Question: When you use Epsom salts to flocculate a glaze, how long does that effect last? How often do you have to add more Epsom salts?

Using Epsom salts to flocculate glazes

Epsom salts are magnesium sulphate. When they’re dissolved in water, the magnesium breaks free from the sulphate. The magnesium ions attach to the clay particles, which have a flocculating effect, increasing a glaze’s viscosity. Generally you don’t have to replenish the Epsom salts unless you’re adding more water or more glaze materials. The flocculation should hold.

I worked in a public studio where we had 20 large garbage can size buckets of glaze. We would flocculate certain glazes at the time of mixing the glaze from dry materials. Every month we would sieve the glazes, measure specific gravity and add water if needed. We didn’t really need to re-flocculate unless the glaze was thrown out of whack somehow.

Insufficient mixing could affect flocculation

For example, say people aren’t mixing the glazes very well. If you don’t mix a glaze well, each time you dip a piece you’re removing a disproportionate amount of water which means essentially you could be removing a disproportionate amount of Epsom salts out of the bucket. And then we’re replenishing the water with fresh water that contains no Epsom salts. So that is one scenario where a glaze may need to be re-flocculated. But generally, if nothing like that happens to the glaze bucket, then the glaze will stay flocculated.

Soluble materials could affect flocculation

Another thing that can happen is, say you mix a glaze from scratch and flocculate it right away. In the first couple of days some of your glaze materials might start to dissolve into the water and have a deflocculating effect. This can cause you to need to flocculate again. 

After a few days, anything that was going to dissolve has likely dissolved and after that point, the glaze should theoretically just stay as it is. So the flocculation shouldn’t change over time. The specific gravity is more likely to change over time as we’re using the glaze and water is being removed, or water can evaporate, but generally we’re not having to continually flocculate the glaze.

We just flocculate it, and that’s it until we are either adding more water to the glaze or we’re adding more glaze to the bucket, like mixing a new batch and adding it to the bucket.

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