"Plaster won't dry out" Topic

I've been casting some rock formations with plaster. I ran out halfway through but then discovered another bag of plaster that I must have had for nearly twenty years. The casts made with the new plaster have dried out just fine. However, the casts using the old plaster have still not dried out properly after four days – I can push my finger into the plaster as if it's clay.

This old bag of plaster has been kept sealed in dry, room-temperature conditions for all this time, and I thought it would keep indefinitely when stored properly, but is plaster known to 'go off' after a certain period of time?

Not that I know of, but I've never used any that old.
Could there have been some other impurity that got into the mix? As you were making it?

Nope, that's definitely not it. The two batches were made in exactly the same way under the same conditions.

Plaster does have a shelf life however, old plaster tends to set a lot quicker than new, so that's no help to you.

Since you are from the UK – you can generally count on the shelf life being a year or less. Even in a sealed, dry, room-temperature room…there is still too much ambient humidity that you will have a hard time dealing with (without using a 0 perm container like stainless steel drums with rubber gaskets).

In a dry locale like Arizona here in the States, you can keep it for much longer. I know I have used 5+ year old plaster when I was in SW Texas.

You have to remember that most plastics are still permeable to moisture and it doesn't take a lot of moisture to kick off the chemical reaction that causes plaster to cure. Over time, enough moisture gets into the bag (through infiltration and every time you open the bag). It causes a few particles to cure…then more gets in and causes a few more particles to cure. After enough time (based on humidity…temperature really doesn't matter, however cold air holds less moisture than warm) you have something that looks like plaster mix. It mixes like the regular plaster. However, it will not cure. Eventually it will dry out – however it is sort of like a dried out mud pie. You can poke at it and it is sort of hard, but it has no strength (and will likely crumble when you attempt to demold).

The plaster hasn't failed to dry, it has failed to cure. Throw it away along with the whole bag – it is useless.

Some types of plaster do store better than others, I find 'stone' plasters last longer than those resembling Dental plaster or Herculite while plaster of paris is very poor and goes 'bad' very quickly.

I actually still have some of the original Linka plaster from over 20 years ago and it still works perfectly – I'm in the UK and it certainly ain't desert where I live. In general plaster of any type, unless stored in airtight containers, won't last much over a year.

I know that plaster bandages used for casts had a shelf life as when I was a kid I was given some which a hospital was throwing out as out of date. It got worse and worse at actually working so that hills I made eventually looked like hessian.

I'd try using a blow dryer on it first, to see if that helps it cure.

No, heat will not work. Setting plaster is a chemical process and, as has been explained above, this process has already started when the plaster gets moisture in it. If there isn't enough water to fully complete the process then the set will never happen properly in the future, no matter what you do.

Actually, applying more heat has worked. I left the moulds on a radiator and the plaster is dry now. The casts are rather chalky now though so I guess there must have been too much plaster in the mix. I have a new bag of plaster here but I think I'll have one more crack at using the old stuff (it's a rather large bag that I'm loath to just throw out) before moving on the the new stuff.

Thanks for the interesting replies. I've used plaster that's well over a year old many times before with no problems, which is why it had never even occurred to me until now that it might have a 'shelf life'. Perhaps I've just been lucky.

Here's a tip. Perhaps you already do this. Mix some brown tempera paint into the water before you mix the plaster. It will dye it a brownish color and if it ever chips or cracks, there won't be any white showing through. I also have used plain black india ink as well as black tempera for a greyish cast.

In a former life I worked in the pottery industry producing plaster moulds for ceramics. Plaster does indeed 'go off'. Plaster shouldn't actually be cast as soon as it is mixed, which sound strange I know but we were actually trained to 'feel' the texture of the plaster to know when it was ready to pour. It started like milk but at the correct point the plaster turned creamy and smooth rather than granular.To me this sounds like the plaster has perished.