Developing and Lifting a Latent Print
Chapter Eighty-Nine
Syllabus topic 3, "An Introduction to :"
Pages 351 to 354 of 654
In one line
A latent print is invisible, and each method of making it visible works on a different kind of surface.
In the wording a student can write in an exam: prints found at a scene are of three kinds, the visible or patent print left in a coloured medium such as blood, ink or grease, the plastic or moulded print impressed in a soft material such as wax, putty or soap, and the latent print, which is invisible and consists of the sweat, oil and other residue transferred from the ridges; a latent print is developed by a method chosen for the surface, powder and magnetic powder on smooth non-porous surfaces, ninhydrin and its successors on porous surfaces such as paper, cyanoacrylate fuming on non-porous surfaces including plastics, small particle reagent on wet surfaces, and physical developer on wet or previously wetted paper; and after development the print is photographed with a scale before any attempt is made to lift it.
What a latent print is made of
Sweat, from the eccrine glands that open on the friction ridges, which is mostly water with salts and amino acids.
Oil and lipid, picked up from the face, the hair and the skin elsewhere, because people touch their faces.
And whatever else was on the hands.
Why that matters. The composition decides the method. Amino acids are what ninhydrin reacts with, so it works on paper where the residue soaks in. Lipids and moisture are what powder adheres to, so powder works on a smooth surface where the residue sits on top.
The three kinds of print at a scene
Patent, or visible. Left in a coloured medium: blood, ink, paint, grease, dust. It is photographed and needs no development.
Plastic, or moulded. Impressed into a soft material: wax, putty, soap, fresh paint, chocolate. It is photographed with oblique lighting and may be cast.
Latent. Invisible, and the subject of this chapter.
The methods, matched to the surface
| Surface | Method | How it works |
|---|---|---|
| Smooth, hard, non-porous: glass, metal, tile, painted wood | powder, applied with a soft brush | particles adhere to the moisture and oil of the residue |
| The same, where a coloured powder would not contrast | magnetic powder with a magnetic applicator | no bristles touch the print, so a fragile print survives |
| Porous: paper, cardboard, untreated wood | ninhydrin, and its more sensitive successors | reacts with the amino acids in the residue, producing a purple colour |
| Non-porous, including plastics and adhesive tape | cyanoacrylate fuming, using the vapour of superglue in a chamber | the vapour polymerises on the ridge residue, forming a white deposit which can then be dyed or powdered |
| Wet, non-porous | small particle reagent | fine particles suspended in a solution adhere to the lipid |
| Wet or previously wetted paper | physical developer | deposits silver on the lipid component, which survives wetting when the amino acids do not |
| Adhesive side of tape | gentian violet or a specific reagent | stains the lipid |
| Any, before anything else | oblique lighting and photography, and where available a forensic light source with filters | shows a print by reflection or fluorescence without touching it |
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