The following method for mixing plaster was shown to me by a sculptor who regularly carves large stones in Italy. Apparently this is the method used by Italian stone carvers when they mix plaster to temporarily securing large blocks of stone. Regardless of it’s origins, I have used this method for years - typically for mold making and alginate casting. It’s fast, easy, and always produces a strong and workable plaster mixture.
Note: In the following photos you will see my plaster is light brown. That’s only because someone donated a large supply of brown dental plaster. Your plaster is almost certainly white. It all mixes the same.
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Step 1: Gather Supplies
Plaster, water, mixing bucket (preferably rubber), electric mixer (optional)
It's also helpful if you have a large sheet of craft paper you can work on. Mixing plaster is messy and a disposable work surface makes cleanup simple.
Step 2: Water
Measure the amount of water you need. I alway mix extra. It’s better to have a little too much than to run short and plaster is relatively inexpensive.
Step 3: Plaster
Gradually sprinkle handfuls of plaster into the bowl. Keep adding plaster until a small island of dry plaster forms on the surface and refuses to sink.
Step 4: Wait
Walk away from the plaster for about 5 minutes. If you’re an Italian stone carver, this is the time you take your cigarette break. Letting the plaster reset is important because water gradually absorbs into the powder. If you rush it and mix too soon, you’ll get lumps in your plaster. When you come back from you break you should find the little island of dry plaster has wicked up water. If you can still see dry plaster, check your email again. If not, it’s time to start mixing.
Step 5: Mix
If you plan to pouring pour your plaster into a mold, you don't want a lot of air mixed into the plaster and you should gently mix the plaster together with your hand. Keep going until you have a bowl full of creamy smooth plaster. It shouldn’t take long.
In this example, I’m mixing plaster for a mother mold. What's a mother mold? Don’t worry about it. My point is that I’m not worried about getting air in the plaster so I’m going to hit it with a paint mixer to speed things up.
Step 6: Use It
Regardless of how you mixed your plaster, you want to tap your mixing bucket on the ground several times. This helps dislodge air bubbles. Dispense fresh plaster as required for your application. Just remember to cleanup.
Step 7: Cleanup
If you mixed plaster in a normal bucket, your stuck trying to clean your bucket before the plaster sets. Even if you quickly rinse your bucket you have no place to dump the water because - super important - you really don't want to pour it down the drain. I know, it looks like milky white water in your bucket now, but when you dump it in the drain all that plaster is going to settle and harden in your pipes. You will have one super angry landlord. Your best bet is to dump any extra plaster into the garbage and wipe out the bucket with paper towels.
If you used a rubber mixing bucket, life is much easier. Put it off in the corner and let the plaster dry. When you come back in an hour (or month) just wack the bucket wall and the plaster will crumble to the bottom of the bowl. Dump it in the garbage and you are done.