Alginate is used to make molds, most commonly in
lifecasting, on people's parts. I get it in powder form, and mix with water until its a bit like peanut butter. Its applied to the area to be

molded, lets say, my
wifes face, and it slides. Droops down, has to constantly be monitored, and finagled, and reapplied, until its set, about 20 minutes or so. Its a racing game, like with foam latex, to get it where you want it before it sets. About halfway through, when it just begins to set up, I apply strips of terrycloth, which provide grab-on points for the plaster bandage mother mold.

The plaster shell will keep the
alginate in position, because it sets up sort of floppy, like rubber latex without the tensile strength. Once it sets its ready to be used. Now here's the cool thing......you can pour in plaster or melted clay.....hard cast (plaster) would result in a soft mold (latex), or soft cast(clay) to hard mold (
ultracal), then to foam latex, for example.
The
alginate will start to dry out quickly, but you can store it in a bucket of water and it will be fine. I had my wife's face in a bucket for almost two months, took it out this week, stuck it back onto the plaster shell, and pulled a new clay copy out of it, which looks virtually the same as the original.
Then I got curious, and here's where the fun begins. I left the alginate mold out, didnt put it back in the water. Over the next day I watched it harden and SHRINK....it started to curl badly, so I stuck a little piece of dowel inside, from cheek to cheek, to keep it in shape, and now I have a miniature mold of the face. About half-size.
The clay mini-copy doesn't look great, because I wasn't thinking about what I was doing, and I poured the melted clay directly into the hardened alginate mold with no release agent, so it distorted some when pulling out. And you can see where I had to resculpt the nose a bit, around the nostrils, because I didn't put enough alginate in that area.
But here's the kicker....I take this clay mini-copy, make an alginate mold of it, and let that shrink.....should end up, oh I don't know.......about puppet size.......!!! Makes me think about a little 'self-puppet' idea I had, many moons ago....
Let me take it a step further.....suppose I need a smaller version of Jenny for a long shot in the 'Greenteeth' short....I could pull a rubber latex version from the Jenny mold (whenever the sculpt is done :D, then alginate-mold that, let it shrink, and pull a new teeny Jenny from the dried mold...possible? We'll just have to wait and see.....Requires further study.......