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Casting Processed Latents with Dental Stone

An innovative method for lifting two-dimensional footwear and fingerprint impressions has been reported. This method can be used to recover impres­sions from difficult dimpled surfaces - such as bank counters, computers, VCRs and cash registers - where other standard methods fail.

The discovery was made when Detec­tive Constable Wade Knaap, of the Toronto Police Service was examining a break-and-enter scene at a business premise and developed a footwear impression on a textured Arborite® counter, using black magnetic powder to enhance this impression. After the usual photographic session, D/C Knaap (knowing that poor results are often obtained using fingerprint tape on such surfaces), decided to use dental stone in an attempt to "lift" the impression. (All Toronto Police Service forensic vehicles are stocked with baggies of dry dental stone, ready to mix.) The dental stone was poured onto the enhanced impres­sion and allowed to cure. The hardened dental stone readily released from the counter with a slight nudge. The result was an indelible, two-dimensional, exceptional quality footwear impression embedded into the dental stone.

Detective Ed Adach and D/C Wade Knaap conducted the following experimentations at the Toronto Police Service Foren­sics Identification Services laboratory.

Footwear Impressions

Purpose: To determine the ability of dental stone to lift two-dimensional footwear impressions.

Setup: Twenty-one tests were conducted, using dental stone to lift footwear impressions.

The footwear impressions were placed on six surfaces of various texture, such as a dimpled plastic Sterilit & container lid, a dimpled plastic Rubbermaid® container lid, unfinished oak, unfinished pine, a dimpled binder cover, and a smooth counter top.

Test impressions were made using dirty/slightly greasy footwear, dirty/ slightly muddy footwear, and damp footwear. Black Peavey® brand magnetic powder was used on some of the impres­sions, while the remainder had no further enhancement.

Lifting

The prepared dental stone was poured onto the footwear impression. Once cured, the dental stone readily released from the dimpled surfaces. Dental stone does adhere well to smooth surfaces, but will "pop" off with sufficient force.

Results

1. In all tests, the dental stone produced satisfactory lifts.
2. The quality of detail in the lifts
appeared to be equivalent to the orig­inal impressions.
3. White dental stone provided good con­trast.
4. The grit and dirt that impede other methods do not obstruct dental stone.
5. Lifted impressions are indelibly embedded into the dental stone.
6. Pen and pencil markings are also lifted and preserved in the dental stone casting.
7. Permanent marker remains on the sub­strate and is not lifted into the dental stone.
8. Castings of footwear impressions can be easily photocopied 1:1.

Fingerprint Impressions

Purpose:Todetermine the ability of dental stone to lift fingerprint impres­sions. To compare dental stone with other methods of lifting

Setup: The fingerprint impressions were placed on 13 surfaces of varying textures such as, a computer hard-drive case (dimpled), unfinished pine, a telephone jack wall cover (dimpled), primer paint on baseboard, a license plate, the body of a Dustbuster©, factory urethaned oak an IBM© keyboard (dimpled), semi-gloss painted drywall, a Dell© computer moni­tor case (dimpled), vinyl coated fiber board shelf (dimpled), a Rubbermaid© plastic container lid (dimpled), and an Arborite© counter (dimpled).

Five fingerprint impressions were deposited onto each of the substrates to be tested. These 65 fingerprints were then enhanced using Peavey© brand black magnetic fingerprint powder.

Lifting

The five fingerprint impressions on each surface were individually lifted using the five methods listed below:
1. Dental stone (buff), item No. T02G04G0002 - Supplier: Ash Temple
2. Dental stone (white), Item No. T249G470031 - Supplier: Ash Temple 3. Stretchy fingerprint tape (poly-200),
4. Rubber lifter
5. Regular fingerprint tape (LP-200)

Comparisons

All 65 fingerprint impressions were photographed prior to lifting and subse­quently all 65 lifts were photographed.
Since all fingerprint impressions natur­ally vary in quality of detail, a fair and impartial comparison between the lifted impressions could not be made. All fingerprint impressions were compared with photographs. The dental stone cast impressions of the fingerprints were com­pared to the photographs taken prior to their being lifted. The quality of the lift method was determined by the degree of detail retained on the lift.

Results

1. The dental stone consistently provided satisfactory results in all cases.
2. The stretchy tape provided satisfactory results in all tests but one - being the dimpled Rubbermaid 0 container lid.
3. All methods provided satisfactory results when used on the smoother surfaces.
4. The rougher textured surfaces required the use of dental stone or stretchy tape to provide adequate results.
5. Lifted impressions are indelibly embedded into the dental stone.
6. White dental stone provided good con­trast.
7. Identifiable matrices often remained on substrates, even after initial cast was lifted.
8. No damage was sustained to any sub­strate during the tests conducted using the dental stone "lift" technique.

Conclusions

Advantages of Dental Stone Lifts:
- Excellent quality of detail in lifted impressions
- Excellent method of lifting from dimpled surfaces
- Excellent preservation of evidence
- Much lower cost that stretchy tape and rubber lifters

Disadvantages of Dental Stone:
- Requires more effort than other methods
- Cast impression is too thick to load directly on Automated Fingerprint Identification System, (AFIS) - impression must be photocopied or photographed at 1:1.
- Storage of lifted cast impressions is required.

Based on the research performed, dental stone casting appears to be a viable alternative to other more estab­lished techniques which are presently being used for the lifting of impressions and preservation of footwear and finger­print impressions. It is an excellent tool for use on surfaces which were previously considered to be unsuitable.




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