Article


by Mary Ellin D’Agostino

Ah, a brand new package of metal clay! There are no bumps, lumps, bits of toothpick, cat hairs, or other detritus in it. The moisture is just right and I can make it do what I want it to do. It is always so tempting to open up a new package of clay for my projects. But what about all those abandoned un-fired projects? The lump of too-dry clay in the package that didn’t get quite closed? The trimmings from the last several projects? And what about the clay that got too much badger balm on it? I gave up on that bit a while ago and it is still waiting for me to get back to it. Not to mention the earlier attempt at re-hydrating old clay that I got too much water in.

I already have scads of paste and don’t have any paste-only projects planned.

With the price of silver so high, it is time to get back to re-constituting that old clay. Even the bronze and copper clay scraps have piled up and it is hard to justify opening a new package when I have so very much clay already opened.

No matter what type of metal clay you have, being able to get it back to a good working consistency is a great skill to have and there are many approaches to the project.

Rehydrating Dried Clay
The basics are to grind the clay into a fine powder and slowly add distilled water while mixing or kneading until you get it to the right consistency. Personally, I use a mortar and pestle to break up big lumps, then use a coffee grinder to turn it into powder, sift through a bit of brass screen, then spritz with water and mix. Too much water and you will need to either add more powdered clay or leave the mess open so the excess water will evaporate. Whenever I have this problem, I usually forget it, and come back after it has become too dry! LOL.

There are good instructions on how to reconstitute completely dried clay at

Once you have it in powder form, Hadar Jacobson’s directions for mixing metal clay are fantastic as well.

Maggie Bergman’s method is great for rehydrating whole pieces that you abandoned by putting it between layers of moistened sponge until it re-absorbs the water. Try one or more methods and see which works better for you.

Once the clay has enough (or almost enough) moisture, use the methods described below.

Too Dry to Work
Sometimes clay is too dry to work with—too stiff or crumbly—but not completely dry. My favorite way to deal with this problem (or to continue the rehydration process for dried clay) is to place the offending clay on a cut up plastic bag (this is thicker and sturdier than most plastic wraps), add a spritz or drop of water (sparingly!), fold the plastic over the clay and roll until it is flattened. Open the plastic, fold up the clay, and repeat until the clay seems to be consistent and all lumps are crushed into submission. You can add a little water as you go if it seems necessary, but use less than you think you need.

Finally, and I have never found a substitute for this step, roll and knead the clay in your (lightly greased) hands until it is the desired consistency. You will quickly find out if it is still too dry or too wet. If it is too wet, use one of your tools to scrape the mess off your hands and put back onto the plastic. Then, either add some powdered clay to the mess and go back to working in the plastic or leave it exposed to the air so it dries a bit and try again later.

Crumbly or Hard to Work
If you overwork even new clay it can become cracky and difficult to make it do what you want. Sometimes even when it has the right amount of moisture, the clay just doesn’t work right; it is overly cracky and you can’t make a coil or piece you can manipulate because it just breaks. The easy, but inelegant solution to this is to make a form that doesn’t require a lot of manipulation or finishing. A better solution is to work a few drops of glycerin into the clay. A drop or two of lavender or citrus essential oil is beneficial and will keep mold and mildew from growing on the clay.

It may help to let the clay rest and the water and/or new ingredients to meld into the binders overnight. While “resting” the binders fully absorb the water, glycerin, and essential oil and are re-activated so they are effective again.

Too Much Oil or Balm
Overworked clay often has too much oil or balm on it from your heavily greased tools or hands. This will give you the crumbly, hard to work, cracky problem. Often, if you just spritz a bit of water onto it, wrap in plastic, store and let sit overnight, the clay will be workable. The next thing to try is to add a bit of glycerin. If letting the clay rest and the addition of glycerin doesn’t work, your next best bet is to add more clay to the lump to dilute the grease. This can be fresh clay or reconstituted clay. Mix them together then knead/fold/roll in plastic until it becomes workable again. Letting it rest after mixing is a good idea.

