Preparing for alginate casting

So the idea for this - make a two part mould. I'm masking out the back half of the head with clay, so I can then pour alginate (I discovered it's the kind of brittle rubbery plastic dentist's use) over the front of the head so there's a layer about 1.5 cm thick, which I then have to back with modrok and plaster to support the alginate when I remove it. When it has fully cured I can pour plaster into the mould to make a really solid cast of the face. Removing this from the alginate will be tough because the alginate is so brittle, and if I want to take more than one cast I have to be really careful not to break it. I mean, I don't need more than one cast, but I'd like a spare really. Then comes the scary bit, I take the plaster to Tremough and thermo-form plastic over the top (clear or translucent, I haven't decided yet) so I'm left with a sturdy plastic mould of the face. Then, repeat with the back. Then, repeat with other heads!