smell terrible, thus, the Epsom solution is for rapidly used glaze that is not stored for more than a couple of weeks.

Here are some thoughts about unfired glaze cracks:

When a glaze cracks as it dries on a pot, it usually means that the glaze is shrinking too much. This is normally caused by having too much plastic material (ball clay) in the glaze. If this is the problem, it should exist from the beginning (not appear two months later). Still, you could try the following things.

1-Remove a small amount of ball clay and add kaolin.

2-Remove bentonite. Bentonite is extremely plastic and has a very high shrinkage rate that could cause the glaze to crack as it dries.

3-You can add about 0.5% CMC gum. It should be added while the glaze is still a powder. The addition of CMC gum will harden the unfired glaze. CMC will also cause the glaze to dry much slower than normal and reducing the strain on the glaze as it dries.

If starts to crack a few months after being mixed. Here are a few theories.

Bisqued pots may be contaminating the glaze. Bisque powder, dust, salts, and who knows what else could be collecting on the pots. You could try rinsing the bisqued pots a few hours before they are glazed.

Water may be evaporating out of the glaze, making it too thick.

When glazes are mixed frequently with drill mixers, the particles can actually break down (similar to the effect of ball milling). This makes the particles smaller and more plastic. You could try adding small amounts of kaolin to the glaze and see what happens.


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