Contamination
But what about the fibers, hair, and other bits of stuff that get in the clay? If you are grinding and sifting the clay, you can remove many of these bits of fluff and stuff during the process. Little dust bunnies form and can be picked out of the clay powder with a tweezers.

Another approach that is especially good if your slip has too many fibers or contaminants in it is to add water until the clay/paste is thin enough to force through a piece of gauze (the kind of gauzy nylon used for sheer window curtains) into a clean container. I usually stretch the fabric over a jar and hold in place with a rubber band. Then I use a pallet knife to force the paste through the mesh. All the lumps and contaminants stay behind and the good paste ends up in the jar. You can either use this as paste or let it dry back to clay consistency. As for the contaminated mess left on the cloth, you can scrape it off and use it for texture, heat bits of it with a torch until they melt to create your own casting grain, or just add it to your recycle bin to be sent to the refinery.

I have found that these steps will revitalize all my problem clays. I hope this helps. Let me know if I have left anything out or if you have a different approach to solving these problems.

by Mary Ellin D’Agostino

While it is clear that we almost always need a bit of slippery stuff when working with metal clays, I am often asked questions about what type, how much, where, and when to use it. Feel free to chime in if I leave anything out!

Why Oil?
Why do we need to grease up? Metal clay can be sticky. It can cling to your hands, tools, textures, and work surfaces. Some days it behaves politely and doesn’t seem to stick much; other days it gets all over everything. Some brands or formulations of clay are stickier than others. Reconstituting clay is a particularly messy and gummy process.

Very often, the answer is to grease up. But not too much. Too much oil will make it hard for you to make strong connections between pieces of clay or to get the lines out of that clay you just wadded up and are re-rolling for another try. What you really want is a light coating of grease.

The type you use is not very important, but there are a few do’s and don’ts. Do use natural or organic oils or balms. These can be plain olive or other cooking oils, spray cooking oils, hand-lip-body balms, or “official” metal clay “release agents.”

Don’t use petroleum based products—anything that includes petrolatum or similar ingredients because these can degrade the binders in metal clays. If you do use a petroleum based product and you form and fire your metal clay object soon after, you may not have a problem. If you store the clay for some time and the binders have degraded, you may find the clay ends up crumbly instead of workable. If you have this problem you can often shape the clay, but may not be able to do fine finishing without damaging the piece. If the binder is degraded and the clay is too fragile to shape after drying, you can make simple forms or elements that don’t need fine finishing or that can be finished after firing.

I like balms because they don’t spill in my kit or on my work table. The brand doesn’t really matter (I use Badger Balm) as long as they are not petroleum based. Most contain ingredients like olive oil, beeswax, and essential oils. You can get them scented or unscented. These are great for hands, tools, work surfaces, and textures.

Cooking oils are great. They are cheap and found in almost any kitchen. I tend to spill bottles of oil. To keep your work area safe from a large oil slick, place a piece of sponge in a shallow container (preferably one you have a lid for) and saturate it with the cooking oil. This will give you a non-spillable source of oil.

Spray oils, whether of the cooking type or a metal clay specific brand can work well, but they are at the mercy of the spray nozzle. Some of these give a fine mist. Others spit gobs of oil. For safety sake, always spray against a solid surface (table, paper towel, or whatever) rather than over the floor. Keep in mind that the extra oil will end up on the floor and create a slipping hazard.

Hands
If you do have dry hands, be sure to moisten your hands with warm water and pat them dry before applying any of the hand creams, balms, or barriers. If you just go straight to the grease, you will have dry greasy hands that will suck the moisture from your metal clay. It is also very important to let the cream or barrier to soak into your hands before doing anything else. This makes a huge difference and is something you should do anytime you use moisturizer—not just when working with metal clays.

Spend the minute or two it takes the balm to soak in before you start working meditating, mentally rehearsing your design plan, or visually checking to make sure you have all the tools and supplies you need. You will know you are ready when you don’t have any greasy or visible residue on your hands. If you really over-do it with the grease and it hasn’t soaked in after 5 minutes, go ahead and rub the extra onto your arms or wherever else you need a little moisturizer. If you get into this habit, you will have a much easier time with the clay and cleaning your hands after working.

