This is what I use to mold masks for now. I used to be hair and makeup designer (and production assistant!) for a local school. This cast was made using dental alginate, which I put thin bits of cotton like cotton balls pulled apart into, then a good layer of plaster bandages over. As soon as it was set and removed, I poured a generous amount of plaster to make the lifecast. The lines were from the mask design and the clay is what I molded the prosthetic from. Since I can't bring myself to chip that clay off, I work around it for now! My husband and son both have nice strong features, so I plan on life casting them for my mask work.
There are some great online tutorials for life casting.
BTW, I love the mask you did!