Hydrocal

I would keep it out of high humidity and moisture.

Manny R. posted:

Does Hydrocal have a usable shelf life? 

Kept in a sealed container it's good for just about forever. 

According to USG:  When properly used, USG Hydrocal White Gypsum Cement is easy to work with and complies with the federal Labeling of Hazardous Art Materials Act, 12 U.S.C. Section 1277 and ASTM D4236. Keep indoors at temperatures between 65 °F - 75 °F (18 °C - 24 °C) and 45% - 55% RH. Do not stack more than two pallets high. Keep from drafts. Rotate stock. USG Hydrocal White Gypsum Cement should be used within 6 months of the manufacturing date located on the package. Always follow handling and use directions and safety warnings on the package.

I have used the same commercial bags (100#) of Hydrocal and Ultacal for the last 30+ years and have always stored it in a sealed plastic container that my drywall mud came in.  I have never had a problem with it as long as it is kept dry.  I buy mine at a local supplier here in Michigan.  I am now down to my last 10# of it and it is kept in a sealed jar.  When I did work in the trade the stuff we had for pouring was usually fresh and I find that mine that was quite older it had the same properties when dry.  Hard to sand but rock hard.  It also comes in different drying times which are usually 30 or 90 minute.  It does not accept stain like most plaster does because of its density and I usually put a coat of drywall sealer on it b4 adding my colors to it.  Great stuff to work with.  I have about 200# of it on my layout.  A couple projects shown below include the rocks for the trestle bridge that were made by pouring it in commercial molds and the mountain with the house on it and that was made back in the early 80's when I could not afford rock molds.  I used 8 inch squares of heavy duty tin foil crumpled up or formed over a log with good looking humps and bumps and then bent the sides up about 1/2 inch poured  my plaster into the mold and as it started to set I put it into the mountain frame work and removed the tin foil as it set.  I did use Pam as a release agent and of course had to wash it off the mountain after the plaster dried.  Old school method from an old school guy.  the tunnel portal is also made from Hydrocal using one time use forms.  Most of my tunnel portals on my layout are hand poured with Hydrocal.

I have a 50 pound sack of plaster sitting open in a corner of my shop, in the paper sack that it came in. When I need some, I just dip a cup into the bag. It is at least 5 years old and is none the worse for the wear. I'm sure that it isn't optimal (neither is the original packaging), but I am not the anal type, and if it ever spoils, I will spring for a fresh batch.