Platinum or addition cure silicones are a unique and useful material, however more expensive and tricky to use. The main advantages of this stuff are that it does not give of anything after it has cured (therefore safe to use for food and on skin and eliminating a major cause of cure inhibition against other materials) and no shrinkage, which makes it perfect for accurate moulds - that is why I am using it for eye-moulds for example: it does not ’shrink’ my eyes and Crystal Clear or Clear Cast will fully cure against it (also, the prototyping Silicones like Dow Corning Silastic T4 or Smooth On Sorta Clear 40 are more or less transparent, which - if degassed - allow casting errors to be spotted right away). However, some things are to be kept in mind when working with it.
It does not cure against anything containing sulphur (like plaster or many clays)
It is easily contaminated by water or humidity (seal wet clay)
Also it will not cure against Polyesther resin, because of the styrene it gives of. However, “regular” Polystyrene moulds can be tempered to reduce the amount of styrene it gives of and also be sealed to create a barrier.
Dust and other airborne particles are also a pretty mean killer - I had a ‘night before deadline’-desaster when I poured some Plat Gel from a very high point into a mould (a little trick to get at least some air out of your silicone if you have no degasser) and someone next to me was using quite a lot of spray-adhesive.
This is also why powdered latex gloves are a very bad idea - better use vinyl gloves!
But: Do use it - it is good and actually pretty reliable when keeping in mind the above. Work very clean, prepare your workspace and if in doubt, test - there is an indefinate amount of things that can go wrong or actually might work.
Personally, I like to place pretty much everything containing uncured silicone in an oven. Silicone itself is pretty heat resistant, the only danger is to melt other materials or to distort your mould. I do it because it speeds things up a lot and also the faster a silicone cures, the lower the risk of a contamination causing cure inhibition. I am using a pretty big foam oven set at around 70°C
