Bodziak, William J. Casting a Footwear or Tire Impression with Dental Stone. (paper) March 1986.

Abstract: The procedures for mixing dental stone in both the Zip-Loc bag method and mixing bowl method are outlined. The the Zip-Loc method, powdered dental stone is premeasured and stored until ready to cast. At the scene, the correct amount of water is added and the bag closed. Massaging the bag mixes the materials thoroughly with no lumps. Viscosity should be adjusted to achieve a good consistency for a proper pour. The mixture can then be poured indirectly from the bag into the impression. In the mixing bowl method, the proper amounts of dental stone and water are added (water first) and stirred with a clean paint stirrer or spoon. Again, no lumps and proper consistency should be achieved. Dental stone, because of its hardness will not require reinforcement, but a form may be used to contain excess stone especially on a slope. Do not pour the mixture directly into the impression as it may damage it. Pour it next to the impression and allow it to flow into the impression or pour it onto a spoon just to the side of the impression. Fill to a proper depth to cover the impression. Allow the cast to harden for about thirty minutes, scratching in the identifying information onto the back before it dries completely. After you remove the cast, let it air dry for at least 48 hours. Do not clean the cast, leave this to the laboratory analyst. Dental stone is preferable to other casting mediums because it is harder and more easily cleaned. Depth need only be .5 to 1 inch thick and cleaning can be achieved with potassium sulphate solution and a fine brush.