On reflection this is something i really should have mentioned way back when i first posted about making plaster molds (how to plaster cast a dolphin) that's got to be over 200 posts ago now - Sorry folks. Basically when using plaster when the project goes wrong, breaks when de-molding etc you need not throw away your failed project. In fact you can re-use the plaster.
Okay here is a quick run down of what i am talking about.
Gypsum, plaster of Paris, sculpting or molding plaster: It's all the same thing, well technically anyway. Gypsum is a form of calcium formed by sea shells buried millions of years ago crushed under countless tones of bedrock and top soil. The calcium of the gypsum whilst it feels dry to the touch is in fact a little wet. The water is trapped in a crystalline form within the gypsum. In this state it still has many uses. Plaster boards, fertiliser etc are two of the most well known forms of use in its unchanged state. Gypsum is sometimes called plaster of Paris because that's where it was first mined, refined and used. Today plaster of Paris or sculpting/modeling plaster is the more accepted name for refined gypsum.
Gypsum comes in a grey colour its natural state. When refined which simply involves heating the gypsum the little water that is held in the crystals is vapourised, effectively bleaching the gypsum giving it both it's white appearance and much stronger form which we call plaster of Paris etc..
When you add water to normal gypsum the crystals swell as they absorb some of the water. As they dry out again they both retain some water and bond in they're now position after swelling. Because they don't absorb much more water than they already hold they don't bond together totally leaving micro gaps between the crystals. Whilst this is still a relatively strong form it's far from good enough for quality sculptures etc. However because plaster of Paris is much drier it then absorbs more water and then swells much more bonding the crystals more tightly. Thus creating a much stronger harder smoother more ceramic like finish that we know.
Now gypsum is one of those weird substances we rarely think twice about in daily life. It is one of those elements around us that can be altered whilst used in some fashion yet it can be converted back to its natural state too.
So with that in mind here is how you can re-use your broken or faulty plaster creations. Lets assume you cast some plaster form(s) in one of the many available mold mediums. Either the details hadn't shown up clearly, there were air bubbles making the creation is weak or malformed or the creation broke during demolding etc. Essentially the cast form is no good for end use so you scrap it. Normally most people would simply bin the creation and start over. This can be a costly waste of materials.
Simply allow the creation to dry out. Either place it in a tough cloth bag burlap etc then break it down to one centimeter sized fragments. Further reduce those fragments to dust as you can either with a blender or better still a coffee grinder. Not having a coffee grinder i simply used my three speed blender. I had to run it through twice in small amounts but the blender coped pretty well. The results can be seen with the grater i used blade below.
Now because this pile of plaster contains tissue paper pulp that i want to keep as it is i will not be carrying out the next stage as it will burn away the tissue paper . I will simply add some new fresh plaster to this in small portions and re-use it in this form.
TO SALVAGE PURE PLASTER CARRY ON WITH THE FOLLOWING
After breaking down and grinding down your recovered 'pure' plaster bake it either in a conventional oven or microwave. Use the largest tray or container you have to save time. Remember however you don't want to make huge mounds for each baking session. As much as you can get on/in the containers as possible without heaping it up works best.
CONVENTIONAL OVEN METHOD:
1: Pre-heat the oven to 220*c
2: Place the tray of recovered PURE plaster (no paint, sealer, varnish etc) in the middle of the oven for one hour. Fan assisted ovens are better as they constantly rotate the heat during baking times and then afterwards dispel the heat when cooling speeding up the process for you.
3: allow to cool thoroughly before handling.
4: bag or box it up in an air tight fashion. Use as normal when ready.
MICROWAVE OVEN METHOD:
1: Use a 600w microwave oven set at its highest power setting.
2: Place the tray/container etc of recovered PURE plaster (no paint, sealer, varnish etc) in the microwave oven for ten minutes.
3: allow to cool thoroughly before handling.
4: bag or box it up in an air tight fashion. Use as normal when ready.
You can repeat this process as many times as you like.
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