Spring is here and lots of things are growing, including mold. Questions about mold in metal clay come up regularly this time of year.  Here is probably more than you ever wanted to know about mold in metal clay.

 The health issues related to mold revolve around breathing the mold spores and the mycotoxins they produce. Unless you have allergies, asthma, or other respiratory problems, minor exposure is usually just irritating. Mold is everywhere, so I don’t want to be an alarmist, but some molds are toxic and you do want to take some care (links to info on toxic mold are at the end of this missive). Wear a dust mask when you start working with the moldy clay–if you are “smelling” the musty mold, you are breathing mold spores. If you are sensitive, wear a HEPA grade dust mask. Be sure to wash hands and use a good air filter or positive ventilation to keep from breathing mold dust.

Personally, I am very sensitive to mold and dust and try to open up and do the initial working of moldy clay outside to keep from adding mold spores into my studio. If there is a breeze, I may not bother with a dust mask. I usually scrape off and dispose of the worst of the mold for aesthetic reasons and then knead the clay. Once it is thoroughly mixed in, the chances of getting mold spores in the air is a lot lower, and I take it back into my workshop and try to use it up quickly (within a few days).

Keeping the clay tightly wrapped so no air gets to it will help limit the mold growth as well. White vinegar can be added to kill the mold. Simply add a few drops of white vinegar to the clay and let it sit overnight; small spots of mold will often completely disappear.  Larger incursions may still be visible, but can just be mixed into the clay.

Lavender essential oil also seems to retard mold growth (it may kill it as well).  I haven’t had a problem with mold since I started using the lavender oil.  I add it to all my slip because it makes the slip stick better to both fired and unfired silver clay.

After working with moldy clay, you will want to clean your tools and work surfaces to remove mold spores. Bleach or strong white vinegar can be used to sterilize tools and work surfaces. If you have a HEPA air filter, you should run it in your working area before, during, and after in order to remove as many of the spoors from the environment as possible. These steps may help prevent future mold outbreaks.

The absolute best way to prevent moldy clay is to not let wet clay sit around for long periods of time after it has been opened (factory sealed packages are usually sterile). So use that clay up! Alternately, if you have leftover clay that is pretty dry and do not intend to use it for some time, let it dry out completely and store that way. Dry clay, will not grow mold, but does give you the challenge of reconstituting it–break it up into small pieces or grind it in a silver clay dedicated coffee mill, add (distilled) water, mix, cover tightly, and allow to sit for one or more days to allow the moisture to fully penetrate the binder. After that, it is a matter of kneading (in plastic), and adjusting the moisture content to the desired consistency. Glycerin can be added to make the clay more workable.  Adding a few drops of lavender oil or white vinegar can give you some mold insurance.  Pick your smell!  Other essential oils (such as citrus) will probably do the trick, but I haven’t tested them.

You can also sterilize moldy clay by heating it. This means that you are going to dry out the clay and will have to reconstitute it. To sterilize the clay using heat, you will need to heat it over 130F (56C) for a minimum of 30 minutes. Heat for longer to ensure the entire piece has been sterilized. I usually use my oven or toaster oven to do this at its lowest setting–about 200-250F. A good dehydrator will allow you to adjust the heat to specific temperatures as well and will give good positive airflow. Don’t heat much hotter than this or the binder will begin to darken and burn. To read more about heat sterilization, check out: http://www.ces.ncsu.edu/nreos/wood/wpn/pallets.html.. This is written for heating and sterilizing wood, but will give you a lot of information on sterilization, heating and drying that can be applied to metal clays (or at least their binders). Apparently this is a big issue in the wood pallet industry! Who knew?

Finally, keep in mind that the mold is using the binder in your clay as a food source. If you let it keep growing on your clay, it is degrading the binder. This is probably not an issue unless you let it grow for a *really* long time though! 😉  I had a tub full of moldy clay that has been sitting in my studio for a couple of years and it was still ok to work.  Keep in mind that the binder in the metal clay does start to break down after 6+ years, so any *really* old clay you have should be used as soon as possible.  Don’t let it sit around too long.

If you know that your working environment is contaminated with toxic mold, you need to take appropriate precautions. A quick Google search found the following sources on toxic mold:
http://homebuying.about.com/cs/mold/a/toxic_mold.htm
http://www.mold-help.org
http://www.homehealthscience.com/toxicmold_inhouse.asp
http://www.toxic-black-mold-info.com/
http://www.inspect-ny.com/sickhouse.htm

I originally wrote this post in February of 2008 for the Metal Clay Yahoo group and have updated it.  It was written with silver clay in mind, but the advice should transfer to the other metal clays as well.  If anyone has additional knowledge regarding mold in the base metal clays, please let me know and I can include it here.  Bronze and copper clays are probably resistant to mold as copper tends to be toxic to growing things—it used to be used in bottom paint for boats to prevent stuff from growing on them, but has been banned because it sheds toxic residue wherever boats are regularly stored.  The replacement paints are not nearly as effective at retarding marine growth.

Send me your questions regarding this and other issues and I will research and answer them.

Mary Ellin D’Agostino 

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