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## The Fingerprint Procedure: What Loeser Did to the Hands
The fingerprint portion of the procedure was separate from the facial work and was based on an understanding — widespread in criminal circles, though incomplete — that the ridge patterns of fingerprints could be permanently destroyed by removing or damaging the outer layer of skin that bore them.
Loeser’s approach combined chemical and mechanical methods. He applied acid to the fingertips — hydrochloric acid was the chemical identified in accounts of the procedure — which chemically damaged the epidermis and the ridge patterns it contained. He then scraped away the remaining visible ridges that the acid had not fully destroyed.
The procedure was painful. Dillinger could not use his hands for several days afterward. The fingertips were raw and damaged in ways that were immediately visible.
What Loeser, Dillinger, and others in the criminal underworld who attempted similar procedures did not fully understand was the biology of fingerprint formation. The ridge patterns of fingerprints originate in the dermis — the deeper layer of skin — not only in the epidermis that sits above it. When surface damage heals, the ridges grow back guided by the same dermal structures that formed them originally. Destroying only the epidermis, even aggressively, does not destroy the underlying pattern.
The acid was applied with particular concentration on the central areas of the fingertips — the areas containing the core pattern structures (loops, whorls, arches) that fingerprint classifiers primarily examine. These central areas bore the most visible damage. But the edges of the fingertips, where the ridges run outward toward the sides of the fingers, received less concentrated treatment and retained clearer ridge detail.