|
|
|
Vacuum Metal Deposition (The following paper is an abridgement of the text presented at the March 5, 1994 SCAFO meeting)
Speaker LISA DIMEO About a year ago my homicide division asked about vacuum metal deposition. I explained the technique to them, and its value in homicide investigations which involve polyethylene bags. “Vacuum metal deposition utilizes vacuum--coating technology for the evaporation of metals and the deposition of thin metal films. Thin layers of gold and zinc are deposited onto the surface of articles being examined. Disturbances in the physical and chemical nature of the surface are revealed by different rates of growth of the zinc metal film. Fingerprints appear either as positive or negative images in a grey film of metallic zinc. The process is very sensitive and can detect monolayers of fats on smooth surfaces. It is most commonly used on plastic packaging films, particularly polythene, but is also effective on many other smooth surfaces.”1 I decided to go back to our archival cases which involved plastic bags. I needed to find cases where, if prints were able to be developed, we would have suspects in mind and would be able to go forward with the case. I weaned it down to eight cases. The oldest was from 1977 and the newest was one of my cases which was only a year old. My information as to the processing of these bags was sketchy. The reports might just say that it was processed and no latents were found. That was a real trick in itself, trying to bring up the past and figuring out what had been previously done to the exhibit. Most of them had blood products and contaminants on them. Some of the bags appeared to have been processed with black powder, CA, and even silver powder. (It is uncertain whether the technique is useful as a sequence after CA processing.) Taking my evidence with me, I went to the Royal Canadian Mounted Police in Ottawa, Canada, to use their vacuum metal deposition chamber, which is an Edwards E--600 chamber. This type of chamber was actually being used a few decades ago to metal coat or anodize electronic components. At some point it was discovered that fingerprints could be developed with the use of these chambers. Much red tape was involved to enable us to take our evidence to Canada (mostly on our part). I had to hand walk it in to them and work alongside the investigator so that the chain of evidence would not be broken and the investigator would not have to come to the States to testify. I arrived and met with Sgt. Pat Laturnus. (I don't want to say he is the sole operator of this piece of equipment, but when he gets moved to Saskatchewan in about a year, it's going with him.) We were able to develop latent prints on 3 of the 8 cases, including the 1977 case. (Prior to my trip, the oldest print developed by the RCMP was 6 years old--—a case investigated by Diane Donnelly, then of the San Diego Police Dept.) Through my experience with this technique, I have found it works well on evidence which is multi--colored, such as orange juice or milk cartons. It works on fine weave fabric (i.e. synethic panties, windbreaker fabric), paper, and many other surfaces, even if they have been wet. The RCMP's chamber was custom made for their building at an approximate cost of $100,000. Smaller chambers are available for approximately $30,000. The Home Office recommends photographing the prints immediately after development because zinc will eventually oxidize, possibly as quickly as within one hour. However, I have been back since November and the prints are still beautiful.
Endnote:
Additional Reading: (copies distributed at the meeting) (Editor--—In addition to this informative lecture, and the handout literature, Lisa showed slides of cases and some prints which are truly beautiful. Lisa is also planning to write a paper on this subject in the future.)
This article was originally published in “THE PRINT” 10(3), March 1994,
pg 4
|