Conditioning is the process that makes polymer clay ready to work with. Most conditioning is done simply by working the clay with your hands until it reaches a good working consistency. The warmth of your hands combined with the physical process of stretching and compressing the clay changes its texture, making it softer and more pliable.

Conditioning clay softens it, making it easy to work with, mold, and roll into canes. It also makes the clay stickier and less brittle, letting you roll thin sheets and reduce canes without causing the clay to crack and break.

(There are a few techniques that don't require clay to be conditioned beforehand. These are mostly techniques where the brittleness of unconditioned clay is desired.)

Some clay brands are so soft out of the package that you'll be tempted not to bother with conditioning, but you will find that taking the time will improve both the workability of the clay and the strength of your fired pieces.

The simplest way to condition polymer clay is to work it with your hands for several minutes. Take a chunk of clay of a size you feel comfortable working with, half an ounce to an ounce or so, and begin squidging it between your fingers. As the clay warms and softens, start rolling it between your palms into a snake shape. Then move your hands against each other in a circular motion to compress the snake back into a ball. Repeat this process several times.

As a rule of thumb, you should condition clay for the amount of time it takes to thoroughly blend two colors. (Take a bit of clay of each of two colors and condition them together, timing the process; when you can't see the separate colors any more in the blended lump of clay, the clay is conditioned.) This time can vary depending on the type of clay you're using, your style of conditioning, the amount of clay, and the temperature, but usually it takes something between three and ten minutes.

The clay's texture changes during the conditioning process. As you get more experienced, you'll be able to see and feel this change to tell when your clay is completely conditioned.

Conditioning large amounts of clay can take enough time and effort to be annoying, particularly if you have arthritis in the hands, or are using a stiff clay. There are a number of ways to make conditioning faster and less hard on the hands:

Pre-warming the clay

You can start the conditioning process by putting the clay in a warm place for fifteen or twenty minutes; for example, you can stick the packages you'll be working with in your clothes (some people sit on the clay packages), or use a hot-water bottle. Warning: Excessive heat or ultraviolet light will cause the clay to start curing, making it unusable except as scrap. Don't put the clay in the sun. If you're using a heat source such as a lamp or heating pad, make sure the clay does not get much warmer than your body temperature.

Chopping the clay

Many clayworkers use a food processor to chop up the clay. The small chopped bits are easier to work with than large chunks, and the friction of the blade warms the clay. After chopping the clay, you dump out the chopped bits, press them together with your fingers, and continue the conditioning as described above.

Adding softening agents to the clay

There are a number of things you can add to polymer clay to make it softer:

• Eberhard-Faber (makers of Fimo) make an item called "Mix-Quick", which is a solid block of extra plasticizer; you can add Mix-Quick to your clay up to a third of the total.
• Sculpey Diluent is a liquid that does essentially the same thing, softening clay when you add a few drops.
• If you're using a stiff clay, you can add as much as a third of Sculpey transparent to soften the clay without changing the color.
• You can also mix a few drops of mineral oil or a dab of petroleum jelly into polymer clay. (Add only a small amount; it's easy to overestimate how much you need.)

Using a pasta machine

Another method of conditioning is to press soft clay into a sheet and put it through a pasta machine, on the widest setting. Fold the sheet in half and put it through, fold-first, and repeat this process ten or fifteen times. (This method can only be used with soft brands of clay, or clay that's already partly conditioned; putting stiff clay through a pasta machine will only shred it.)

Whichever method you use, make sure your clay is well-conditioned before you use it, since under-conditioned clay can cause you problems in workability and reduce the strength of your finished items.

There's no way for clay to be "too well-conditioned", but some clays (particularly Sculpey) get softer and softer the longer they're worked and the warmer they get. If your clay is getting too soft for the technique you want to use, you can let it rest overnight, or chill it for an hour or so, to firm it up. (Chilling or resting will not make the clay lose its conditioning, so you don't need to re-condition after this.)

If you've added too much Diluent or mineral oil and have over-softened the clay, try flattening it into a sheet and leaving it between sheets of blank newsprint overnight. The newsprint will absorb some of the plasticizer, making the clay firmer. (Don't use printed newsprint; the clay will pick up the ink.)

If clay has gotten partially cured, it will be crumbly and refuse to condition no matter what you do to it. This can happen if the clay has been exposed to excessive heat or ultraviolet light - it's as though it had been partly fired, and the process can't be undone. You may be able to use it for special effects (such as embedding the hardened chunks in other clay for an interesting texture). But you can't use it the way you'd use fresh clay.

Some clay brands are just naturally stiff, however, even when fully conditioned. If you're using a stiff clay and find you don't like the texture, try using a softer clay, or mixing a soft and a stiff clay together to get an intermediate degree of softness.