Chavant NSP is the most popular choice for prosthetics sculpture. It is wax based and melts well at low temperatures until fluid enough to pour or brush in to moulds. Holds very fine detail well. Can be smoothed with solvents such as Naphtha or 99% IPA. Choice of hardness should be made based on your requirements and personal working situation. Generally I use the medium for my prosthetics but in the summer I have considered switching to the hard as the clay can get a little sticky in hot weather. Equally in the winter, if you're working in a workshop without good heating, it may be better to use the soft. While Sulphur has not been added to this clay there may be tiny traces in evidence. While there's not enough to effect the cure of Platsil Gel 10 or Platsil Gel 25, this can be problematic when making moulds in some other platinum addition cure silicones. A dusting of some kind of sealer such as Kryolan Fixier spray may be enough to prevent inhibition. In addition Inhibit X may be sprayed for extra security. Chavant NSP is available in soft, medium and hard.