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From- "Criminal Investigations" third edition by Charles R. Swanson, Niel C. Chamelin, and Leonard Territo.
First Step is to photograph impression(s) from all four axis's (north, south, east and west). The print should be photographed as found. Any material found in the print should be left.
Then, using a red-colored product called Snow Print Wax, the impression is sprayed until it is lightly tinted (the tint is normally a red tint but a dark colored spray paint will also serve the same purpose). The spray must be held far enough away so that the force of the aresol doesn't disturb the details of the impression.
Next, the impression is rephotographed from all axis points and then continued to be sprayed with Snow Print Wax until a layer of wax is formed.
Plaster paris is no longer recommended. Dental Stone is perferred because of its greater strength, quicker setting time, ease of use, and the fact that it provides more detailed impressions.
If a casting form is used, there should be a 2-inch space around the impression and it should be of sufficient height to allow at least a 2-inch thick cast to be made.
About 2 pounds of dental stone and 12 ounces of water are needed to cast an impression (note: this is for a normal human shoe impression, so I guessimate that you would double this for a 17 inch BF print).
When mixing the dental stone, initially use 3/4 the amount of water needed in the mixture, mix and then add additional water until the mixture is of the desired texture (should be like pancake batter).
Then pour the dental stone alongside the impression and allow it to flow into it or it can be gently laid onto the impression. To prevent the accidental destruction of detail, the of the dental stone into the impression can be broken by using a spoon or tongue depressor. The impression should be filled until the dental stone overflows the impression.
Dental stone sets fairly rapidly. In warm weather it can be moved within 20 minutes, but a longer time should be allowed in colder weather. When the cast is firm, but still soft, basic identifying information should be scratched on its back. The cast should not be cleaned or examined until it has dried for at least 48 hours.