At any rate, a ceramic artist friend of mine said that plaster actually has a shelf life, and only typically lasts about 3 to 6 months. She mentioned that I might take some and test it out, before trying to do anything else with it. So tonight, I decided to gather some supplies and give it a go. I filled a cruet (the closest thing on hand) with some water, and also grabbed up a metal chopstick (for mixing) and an Altoids tin. If you're intending to duplicate this, grab whatever suitable items are close enough.
This is the box of mold-making plaster.
I checked for spiders and eggs, and then grabbed the box and ripped it open.
There was the bag of plaster, and a piece of paper. The paper talks about the brand Vigor-Vest. Turns out, this mold-making plaster is particularly of use for jewelry. Neat.
I put some of the material into the Altoids tin.
And added some water, and then mixed until it had a pancake mix-like consistency.
At the time of writing, I am letting it set, and harden a bit.
I let it set for about ten minutes or so, and by that time, it seemed fairly solid. So I took the shell I had, and pressed it into the material.
I took another ten minutes and did some dishes while I waited. Progress! Then, I checked the mold. The shell did take a little prying, but I was able to get it out, and I was left with a beautiful shell mold.
So it looks like the plaster's still good - and now I know how to make a mold out of an Altoid's tin - and so do you!