The options listed above are great for all metal clay applications. Here are a couple more that are just for the hands that are a boon for those of us with dry skin: Some people swear by Gloves In a Bottle, Bee Balm, and similar products. Some of these products are petroleum based, so you really don’t want to over use them and you want to be sure the lotion has completely soaked into your hands before starting to work.

My own personal favorite for dry hands is lanolin. This is an inexpensive natural product that is used to protect and seal the skin. It comes from sheep and is what keeps sheep’s wool water resistant. I have found lanolin to be the best and longest lasting product available. Rub on your just-washed and lightly dried hands until it is worked into your skin. I usually find this in the baby care section of the drug store. Just be sure to get the pure lanolin (lanolin usp), or you might end up smelling like diaper rash cream. You don’t need the extra expensive one for nursing mothers as that one is food-grade. You can buy pure lanolin online as well.

Where Else?
Rollers, textures and work surfaces may also need some grease when they are new. Once you get a very thin coating of grease on your tools, you don’t usually have to re-apply unless you have cleaned off all the lubricant. Also, some work surfaces need lubricant, others don’t. I use cheap ceramic tiles in my studio and these do need a little lubrication, but once they are “seasoned” I never have to re-lube unless I determinedly wash with hot water and dish soap. Rollers and most textures (brass, rubber, cloth, etc.) will also need some release agent applied to them before being used the first time.

When applying balm to deep textures, you may find Sherry Fotopolous’ trick of using an old toothbrush helpful for lubing up all the recesses and not leaving too much balm clogging up the texture. The toothbrush is great for cleaning those deep rubber stamp textures as well.

When I use cloth for a texture, I add some oil and scrunch and roll the cloth in my hands until it the entire surface is well coated. Nylon and polyester lace and cloth are best because the clay doesn’t usually stick to them. Cotton and other natural fibers are a bigger problem, but if you coat natural fibers with a release, you can often get a good impression without the clay sticking and becoming embedded in the cloth. Be sure to roll the slab out most of the way on a smooth work surface and only do the final rolling on the cloth. This technique of pre-rolling your slab is good practice no matter what type of texture you are using.

On the Clay?!
Alternatively, you can apply the balm to your slab of clay before the final roll on the texture. This is my favorite technique. I roll my clay out a little thicker (1-2 playing cards) than I intend the final piece to be on my smooth work surface, apply a thin layer of balm or oil on the entire surface of clay with my finger, then lay it on the texture (or vice versa), and make the final roll. The balm spreads smoothly and evenly with the clay and works as a barrier/lubricant between the clay and the texture. Apply balm to both sides of the clay if you both sides of the clay if you are rolling between two layers of texture.

The downside to this last technique is that if you do it too often to the same piece of clay, you can end up adding too much lubricant to the clay. So don’t think you can oil, roll, scrunch up, and repeat indefinitely! If you do find you have added too much lubricant, check out my post on Clay Consistency for tips on reconditioning your over-lubed clay.

by Mary Ellin D’Agostino

Where there is fire…

…there are smoke and fumes. Plus, we have to deal with dust from metal clays, enamels, and other materials we work with in our studios.  This is the first installment in a series on the health and safety issues metal clay artists need to consider.

While the binders in silver clay are non-toxic, fumes and particulates from firing are not great to breathe.  Smoke from metal clays is similar to those you get when burning food.  Those are not great to breathe either, but not necessarily something to cause panic (unless the burning goes too far and your house catches on fire!).  You can read reports on organic gasses released during firing on the PMC Guild website.  The Manufacturers’ Data Safety Sheets (MSDS) for PMC Silver and Art Clay Silver may also be useful.  All confirm the non-toxic nature of silver clay binders.

