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Mold in Clay?
The following is reprinted with permission from the Questions Department of the January 2003 issue of "Ceramics Monthly", page 118. Copyright 2003 by The American Ceramic Society. All Rights Reserved. Visit "Ceramics Monthly" at

Q: I am an elementary art teacher. I need to know what can prevent mold from growing on the clay. I have a bucket lined with two bags, and it is always covered. I have heard Epsom salt is a better alternative than vinegar, but would like some advice. –K.M.
A: I have been a potter for over 40 years now and I always welcome that smell when I open a box of clay. It means the clay has "aged" – enough time has passed between the mixing of the clay and its use for bacteria and mold to develop. This is a good thing, because mold and bacteria contribute to the workability of the clay.

The way these by-products of the aging process contribute to workability is by increasing the flocculation of the clay body. This means that the clay particles will stay together better. Handles and other attachments will stick on better and the clay has less of a tendency to crack. The clay’s strength, plasticity, and resistance to stress are improved. Some potters even add some of their aged clay to freshly mixed batches to get the new clay to produce bacteria and mold faster than it would if left on its own.

My answer to your question is: Welcome that smell! Whenever you smell it, it means that your job as a potter has just become easier.

Some clay makers will occasionally add vinegar to clay to improve its workability. However, if clay that has had vinegar added is not used within a few weeks, then the smell can get to be too much. I don’t recommend it at all, because if you are trying to get rid of the smell, this will make it worse in the long run. Epsom salts will not cause the strong odor associated with vinegar, but will have the beneficial effect of helping to flocculate the clay body. Keep in mind that clay may contain bacteria anyway, and Epsom salts will not prevent their growth.

There are some individuals who are allergic to molds and mildew. They should not be around clay to start with. If aged clay is making anyone sick, and for some reason they have to be there, I suggest using bleach to cut down the bacterial and mold activity. This is not the best solution, because chlorine has its own drawbacks. Perhaps a few drops of diluted bleach in the bucket now and then might be a solution, but it might cause reactions more than the mold or bacteria. I certainly would not use bleach unless I absolutely had to.

In the end, my suggestion would be to get used to the moldy smell. Or, do as most potters do – welcome it!

Ron Roy
Ceramics Consultant
Brighton, Ontario, Canada
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