This is the information I couldn't find when I started the project, and had to guess at, measure, and ask about before embarking on this project. These materials are sold by weight, and I could only find oblique references to possible volume. I ended up calling the company I planned on purchasing from for tips, and they suggested that 5 pounds of alginate and 10 pounds of hydrostone would be more than adequate to give me at least 5 small hand molds, and 12 or more castings. I purchased according to their recommendation, and you can see that the volume of the plaster powder in a 1 gallon tub is significantly less than the volume of the molding compound in a 2 gallon tub. I'll re-summarize my findings here:
AlginateMix 3 parts water to one part alginate by weight. For loosely packed material this is 1 part water to 1 part alginate to get 1.5 parts molding compound by volume, including some air entrapped in the compound. This means that for each cup of mixed molding compound you need 2/3 cup of alginate, which corresponds to about 3oz of alginate. A 5 pound tub of alginate will make somewhere in the vicinity of 48 cups of molding compound, or 3 gallons. Note that the mixture volume is greater than the volume of the powder - 1 part of powder properly mixed with the water yields 1.5 parts of molding compound.
The bottom line here is that 3 ounces (weight) of this alginate makes about one cup of molding material once mixed. So if you need four cups to make your mold, you should purchase at least 3/4 pound (12 ounces) of alginate powder.
Hydrostone PlasterMix 1 part water to 3 parts hydrostone by weight. For loosely packed material this is 1 part water to 3 parts hydrostone to get 2.5 parts casting compound by volume. This means that if you need 1 cup of mixed casting compound, you'll need 1.2 cups of hydrostone powder, or about 9oz of hydrostone powder. The ten pounds of hydrostone that I purchased makes close to 15 cups of casting compound, which is just under a gallon. The volume decreases when water is added - 1 cup of hydrostone, mixed properly, yields .83 cups of casting compound.
The bottom line here is that 9 ounces (weight) of this alginate makes about one cup of mixed casting material.
How much I usedI tended to make more than needed, as I would rather waste a little now for a lower chance of making a mistake than mix less now for a higher chance of having to do it over again wasting even more time and material. Now that I've done it once, though, I expect I'll be more frugal in the future.
For the 6 molds I made (re-did the eldest's mold due to missing fingers in the casting, and did the neighbor's child when he visited as payment to keep the secret) the most alginate I used was 15oz, which made over 5 cups.
The smallest amount of alginate I used was 10oz, which made just over 3 cups of molding compound. In total the six molds took about 4.5 pounds of alginate.
Each time I made more hydrostone than needed - each hand needed less than a cup. I used nearly 5 pounds of hydrostone on two pours, one for 4 hands, and one for 2 hands. In both cases I ended up with enough extra to make several sand castings.
CostI spent about $85 after shipping for the materials. I have twice as much plaster as I need, and enough molding compound for another hand or two. Assuming I simply throw them away, each hand cost me about $14. I would consider this an upper-limit for the expense.
However, to save money, next time I would plan more carefully and do the following for each hand:
Find/prepare molding containers that suit the hand volume better.
Use about 8oz (5 cups mixed) alginate, which costs about $5
Use about 10oz (Just under one cup mixed) hydrostone, costing about $1
So the lower limit for a child's hand would be $6 each, not including shipping of materials. In the future I expect that I'll probably still use more than needed, and will simply round each hand to costing about $10, which is half what carnival and art fair hand casting booths seem to charge in this area.