Oil based clay is clay mixed with some kind of oil or combinations of oil. It will not dry out, but it will oxidize over time and become difficult to work with. Sometimes, you can manipulate the consistency of the clay by mixing it with petroleum jelly, but this is difficult. Some oil based clays, like Plastina, contain sulfur and can't be used with silicone molding compounds. Plastina can be sealed with a sealer or shellac, but any coating of this type will take away some of the details of your sculpture. We recommend and use our Industrial Tooling clay, which is easy to work and won't react with silicones. Some oil based clays can be melted and poured into a mold. Preheating oil based clay will soften it and make it easy to work with.
Klean clay has gone the way of the Earth. The old gentleman who made it used up the last of his chemicals and retired. He did, however, sell his formulas and equipment to a competitor who has taken up the cause. The replacement clay is as follows: