|It’s getting to be that time of the year when winter comes and the snow falls. Footwear tracks can easily be found in the snow. Casting with plaster of Paris and now dental stone is commonly used by crime scene officers. Some of the problems that arise is the snow would collapse under the weight of the dental stone, destroying the impression. Snow print wax is commonly used prior to using the dental stone to help support the weight of the dental stone casting.
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. An alternative method to using dental stone is to use a method of sulfur casting to make a mold of the impression left in the snow.
I have read several articles on sulfur casting and it sounded difficult, complicated and I thought I could not stand the odor of the sulfur. I attended a class in Utah that was put on by James May and Jason Cole of the West Valley Utah Police Department. The sulfur casting method is simple and not complicated at all. The results are fantastic.
Casting with melted sulfur involves melting the sulfur, cooling it to a temperature just above its crystallization point, and then rapidly pouring the liquid sulfur in the snow impression. The sulfur quickly cools as it comes in contact with the snow and immediately re-crystallizes, capturing the impression. The cast must sit undisturbed for at least 30 minutes to allow the inner areas to cool completely. Sulfur casts should be handled with care, as they are fragile and brittle.
James May and Jason Cole of the West Valley Utah Police Department supplied the following instructions and photographs for casting with sulfur.
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 Sulfur:
Information courtesy of Anja Yitti, National Bureau of Investigation, Vantaa, Finland.
Sulfur pellets (1.5 kg for one impression, or approx. 3 cups of pellets), Pot, Spoon, and Heating plate or burner
1. Melting the sulfur
- the melting point of sulfur is +119 C
- the flash point of sulfur is +207.2 C
- the self ignition point of sulfur is +232.2 C
- be careful the sulfur does not catch fire while melting
- the gas caused while melting the sulfur is harmful and poisonous
- always melt the sulfur outdoors
- the sulfur must be totally melted in the pot
Read the MSDS for sulfur before using!
- if the temperature of melted sulfur exceeds +160 C, it becomes dark, elastic and chewy. If the temperature exceeds +220 C it becomes liquid again. When cooling the sulfur it goes through the same alterations.
- with an effective heating plate or burner the sulfur melts within 10 minutes
Total time for the procedure, for melting and cooling, before the sulfur is ready for pouring will take nearly half an hour.
2. Cooling the sulfur
- cool the melted sulfur by stirring it with a big spoon or scoop continuously so long that the sulfur becomes grainy. The sulfur is then a liquid, non-elastic gruel and its color has lightened
- stir the sulfur all the time so that there is no possibility for the sulfur to begin to harden in the middle of the pot
- do not put the pot in the snow, water or any place where it begins to cool quickly because then the sulfur hardens too soon and you may not have time enough to pour it into the impression
- if this happens you have to melt the sulfur again
3. Pouring the sulfur into the impression
- when you pour the sulfur into the impression, it must be grainy and the temperature must be as close as possible/near the melting point, this is because it then hardens instantly after pouring
- cool the sulfur always close to the impression you are going to cast
- surround the impression with a frame is needed
- start pouring on the lower part of the impression and direct the melted sulfur evenly over the impression
- pour the melted sulfur close the to snow surface
- the casting must be at least 2cm thick
- do not pour another layer
- do not use and extra things to strengthen the casting (for example sticks or wires)
4. Lifting the sulfur cast
- lift up the cast from the snow after it has cooled, that means after you can handle it with bare hands
- do not leave the sulfur cast to the impression for a long time because it can freeze to the ground
- put the cast on a firm surface
Note: Handle the cast very carefully, it will be fragile and will break easily.