I know that this is getting a little late for casting in snow, but here is the procedure for casting in snow with dental stone.
Casting can retrieve a three-dimensional footwear impression. In the past, the most common casting technique involved the use of plaster of Paris. This was a messy and not always successful process. Newer and better quality dental stone casting materials and simpler casting methods developed in recent years have made casting footwear impressions easier and more successful. Snow print wax is commonly used for warmer wet snow or slush prior to using the dental stone to help support the weight of the dental stone casting.
James May and Jason Cole of the West Valley Utah Police Department supplied the following instructions and photographs for casting in snow with dental stone.
Documenting and Collecting Snow Impressions
Photographing and Preparing a Snow Impression for Casting:
Spray pain (gray primer or other suitable color)
Snowprint wax (for warmer wet snow or slush)
Camera/film/scales/labels (detachable flash is useful)
1. Photograph the impression with a scale and label.
2. Spray to coat the impression with primer or snow print wax.
3. Re-photograph the impression.
4. Use appropriate oblique lighting from several angles for all photographs.
5. If impression is only partially coated, apply additional coats of spray paint/was prior to casting.
Casting with Dental Stone:
Ziplock bags (gallon size)
Wide mouth jar for water
Place approximately 2 cups of dental stone in a Ziplock bag. Add a tablespoon of potassium sulfate to the dental stone and mix the dry powders. The purpose of the potassium sulfate is to accelerate the curing process, either before the cast material freezes, or as it thaws out.
Cool the dental stone by laying the bag in the snow. Give the powder enough time to cool to the snow temperature.
Cool the mixing water by adding snow until a slight amount of slush is present.
Photograph impression, the spay impression with paint (cold/dry snow) or snow print wax (warm/ wet snow). Re-photograph impression.
Add the cold water to the cold dental stone and mix to a consistency of thick pancake batter. Try not to get ice crystals in the mix. Cut a hole in a corner of the bag. Carefully pour the mixture into impression, taking care not to damage the impression with the initial drop of material. Pour down the center of the impression. Allow the mixture to flow, wiggle with finger if needed.
Allow to thoroughly harden or freeze before lifting, usually about 1 hour.
Allow cast to thaw indoors overnight after lifting cast (keep the cast upside down while it is thawing).