All of them also say to have “adequate ventilation” when firing.  So just what is adequate ventilation?  Linda Kaye-Moses believes that we should be thinking “active” rather than just “adequate” ventilation. An open window and/or doorway make a good start, but not usually enough for even “adequate” where toxic fumes are involved. A fan that blows air from an open window or door toward another open window creating a cross breeze and flow of fresh air is a lot better for general room ventilation.  However, if your work creates a localized source of dust or fumes, a fume hood or filtration system that will suck away the contaminated air is a good idea.  Such a system may also be required in cases where there are not multiple windows and doors that can be opened or when weather makes open windows not an option.  Excellent discussions and advice can be found on the Ganoksin jeweler’s forum by doing a search on “ventilation,” “workshop safety,” or scanning their archives.  Two excellent places to start are Charles Lewton Brain’s articles on Basic Safety Principles, Ventilation in Your Jewelry Workshop,   Jewelry Workshop Safety Report, and Brandney W. Simon’s  Workshop Air Quality.  When you read these articles, you will find links to paid advertisements selling ventilation equipment. There are is a whole lot more on the Ganoksin archives, including discussions and directions for building your own fume hood.  Ganoksin and its Orchid discussion forum is the best resource for jewelers on the web.

If the only concern is dust rather than fumes, a true HEPA air filter of the type sold for home allergy sufferers may be sufficient for your needs.   These can be placed in front of your work area to suck up any airborne dust down to 4 microns in size.  I use HEPA air filters originally purchased for my house-dust allergy.  These are an economical solution for the hobbyist and small scale artisan who is not working with materials that produce fumes.  High production studios, may want to investigate further and purchase filters designed for industrial use.  No matter what type of filter you have, be sure to change the carbon pre- and HEPA filters regularly.  For studio use, you may have to replace them more frequently than for standard household use.

If the concern is also fumes from the materials being used (resins, soldering, firing, & etc.), you should have a filtration system or fume hood that pulls both dust and fumes away and either filters them before returning the air to the room or vents to the outdoors.  Brands include HAKKO, Sentry Air Systems, Electrocorp, Vaniman, Lab-Air, and others—I know, I should include links here, but I am lazy and these names give you a place to start an internet search.  Some of these units are portable and have hoses that can be positioned to focus on your work area.

Further Reading:

The US EPA has an excellent  page with information on comparing the various air purifying technologies and a guide to issues in indoor air quality.  I know the government info is far from perfect, but these sites are great places to start lo0king for reliable information before moving on to the industry, muck-raking, and alarmist sites that pepper the internet.

Coming Soon:
Dust, Fumes, Smoke, and Fire: Part 2:

A discussion of the health and safety risks surrounding the powdered silver, gold, copper, and bronze used in metal clays.

© 2010 Mary Ellin D’Agostino

By Mary Ellin D’Agostino

People often ask about what they need to fire their silver clay. Do they need a kiln?  Can they use their old ceramic or glass kiln? What kiln should they buy?

Let me say first that silver and gold clays do not care what your heat source is. Heat it long enough and hot enough and you will have a good result. Heat it too hot and it will melt. If it doesn’t get hot enough for a long enough time, it will be brittle. There are many firing options available and all have their pros and cons mostly having to do with expense, convenience, and consistency of use. In this article, I discuss several why or why not you might choose any particular firing method.

Base metal clays have additional issues to consider when firing.  These clays are still in their infancy and the most reliable firings methods are kiln based.  Many metal clay artists and manufacturers are working on coming up with reliable firing alternatives.

Hot Pot: The reason to purchase a Hot Pot is if you are a new user of silver clay, don’t know if you will continue using it, are concerned about using a torch (often this means you are concerned about the open flame or of the possibility of melting your piece), and only firing the low fire metal clays. Hot pots are a good choice for people who match one or more of these criteria. The Hot Pot, WHEN USED CORRECTLY, gives a perfectly adequate firing–I do not recommend it for firing rings or other items that need to be very strong unless the maker has no other firing options. These items should be fired in a kiln for a longer/hotter firing schedule than is recommended by the manufacturers. Even when firing such items with a torch, it is highly recommended that they be fired for longer than one might fire a pendant. The choice to fire twice is often made with a torch and kiln as well as the Hot Pot and reflects more on the particular piece and the person making it than on the type of firing system used. Finally, part of the concern over the Hot Pot may be due to people using the wrong fuel for it. When purchasing replacement fuel, it is important to buy the right kind. Not all gel fuels are the same! Unfortunately this means that the safest choice is to buy the Hot Pot branded fuel as the heating characteristics of gel fuels is not listed on the various brands found in hardware or camping stores. Hot pots are only for low fire silver clays (PMC3 or ArtClay650)!

