Making a Cast:
- 2-3 pounds of class- 1 dental stone per cast
- One-gallon zip-lock plastic bag per cast
- One large mixing bowl or coffee can
- One spoon or stirring stick
- 6-9 oz of water per pound of dental stone
- One form (not always necessary, but useful)
- One can of Talcum powder
A form, or other material with which a form can be made around the impression to contain the dental stone, gives the cast thickness and is handy, but is not necessary.
Procedure 1: Place the form around the impression. Be careful not to place the form so close to the impression that it risks distorting it. Place 2-3 pounds of dental stone in the mixing container. Add roughly 6-9 oz. of water per pound of dental stone. Mixing usually takes about 5-7 minutes or until “pancake batter” consistency.
Procedure 2: Take the 2-3 pounds of dental stone that is contained in a one-gallon zip-lock plastic bag and add your water accordingly. Close the zip-lock bag and mix by meshing together.
Pouring: Before pouring the mixture onto the impression, very lightly sprinkle talcum powder over the impression, which gives a fixative. Be sure not to over sprinkle. Pour the mixture onto the spoon or stirring stick while holding the spoon or stick to the side of the impression. By pouring the mixture in this manner ou can direct the flow of the mixture and let it flow back into the impression. If you use the plastic bag technique, cut one of the corners off the bag and pour in the same manner. By using the plastic bag technique there is very little mess and no clean up.
DO NOT POUR THE MIXTURE DIRECTLY INTO THE IMPRESSION!!
Since you are using dental stone you do not need to use reinforcement material. Before the cast can harden, scratch the necessary identifying information on the back side. This information may differ from one police department to another.
- name or initials
- case number
- other important information
The cast must set undisturbed for at least 30 minutes, after which the cast can be carefully lifted from the surface. Place a knife one inch form the cast and stick it directly into the ground and under the cast. Pry upwards. The cast should break free from the surface. Some soil or dirt may adhere to the cast, however DO NOT attempt to clean it. Allow the cast to air dry at least 24-48 hours before cleaning. After this time the cast can be cleaned and washed VERY GENTLY. Once this is completed the cast can be packaged for impounding.
DO NOT impound or package the cast in plastic. Use porous packaging material, making sure to carefully package the cast so that it will not break during transport.
Replacement Casting Material: You can replace dental stone with plaster of paris (which can be purchased at almost any hardware store). The plaster of paris requires 15 oz. of water to 5 lbs. of plaster and will require a form and reinforcement material.
The pouring procedure is the only difference from the dental stone.
Pouring: Using a form, pour the mixture onto the stirrer while holding the stirrer to the side of the impression. After pouring about half of the mixture onto the impression, add the reinforcement material (i.e.; sticks, small pieces of metal wires) and then complete the plaster pouring.