When all else fails, knowledgeable crime lab technicians turn to Physical Developer
In the 1970s, the new kid on the block of latent print development chemicals was Physical Developer.
As effective as they may be, Iodine fuming, DFO and ninhydrin do not always produce desirable results. The blessing here is that these chemicals do not interfere with subsequent development with Physical Developer.
On porous materials, a reaction occurs between the lipids, fats, oils, and waxes found in sebaceous sweat and the silver-based liquid reagent known as Physical Developer (PD), to produce a silver-gray deposit visible to the naked eye. Since these sebaceous components are not soluble in water, the PD latent print development technique is particularly useful on wet paper or paper that has previously been wet. When the amino acids in fingerprints have been washed away or failed to react with Ninhydrin, or DFO; processing with PD may still produce identifiable latents. However, PD is time-consuming, expensive, destructive, and has a short shelf life. Therefore, one should only utilize PD as a final process in circumstances where it will be the most effective. (Forensic Magazine) http://www.forensicmag.com/article/comparison-physical-developer-formulas
The most common commercial formulas for PD include silver nitrate, ammonium iron (II) sulfatehexahydrate and other equally complex reagents.
Recommended Development Procedure
Some labs recommend a pre-wash of the document in a maleic acid solution:
1. Place the mixed chemicals in a clean glass or plastic developing tray. Do not use metal trays.
2. Immerse the evidence in the solution. Prints should appear in about 5 minutes. Allow development to continue for up to 15 minutes or until the background begins to darken.
3. Rinse the evidence under running water for up to 20 minutes. Air dry after rinsing with document lying flat.
4. Once dry, photograph any visible prints. Be certain to include a scale in each photo.
Learn more from the Technical Bulletin: “Overview of Latent Print Development Techniques”
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