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Frequently Asked Questions

  • Q.  Do you have any Plaster of Paris?
  • A.  Yes.  All the plasters we stock are "plaster of Paris" but they vary greatly in their characteristics from very soft to extremely hard, fast, slow, high and low expansion, etc. etc. Once we know what you want to make with the plaster we can suggest a good starting point and back this up with full technical after sales support.
     
  • Q. Can I use plaster outside?
  • A. Usually no because on its own gypsum is slightly soluble in water. But by adding polymer to the mix plaster can be made much more water resistant. This is also advisable for indoor high humidity applications such as swimming pool areas.  (see Polymers section).
     
  • Q.  Can I colour Plaster?
  • A.   Yes, we have a range of liquid pigments that are very effective. This is because they are micro fine particle size unlike powder pigments which get "diluted" on the surface of the plaster by the equally fine particles of white plaster. Their use in polymer/gypsum casts is particularly good because the polymer seals the cast and holds the colour true.
     
  • Q.  I have heard the term alpha and beta referred to plasters what does it mean?
  • A.  All gypsum casting plasters are either alpha or beta or a blend of both. The basic plaster made in open pans in batches of about 4 tonnes by heating the powdered rock becomes a beta plaster. Eg. Fine Casting, Dental, Pottery Plaster etc.  But if the gypsum is made into a slurry and put into large autoclave (pressure cooker) then heated to around 220 oC  a plaster is produced that forms much longer and straighter crystals on setting- an alpha plaster which is very hard. From these two bases everything in between is made by mixing both. So a plaster such as Herculite No2 (50/50 a/b) is half way up the hardness range. The plant needed to make alpha plasters is very complex and the batches are small this is the reason for the much higher price.
     
  • Q. How important is the plaster to water ratio when I mix the plaster?
  • A. The plaster/water ratio is very important especially if you need to get consistent results regarding strength and porosity. Too much water and the set will be longer, the strength lower and the porosity higher. Too little water and the set will be quicker, the strength may be too high and the porosity too low. The higher the alpha content in a plaster the lower the water demand will be. Check the data don't go by the appearance of the mix until you have done numerous mixes at the correct ratio.
     
  • Q. What plaster do I need for casting lead.
  • A. For casting lead you need an investment which is a plaster which has grog and some other bits added.
    Some people make their own from pottery plaster and Molochite -200 or Crystobalite. But you can use a ready-made one and something like the one used for glass casting on this site (SRS GLASS) would work well. The problem with plaster on its own is that you have to raise the temperature well above 100 oC  to dry it and even dry plaster has 2 water in its chemical composition. Once you force that out it shrinks, cracks and returns to something like it was in the bag when you bought it ! Any wetness in the mould will cause the hot lead to explode. Usually other things are added to an investment to hold it together after you have de-natured the plaster such as cements and/or resins.  There are health and safety issues when casting lead. If you want to demonstrate casting to students or children you can use our LM70 low melt metal (700C) which can be poured into damp plaster or clay.

     
  • Q. I'm looking to mould up some garden gnomes and garden stones - can you please let me know which is the best plaster to use for outdoor use for wet weather and frost conditions.
  • A. You can use any of the middle range of plasters by adding polymer (SP201) to the mix but it’s not a cheap way of doing it. We have customers that do it when they need a very good surface finish for detailed painting. One such customer uses Crystacal R with SP201 Polymer diluted 2parts water to one part polymer. To cheapen the system they add about 40% fillers in the form of Trucal 10 limestone. The finished casts are dipped in a sealer made from the polymer diluted 5 parts water to 1 part polymer and the painting detail is done with acrylic paint. A cheaper alternative that requires no polymer is to use cement at 1part to 3 parts aggregate. If this proves to be too slow for the number of moulds you have then our Cerrocrete cement could be used which can give de-mould times as short as 25 minutes at reasonable temperatures. Again limestone aggregates, Trucal 4, 5, 6 & 10, work well, aid the setting and are easy to fettle.

  • Q. Is your plaster of Paris bandage the same as the loose stuff we used to use and is it suitable for use with children to make masks.
  • A.Yes, there used to be bags of P.O.P bandage sold which was the waste product from making surgical quality bandages. This was ok for arts and craft. These days the process is so automated that no waste or off-cuts are produced. So there is really only first quality plaster of Paris bandages readily available in any great quantity. These are fine for making masks, scenery etc. the most important thing to watch for when children use it is that they don’t over work it by messing with it while its trying to set. It only needs a few seconds in water and then it becomes easy to smooth out which should be completed within a few minutes and then left to set. If it is worked too long it will end up with very little strength even when dry.
     
  • Q.I am trying to make a mould for making a porcalin doll. I believe I need either plaster of paris or dentists plaster.
    I would be grateful if you could let me know which one of your plasters is good for porcelain and also what release agent I need to put between the mould parts.
  • A.   You can use any of the beta* (*see FAQ's above) plasters, but for this sort of work Keramicast or Ceram 1 is often used. It is a little bit stronger and has very good surface definition. When slip casting you can't use oily of waxy release agents because they would remain on the mould and stop the suction. So potters always use soft soap (sometimes called black soap). This is applied diluted as a thin film. Because soft soap works by "killing" the plaster on the face it should be used sparingly to avoid getting a powdery face on the mould
  • Q.   YOUR QUESTION ? info@specialplasters.co.uk
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