Torch: That said, the torch is more versatile and I prefer it for that reason, but many people are concerned about using an open flame. A person using a torch to fire metal clay should be very clear on what is meant by “glowing orange” as many people have under-fired their pieces when their “glowing orange” is different than what experienced metalsmiths and torch fire-ers understand the term to mean. This is one of the biggest drawbacks to learning from books and instructional media. Nothing beats learning from a good experienced teacher in person.

Stove Top: A stove sop can be a good firing option if you actually have a gas stove. These are not as common as many people think. While gas is cheaper to run, it is a lot less expensive to install an electric stove and that is what many builders have chosen in the USA. In my experience, you can’t really just set it and forget it because you may need to adjust the flame while firing. Also, you need to be sure to use a stainless steel wire grid and not standard hardware cloth, which is usually galvanized and will deteriorate during firing. Camp stoves are far more problematic than the household range as the flame does not remain consistent–it changes as the level of fuel in the propane or butane canister goes down and as the canister becomes colder while firing. Hence the Speed Fire Cone™, which concentrates the heat AND comes with a pyrometer so you know what your temperature is. The SF cone needs to be monitored as well. Also, the most consistent result will be gained if using a large BBQ type tank for the butane/propane–an adaptor is needed in this case for the SF Cone.

Kiln: Kilns give the most consistent firing assuming one has a reliable pyrometer in the kiln. These are available in a wide range of options, but if one can afford it, a kiln with (or that can be retrofitted with) a digital controller and pyrometer is the way to go. The kilns designed for use with metal clay products–Evenheat (sierra) and Paragon–are the best for using with silver and gold clays because they are designed to have a very consistent temperature throughout the firing chamber.

Lots of people like the Ultralite™ kiln because of its small size and relatively low cost. It is comparable to the SF cone at its base price, but the special metal clay inserts may make it a little more costly. It does take longer to heat than the SF Cone, but does run on electric.

The easiest and most reliable method for firing any type of silver or gold clay is a small programmable kiln with a ceramic fiber or refractory brick muffle. A muffle is the insulating part of the kiln. Ceramic fiber is a good choice for silver and gold clays because of it is an excellent insulator, lightweight, and the heating elements for the kiln can be embedded in the sides. Traditional refractory brick insulation is bulky and kilns made from it tend to weigh a lot, take up a lot of space, and have the kiln heating exposed on the interior of the kiln. Ceramic fiber is a space age material that can be formed in a mold with the kiln elements in place. The embedded elements are not exposed so you can load, unload, and crash cool the kiln without worrying that the elements will be damaged. The exposed elements in a traditional kiln require periodic replacement because they can become damaged if the kiln is opened while hot or by glass, glazes, and other substances in the firing chamber. On the other hand, traditional brick kilns are a lot less expensive and the elements are inexpensive to replace when they wear out. If you plan on firing bronze and copper clays using the “bury it in carbon” long firing schedules, brick kilns with exposed elements are preferred because the long firing times wear out the elements more quickly and, if you do a lot of firing, you will appreciate their easy and inexpensive replacement. When the elements go in a ceramic fiber kiln, you have to replace the whole muffle, which is expensive.

The size of the kiln is an issue because the larger the kiln, the more difficult it is to have even heating throughout the firing chamber. By “small” I mean a kiln that has an interior chamber that has a footprint less than or equal to 12″ at it’s widest point; a little bigger will be ok, but the typical 18″-20″ kiln may give less even heating. A large ceramic kiln will have hot and cool spots which can make firing metal clays chancy. This is not to say that they can’t be used, but you should know your kiln well and place any silver or gold clay objects near the thermocouple or cones so that they are heated to the desired temperature.

