Difference between Plaster of Paris and Regular plaster

[Anonymous]

Anyone know the difference between Plaster of Paris and the regular plaster you can buy at Home Depot. The HD stuff is $10 for 50lb bag which is way cheaper then the prices I’ve seen for Plaster of Paris. The HD stuff is probably heavier but for making scenery (plaster soaked paper towel hard shell) I wonder if this really matters that much.

[fiatfan]

I think the big difference is PoP sets up faster so it’s a matter of what you want to do. If you want ot create castings, use PoP. If you want to be able to work with a rock face (carving, etc) use regular plaster with a longer set up time. The set up time for PoP, I belive, can be slowed by using vinegar. I’ve never tried it myself, though.

Tom

[Benjamin_Maggi]

The plaster of paris has instructions written in French…

[Anonymous]

I have been using HD plaster for years for hardshell and it is absolutely adequate and very strong. It sets very rapidly. Lately I have been using 45 minute plaster (as used for gib-stopping on plasterboard walls) and it gives much more time to work but is not nearly as strong. To create bulk when scenicking I add sawdust to plaster (and colour it with acrylic paints) and that is very effective (although not so strong with the 45 minute plaster).
I have never used Hydrocal as it so very expensive.

[Anonymous]

Maitaivr: when you say 45 minute plaster, are you talking about the 45 sheetrock joint compound? (mix it yourself drywall mud) If so, I have some of that also in the 90 minute variety. (It comes in about a 15 or 20 lb bag. for $7)

For coloring the plaster, I’m quessing your adding powdered tempra paint then mixing in the water.

[Anonymous]

The following is an exert from PlasterMaster.com

QUOTE:
There are two basic categories of plaster that we use today:
Alpha Hemihydrate (alpha gypsum) and Beta Hemihydrate (beta gypsum)

Alpha Gypsums calcined under pressure are called gypsum cements - These Alpha Gypsum casts are harder and stronger with limited absorptive power. They require 22 to 45 lbs of water per 100 lbs of gypsum cement (this is called the Use Consistency Ratio). They are used primarily when greater strength is required. Because of their extreme hardness some Alpha gypsums can not be carved or scraped after hardening. Alpha Hemihydrate is produced in many different formulations. You can mix an alpha gypsum with a beta gypsum such as moulding plaster to increase the plaster’s strength or hardness. I use 3 scoops of plaster to one scoop of HYDRO-STONE in my sculpted model work. To help you choose the ones that best suits your needs, check out the list below.

Beta Hemihydrates are known as industrial plasters, plaster of paris, kettle plaster and kettle stucco. Beta Gypsums are made by calcining in a kettle at atmospheric pressure. They require more water to make a workable slurry because of their irregular crystalline structure. They require 65 to 160 lbs. of water per 100 lbs of plaster ( consistency ratio). Beta Gypsums are not as hard as the alpha gypsums. That’s why they are easier to carve and scrape. Because of their high water absorptive power, they make excellent pottery molds. There are many beta Gypsums from which to choose. Use the list below to help you in making your decision.

Really, check out the site.

[Anonymous]

Thanks gsetter.

[Anonymous]

I use the stuff for making dental molds, the name escapes me at the moment. It’s cheap and strong. Sets up fast and creates a fair amount of hear while setting. Marty

[BR60103]

Another factor that needs consideration is shrinkage. Some plasters give more shrinkage than we’d like.