It's true; people can have mold reactions to clay that has mold in it. We've heard about several potters that have had allergic reactions to mold in clay. Usually for it to kick up, though, there has to be visible mold—such as you get when a bag of moist clay has been hanging around for a fair amount of time.
When the mold begins to grow in the bag or bucket (causing discolorations), you can spray the outer surface of the clay with bleach-water (1/4 cup per gallon of water up to 1 part bleach to 9 parts water) to kill the surface mold. Usually, the mold won't have grown down into the clay too much. (Although given time, it will.)
Cutting Down on Mold Growth
We also recommend cleaning reclamation buckets with bleach-water once all the clay has been removed from them. This cuts way down on mold growth in recycled clay scraps.
If you are mixing your own clay, you can use bleach-water to clean the interior of the mixer. You can also cut down on the mold by adding a small amount of bleach to the water used to wet the dry clay. (The chlorine volatilizes out and will not effect forming or firing results.) The same goes for reclaiming dried clay scraps.
Don't Want to Use Bleach?
If you are not keen on the idea of using bleach, as it is too harsh and abrasive, some potters have been known to add a few drops of vinegar to any clay that has become moldy. This, of course, can make the clay smell, but you won't need to add too much. Another method is using Epsom salts, these attract clay platelets in the moist clay state and causes the clay to become flocculated. The Epsom salts should be diluted before use to prevent the clay from blistering.
Common mold symptoms include respiratory distress (coughing, sneezing, shortness of breath, asthma, etc.), nose and throat irritation, nasal or sinus congestion, runny nose, eye distress (itchiness, redness, burning, watery), headaches, sensitivity to light, and skin rashes, redness, or irritations.
Why Does Mold Grow on Clay?
It is incredibly common for mold to grow on clay as it is a natural material. Different types of mold can grow on different types of clay, and many different variants will determine what type of mold grows. For example, heat and moisture are two big factors for mold growth, alongside the length of time the clay is left there. Two of the most common types of mold that can be found are green mold, which can be seen as a good thing as it can add moisture (and plasticity) to the clay, and black mold, which is more tricky as it has a darker colorant. Porcelain often attracts these dark molds and should be thoroughly wedged before using. The mold often burns off in the kiln, so you should still be left with a pristine white. Often clay that has been left for a very long time will have developed some mold on it.
Preventing Mold on Clay
One of the best ways to prevent mold developing on your clay is to use up your clay quickly and do not leave wet clay for long periods. However, if you do have lots of clay that you haven't used, you can let the clay scraps dry out completely and then when the time comes that you are ready to use it you can reconstitute it with water.