Learning how to mix plaster is easy and relatively clean process as long as you know what your doing, and set up an efficient workspace. The process described in this post is for plaster that is going to be used in mold making and plaster casting. Since molds need a stronger plaster I advise using pottery plaster, or when you get good at mixing you could even use a gypsum cement like Hydrocal. Hydrocal generally sets up very fast but forms a harder plaster, so if durability is an issue you might want to look into this type of plaster.
Step 1: What You Will Need
1.plaster (can be purchased at most art stores)
3.latex or rubber gloves
4.paper or plastic cup for scooping plaster
5.space that can be cleaned easily (plaster can be messy and hard to clean up so use good judgment and don't do it in your living room) outside or a plaster studio are the best places
Step 2: Setup
I advise having a plan of attack for making and then pouring the plaster. You should have a mold totally prepped and ready to accept plaster, don't cut corners and take your time to make sure your bounding box or whatever you use to make molds is not going to leak plaster. That said have an area with all your materials and tools ready to go before you start making plaster.
Step 3: Pour Water
There are a couple ways to make a mixture of plaster, if you want to be super precise about your ratios get a scale and you can measure one and a half pounds of plaster for every pound of water. For this demo i use a less precise but still effective method to make your plaster the correct hardness for molding. Regardless if your measuring ratios or not, you must always ADD THE PLASTER TO THE WATER not the other way around. How much water you ask? Whatever amount of water you add to the bucket expect that amount to double when your done adding plaster. So, don't expect a good plaster mix if you've just filled your bucket up 3/4 the way up with water.
Step 4: Add Plaster
I won't go into how plaster is produced but the nature of the beast is that it loves moisture and actively seeks it out, it is because of this that plaster needs a dry place to live and a clean dry scoop (your plastic cup) so it can be moved without adding moisture to your dry plaster. One should have a cup only for scooping plaster, it makes adding plaster easier and decrease the chance that your going to contaminate your main batch of plaster. At any rate, when pouring the plaster into the water you should use a shifting motion being sure not to concentrate plaster in any particular area .
Step 5: Know When to Stop
Stop adding plaster when islands of dry plaster start to appear at the surface and don't immediately absorb. Another indication that you have added enough plaster is when there is a fine layer of water sitting on top of the absorbed plaster. Let the plaster sit for two to five minutes before stirring, this wait time ensures the plaster has fully absorbed the water and makes the plaster harder.
Step 6: Stir
I always stir the mix with my hand but other instruments can be used. Stirring by hand has the advantage of releasing air bubbles but careful mixing with something else can yield the same results. If your going to hand mix wear a rubber or latex glove and gently stir till the mix becomes thick enough to create lines. (this sometimes sneaks up on you so be watchful)
Step 7: Pour
pour your mix evenly and in a steady manner to not create air bubbles. After you fill your vessel be sure to shake the table your working on gently to release any bubbles that may form in the mold from pouring. Now wait till the plaster sets, also be careful, plaster gives off heat when its setting so wait till it cools down before doing anything with it.