Fingerprints left at scenes are usually latent — invisible deposits of sweat and oils. Before they can be compared, they have to be developed, and the right technique depends almost entirely on the surface.
Non-porous surfaces (glass, metal, plastic)
Here the residue sits on top of the surface:
- Powders — fine powder adheres to the residue and is then lifted with tape. Simple and standard.
- Cyanoacrylate (superglue) fuming — vapour polymerises on the ridges, forming a tough white cast of the print, often then dyed or dusted.
Porous surfaces (paper, card, unfinished wood)
Here the residue soaks in, so chemical reagents are used:
- Ninhydrin — reacts with amino acids in the print to produce a purple colour ("Ruhemann's purple").
- DFO — a fluorescent reagent, more sensitive than ninhydrin, viewed under a laser or forensic light source.
Special cases
Other tools handle awkward situations — small particle reagent for wet surfaces, chemical treatments for prints in blood, and forensic light sources to make some prints fluoresce without any treatment.
Order matters
Development can be a one-shot decision. Powders and reagents can destroy the print if used in the wrong order, and can also compromise DNA in the same deposit. So examiners plan a sequence — often the least destructive and DNA-friendly methods first — before touching the mark. Once developed, the print is photographed at high resolution and, where possible, lifted, ready for comparison.