The hydrochloric acid that Loeser applied to Dillinger’s fingertips was a surface treatment. It damaged the epidermis visibly and painfully. But the dermis beneath — the structural template of the ridge patterns — was not destroyed. When the skin healed, the ridges returned.
The criminal tradition of fingerprint removal by acid was based on a misunderstanding of the biology. The acid made the fingertips look damaged. It did not alter the template from which the fingertips regenerated.
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## The Criminal Underground’s Belief in Print Removal
The persistence of fingerprint removal attempts in the criminal underworld of the 1930s reflected a specific theory about how fingerprint identification worked, and that theory was wrong in an important way.
The wrong theory was this: fingerprints are surface patterns. Remove the surface pattern, and the fingerprint cannot be matched to records. The identification fails.
The correct understanding was more complicated. Fingerprint identification does not rely on a complete, perfect reproduction of a full fingerprint. It relies on the comparison of specific ridge characteristics — called minutiae — in whatever portions of a print are legible. An examiner looking at a partial print, a smudged print, or a damaged print does not need the entire fingerprint to work with. They need enough identifiable ridge characteristics in some portion of the finger surface.
A central alteration that obscures the core pattern area of a fingerprint still leaves the peripheral areas available for comparison. An examiner who cannot classify the central pattern — the loop, whorl, or arch that gives the print its basic identity — can still compare minutiae in the outer regions. If those minutiae match a known record, the identification can be made.