All types of metal clay are made of microscopic particles of metal, some kind of organic binder (each brand and clay type may use a different ingredient), and water. Metal clays are about 90% metal, which only leaves about 10% binder to hold the water that keeps the metal clay malleable. Which means that it has the tendency to dry out quickly.
Metal clay dehydration happens along a continuum. At first it's just slightly dry, and you may notice cracks forming at the edges of the clay as you work with it, later it may be so dry you think it's unusable, but you can still press it with your fingers, lastly if left too long it finally becomes rock hard and is no longer workable. The remedy is to add water. How much water is dependent on how dry it is. (scroll below to see rehydration techniques)
The best technique you can practice to avoid this phenomenon is to keep the clay wrapped in plastic wrap until you're ready to use it! This means having all your tools, textures, cutters, and a design plan in place before you even open the package. Make sure the piece of plastic wrap has been cut to the correct size for the lump of clay you're using. I like to tell my students to tear off a piece about 4" long, and then cut that in half. If you use too large a piece of plastic wrap there's room for air pockets, which leaves the clay vulnerable to drying out - even while wrapped up. When you are done working with your metal clay mist it with a little water, put it in one corner of the plastic wrap, roll it all the way to the other corner, twist the ends opposite ways - like a hard candy wrapper - and tie once. This should protect the clay until the next time you use it. If you'll be away from the clay for a while, I suggest you wrap it in another layer of plastic wrap, and put a piece of wet sponge in the zip lock bag or other container with the wrapped clay. (There's more about plastic wrap in the Tools section of this blog)
Syringe clay should be kept in the type of water vial that you find at the florist. If you put a small piece of sponge in the vial, the water won't evaporate as easily. Keeping the syringe in a vial also makes it easier to transport to a class or workshop.
Slip/paste will eventually dry out too. If you have a jar of commercial slip/paste check it occasionally to make sure it is the viscosity that you like. Slip can be the thickness of nail polish, yogurt, peanut butter, or anything in between depending on the use, and the preference of the artist. Adding a spritz or two of water, covering it and letting it sit overnight will bring it back to a creamy consistency.
REHYDRATING LUMP CLAY
• Stage One - Everyone needs to rehydrate their clay at some point. At first you might notice small cracks along the edges of the clay, or you might find that it's difficult to impress a good texture into it, but the clay is still very malleable. When this happens press the clay flat with your fingers then place it on one end of a piece of plastic wrap. Dip a finger in water and spread it on the clay. With your fingers on the outside of the plastic wrap, fold the clay in half (with the water on the inner layer) and flatten/blend the water into the body of the clay. Fold and blend the water into the clay at least 3 times before adding more water. How much water you add is dependent on how dry the clay is, and you will become better at gauging it with experience. When you think you have blended in enough water, wrap up the lump of clay and let it 'rest' for at least 45 seconds. This is to allow the binder time to absorb the water. If you use the lump right away, it will dry again immediately. The reason you're blending with your fingers on the outside of the wrap is because your hands absorb moisture from the clay, so if you try to rehydrate the clay with just your fingers alone, not only are your fingers are getting really messy but your skin is wicking the moisture away from the clay that you're trying to rehydrate!
• Stage Two - At this point the clay is really dry and difficult to manipulate with your fingers alone. In this case you may want to switch to a roller to flatten the clay. I also suggest you replace the plastic wrap with a heavier piece of flexible plastic - like a freezer bag or an office type page protector that has been cut to open like a book. Although you'll have to use more pressure blend the water into the clay, it will be easier to rehydrate by using these tools. When you have achieved a good consistency, let the clay rest for an hour and up to overnight to allow the binders to fully absorb the moisture. See my YouTube video to view my technique.
• Stage Three - Now your clay has gone a very long time without attention and it is rock hard. Do you throw it out? Of course not! Metal clay is always rehydrate-able. To replace the necessary water in completely dry clay you need to turn it back into powder. There are a couple of ways of doing this. I like to use a tissue blade to chop the hard clay into tiny pieces, then use a mortar and pestle to grind it into dust. The closer you get to dust and the more you grind, you'll see the powder turn into what looks like thin sheets of dry clay - but it's just compacted dust. Stir it with a pin tool and you'll see that it's all loose. Other artists like to use a dedicated coffee grinder to make the powder. Once you get to the powder stage the technique is the same. Transfer the powder to a small bowl and add water a drop or two at a time (if you add too much you'll bypass the lump clay stage and make slip) and stir it in with a spatula. When the dust sticks to itself in a lump and comes away cleanly from the sides of the container, transfer it to plastic and use the same directions as Stage One to fully rehydrate it into malleable clay. Lisa Cain has a good YouTube video showing the technique using a coffee grinder.
The goal when using metal clay, is to not have to rehydrate fresh clay more than once or twice, and never to get to Stage Two or Three. But things happen, clay dries out, and now you know the remedies for each stage of metal clay dehydration!