2 CASTING TECHNIQUES FINAL.mp4 — 102011 KB
Bryan T. Johnson, Major Incident Program Manager, Latent Print Unit, FBI Laboratory Division: Welcome to another installment of the Postmortem Fingerprinting video series.
In this video, we outline the use of casting materials to replicate the prints of hands, which may not be printable by traditional means. Commonly, when the skin is mummified, it may be too three-dimensional for basic printing methods. While photography can often help capture these types of prints, casting is a great way to physically print the friction ridge skin without having to use chemicals for rehydration. Hopefully after watching this video, you will feel educated and able to cast any decedent using a variety of techniques to affect an identification.
Lastly, we would like to thank the District of Columbia Office of the Chief Medical Examiner for their collaboration and their participation in this series, ensuring that everyone watching is provided with the tools needed to identify unknown deceased everywhere.
In the case of moderate mummification, when the fingers really won't easily bend, slow and steady pressure can sometimes allow them to straighten out. In extreme mummification, this may not be possible. And even with cutting, they may not pivot. Care should be taken, however, not to jump right into removing the digits. Examining the fingers, it appears that they're very pruned; the wrinkles are not pliable, and they're very hard to the touch.
One of the options for dealing with this is photography. Another option is to chemically rehydrate the hands, and the third option is to use casting materials. Rehydration solutions that require chemicals can take hours or even days to complete. So this is a good place to start if photography is not viable or did not provide a good print capture.
Casting materials come in different varieties based on the manufacturer. The easiest style to use is one that premixes in the tip of a dispensing gun and often allows for a cleaner application process. Other varieties that require manually mixing two or three parts still work just as well.
To attempt this technique, the best method is to evenly apply black fingerprint powder first to the area being casted and then to apply white casting material over it. This allows for a nice contrast when the lift is removed, so the ridges of the cast are easily scanned or photographed without further alteration or processing required. To apply it using the gun dispenser, the handle is squeezed slowly while the casting material and hardener mixes in the tip as it comes out. It is a two-part polyvinyl siloxane putty or essentially a special type of quick drying silicon mold.
As the mixture leaves the tip of the gun, it should be evenly distributed over the area intending to be casted, ensuring that it gets in all of the wrinkles and crevices and there are no air bubbles below the surface. A tongue depressor or spatula may assist in even distribution as well. Depending on the thickness and temperature, drying times will vary. Refer to manufacturer's instructions to ensure best results. Multiple fingers or areas can be casted at once for efficiency. Care should be taken upon removal to ensure the lifts are labeled appropriately. One easy labeling method is to use a permanent marker on the dried cast prior to removal to indicate the finger number. Alternatively, small cards with numbers on them can be pressed into the casting as it is drying so that it adheres to the cast.
For comparison, one finger will be casted without powder so that the difference can be seen. After it dries, the cast can be slowly removed by peeling gently from one corner, ensuring not to tear the casting. The reason for advising to apply the powder before casting is that it is very difficult to see the contrast of ridges and any color of cast directly. The cast can be powdered after, but there is a risk in over powdering or damaging the cast if it wasn't quite solidified. The cast are not easily cleaned once over powdered. By casting the mummified fingers, it allows a fingerprint that was not previously capture-able through traditional ink and paper or powder and adhesive lifter methods to be captured in a timely manner with limited resources.
Depending on the hue of the skin after the mummification process, it can be just as difficult to obtain the contrast needed to use the print when photographed. Casting it accounts for the different wrinkles of the finger and allows the resulting cast to be searched or compared for identification. For preservation, the cast should be taped onto the back of the acetate sheet so that it can be scanned into a fingerprint database and searched for possible identification. Alternatively, they can be labeled and bagged individually.
After attempting to powder the casting from the previously the unpowdered finger, it can be shown that it does not have the same contrast or quality as the cast of the finger that was powdered in advance. It doesn't have quite the clean look as when it's pre-applied to the finger, because it doesn't have the same even distribution.
The final products can be seen here in these images. The first image is obviously a pristine example of a casting, but casting can be very effective regardless of the surface contour if done properly. Even with casts, which may be of lower quality due to skin wrinkles, it can still provide an identifiable print. As long as the ridges are clear and have contrast, a photograph of the cast will be enough to make the identification.
Mummified fingers are very often discarded as unprintable due to their difficulty and appearance. In this installment, we explored some of the tools that can be used for postmortem print casting. Hopefully through the use of this technique, even daunting cases will become easier to print and the successes will be seen through identifications.
The exact products used are not imperative, but sticking to a white casting material with black powder really does give the best contrast. These lifts are not permanent, but they will last for years if stored properly. We encourage you to watch the remaining videos in this series so that you may learn rehydration and reconditioning techniques that can further assist in obtaining the prints needed to identify the deceased. We also encourage you to reach out to the FBI Laboratory with any questions you may have regarding this content or any case related questions we may assist you with.