try heating it a slight amount, I do this for some of my Apoxie Sculpt, but its two parts and not mixed together, The hand conditioning warms it up. I put mine in the microwave for a very short time just to warm it up to. but too much it will harden they 70 degrees is the best temperature. I found this site which has storing information for for the future,

Not sure how long you have had it. but read this maybe it will help, you.

I googled old hard kato clay and came up with this site that gives suggestions on how to soften

http://www.garieinternational.com.sg/clay/shop/softening_pc.htm

from Donna Kato
Conditioning
Preparing your clay for use. Insufficient conditioning may result in a weakened finished item.

Conditioning Clay by hand:
To condition by hand, cut 2oz bar into cubes. Soften each cube by
squeezing opposing sides with fingers. Roll into a ball - then a snake
shape fold in half. Knead until soft and pliable.

1. Slice the block of clay into approximately 3/16" pieces, then roll them through the pasta machine - this will help prevent the clay from crumbling.

2. If you don't have a pasta machine, cut the block of clay into manageable, small pieces and knead them in your hands.
3. Ambient temperature affects ease of working with your clay! Kato Polyclay behaves best at a temperature of approximately 70 F. If you have very warm hands, a slightly cooler temperature will probably work best for you.

Tools

1. For making uniformly thick or thin sheets, the pasta machine is best. I'd recommend the Atlas brand - it's a bit more costly, but will last longer than the others.

2. Sheets may also be made using a rolling pin or rod. Wooden rolling pins will work but the clay will build up in the wood, making it necessary to frequently clean them. The best rod is an acrylic rod (like the Kato Clay Roller). Clay does not as readily build up on the acrylic surface and it's much easier to clean.

3. A good work surface is a necessity. The oils in polymer clay may damage fine wood surfaces! Glass, acrylic, marble and Formica are good materials to work on. Formica has a very fine texture on which the clay will not stick. You may find your clay sticking to very slick surfaces like glass and marble. In certain instances, you may want the clay to stick, so this isn't necessarily a negative trait. In very warm climates, the coolness of marble may help maintain a level of workability and keep your clay from becoming very soft and sticky.

4. You'll want a good long blade for cutting canes. The Nublade has become the standard among most polymer clay users. Its thickness makes it easier to cut through blocks of clay and large canes with minimal torque and twisting. Clean blades frequently for best results. These blades are not for child use! Single edge razor blades are great for cutting small diameter canes.

5. The Marxit measuring tool makes it possible to mark canes or sheets of clay in 6 different mm sizes. If you're cutting canes and wish for uniform thickness, this is the tool for you.

Millefiori Caning
Technique in which clay is layered or arranged to create images in clay. Think of sushi rolls! When the roll or log is sliced, you will get the same image from one end to the other. These images may be miniaturized by applying pressure to the sides of cane, then gently stretching the cane out.

1. Certain clays lean toward the translucent side. Raw canes will look quite like any other clay canes but, after curing, you may see loss of detail due to the influence of darker colors. The addition of white to these clays will ensure greater color separation and a clearer picture. Kato Polyclay does not require the addition of white and will maintain its true color and separation.

2. For the most even reduction and the least waste, make sure the different colors used are the same softness. Kato Polyclay has been specially formulated so that all colors share the same softness.

3. When you wish to make a cane (like a stained glass cane) composed of both translucent and opaque elements, wrap the translucent components with a thin sheet of white, then the leading color (black). The white will prevent the muddying influence the black might have on the translucent colors.

4. If you're new to caning, begin by making small canes! Reduce these and see how much you can reduce a cane before you lose detail.

5. When making large canes, don't reduce the entire cane to the smallest diameter. It's best to retain pieces of the cane in several sizes. Remember that canes may always be reduced, you can't make a reduced cane large again. It's also easier to store your canes in larger diameters, rather than keeping a cane you've reduced to a length of several feet!

6. If you've made a cane you don't like, don't throw it into your scrap pile. What you've got is the chance to experiment! Try dividing it and reassembling the pieces in new ways. Make kaleidoscope images! Try flattening the cane and rolling it though your pasta machine, then stacking the pieces up.

7. When applying slices to a bead or sheet, it's best to cut thin slices. Roll a few s