Top loading and front loading kilns also have different heating characteristics. When firing bronze and copper clays in a carbon filled box, a top loading kiln is preferred because the box is heated on all sides. Front loading kilns do not usually have heating elements in the door and the side of the firing box facing the kiln door remains cooler than the rest of it.

Glass and silver clay kilns tend to have maximum operating temperatures of 2000°F/1093°C and extended firings at the hotter bronze/copper temperatures will wear out the kiln elements relatively quickly.  These kilns are designed to be fired at glass fusing and silver clay firing temperatures and will last for years when used for these purposes.  If you are doing the long carbon filled box firings for copper and bronze, you may want to look into using a kiln designed for firing ceramics.  These cost more because they are made using materials that are durable at the higher temperatures required for firing stoneware and porcelain.

Happy Firing!

Note:  I have left off links to products and brands in the text because I am being lazy.  However, I would like to note that PMC Connection, our sponsor, sells all of these devices except maybe  the kitchen range.

© 2010 Mary Ellin D’Agostino

by Mary Ellin D’Agostino

Some time ago, I ran some experiments to help one of my former students track down a problem.   She and some of her students experienced curling, warping, surface deformation, spots, and what they thought was incomplete sintering of the silver clay.  Was it a problem with the PMC she was using?  I asked her to come over and bring the problem clay so we could try to figure out what was going on.  On her way over, she had a light-bulb moment and realized that the work surfaces she was using might be made from aluminum.  That gave us an excellent place to start.

Most of us know that silver clays (PMC or ACS) can be contaminated by contact with aluminum, but many of us ask just what is the problem and how long must the contact be to contaminate the silver?

The essence of the problem arises from an electrolytic reaction between the silver and the aluminum (or aluminum oxide).  A full technical description is beyond me, but my understanding is that the reaction between the two is essentially the same as what happens in an alkaline battery.  What we are getting is similar to the corrosion/crud that forms on the battery contacts.

What I can give you is a description of what happens to the silver clay from a practical viewpoint.  The symptoms of aluminum contamination of PMC or ACS are:

  • Discoloration: Dark spots or areas of discoloration may appear on the unfired silver clay.  Discoloration might not appear until after firing your PMC/ACS.  After firing, discoloration can sometimes be polished away, but “ghost spots” will reappear later.
  • Warping: During firing, contaminated pieces may warp, curl, and deform dramatically.  This far exceeds the minor kinds of warping we sometimes see when firing normal silver clay jewelry.
  • Brittleness & Flaking: Pieces may be brittle and surface areas may flake or spaul off of the piece.  This can be confused with incomplete sintering of normal metal clay.  It is, in fact, due to an incomplete sintering because the contaminated metal clay is chemically changed and cannot sinter properly.

The Experiment

Going back to the original problem to test whether the problem was due to “bad clay” or had been contaminated by working it on aluminum surfaces, we tested the aluminum contamination theory by exposing silver clay from a single package to varying lengths of time in contact with aluminum. One package of PMC+ clay was broken into four pieces.  Each piece was rolled out into a slab 1mm thick and marked with a letters A-D and treated as follows:

A) The clay was worked on the aluminum and dried elsewhere,

B) worked and dried on the aluminum,

C) worked on plastic and dried on aluminum,  and

D) worked and dried on non-aluminum surfaces.

All the pieces were dried on a heating tray for about 15 minutes and then fired in a kiln at 1650°F for 10 minutes

Results

The longer the silver clay is left on the aluminum, the worse the reaction.  The contamination caused dark spots and a brittle clay with a flaking surface.  The most affected was the one worked and dried on aluminum, the next was the one dried on aluminum, and the third was only worked on aluminum.  The piece made without contact on aluminum was fine.

Generally, if you are just using a cookie cutter made of aluminum and it is not left in contact with the clay you will be fine.  If you really want to be safe, you can coat aluminum or suspected aluminum cutters with a coat of plastic or varnish.  Make sure all drying and work surfaces are coated or made from non-aluminum metals.

© MED’A Creations & Mary Ellin D’Agostino, 2010

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