I suggest if you go the hydrocolloid route, dissolve 1-5% of the hydrocolloid powder (e.g. xantham gum or sodium alginate) to make a thick solution,
then when you're ready to cast, mix in a small amount of a calcium salt powder straight in with rapid stirring, not solution, this way it will form
instantly a skin of gel around the particles and temporarily prevent the whole thing from setting into a jelly, this skin that forms around the
particles would be permeable to calcium ions and after some time the whole mass will set. With the hydrocolloids, you might need to let the solution
stand for quite long (before adding calcium salt) to let the bubbles escape that formed during dissolving the gums, this could be a problem when what
you have in there is HTP. Then again, if you can pull a vacuum slowly, you will get rid of the bubbles in no time.
[Edited on 8-11-2015 by deltaH]
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