There are a lot of methods to soften clay that has advanced. There are three things that I do. All three methods are successful but have varying degrees of p.i.t.a. factor.
1. Simply mix in plain super sculpey in whatever ratio gives you a workable mix. With a pasta machine this is probably the most simple solution. The drawbacks are that in significant amounts it will change the color of the super sculpey firm, and you have to find the super sculpey that has not also advanced or your degree of softening will be reduced.
2. Mix in Sculpey Clay Softener (formerly called Sculpey Diluent) to the overly stiff Super Sculpey firm. This method is messy at first but requires very little of the additional product to quite quickly soften the clay. The drawbacks are that it is messy and you should not use a pasta machine until it is relatively will mixed in.
3. Mix in Sculpey Moldmaker (and clay conditioner) with the overly firm clay. This is my prefered method for several reasons. It also requires very little product, you can use the pasta machine, and the mess factor is minimal. You start with just a smidge of the moldmaker to start and add until you reach the level of softening that you desire. This method also increases the elasticity of the clay making the finished product less likely to break.
All of these methods share one very important characteristic... That is that they infuse the overly stiff clay with the same plasticiser with which it was manufactured. The chemists spend a lot of time designing the formulas that result in the clays that we use and while mineral oil or vaseline or some such thing may work, there may also be unexpected results sometimes years down the road. I feel far more comfortable using the materials that are actually created to work together, rather than taking a chance on having a catastrophic failure at some point due to using improper products.
Just my opinion, feel free to add anything you want, I just don't like to take that chance.
Tommie
http://tommiej.wordpress.com/the-ult...culpting-tool/