The flame method can be used to process for latent prints; but it is also demonstative of why and how latent prints may be developed on some fire scene evidence.
The torch provides carbon soot, which settles on a surface. The soot sticks to the oily fingerprints. We remove the dry soot. Hopefully a fingerprint made visible by soot sticking to it remains.
This is what we may expect to find in some fire scenes: Soot provided by smoke coating a surface, soot sticking to fingerprints; removing the loose dry soot with a fingerprint brush may show us visible latent fingerprints.
Let's look at how to do this excersise:
Take a non-porous item, seed it with a fingerprint (rub your thumb on the side of your nose or forehead, and place an oily thumbprint on the item)
Make a torch with masking tape. Pull off two arm-lengths of tape. Fold the peices back on themselves, twice. Twist the tape ribon. light the end of this tape torch. Hold the item in the stream of soot / smoke eminating from the torch. Don't be too concerned about the flame, unless you are worried about your item catching on fire.
Continue until the surface with the fingerprint is coated. Now brush the soot with a fingerprint brush. the loose dry soot should be removed easily.
We should have a nice fingerprint left behind
The carbon in the soot is much finer than carbon in fingerprint powder, and develop a print with much finer detail than standard black powder.
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