Fingermarks in blood: Mechanical models and the color of ridges

This article treats fingermarks in blood on non-porous surfaces and addresses the question of “which came first”: the fingermark or the blood. Three mechanical models were systematically examined: (1) A blood-contaminated finger pressed against a clean surface; (2) blood contaminates a latent print that had been placed on a clean surface; (3) A clean finger pressed against a blood-contaminated surface. The questions of reliability and limits of all three models were discussed. The relevancy of the approach to “which came first”, based solely on the color of ridges was questioned. The first mechanical model most simulated a real situation, when previously cleaned, a blood contaminated finger touched a clean Formica or glass surface with pressure of 100-500 g. Concerning the second model, it was observed that in the case of a greasy latent print, placed on an inclined surface and contaminated with appropriate amount of blood, the color of ridges were normally darker than the color of its valleys. As for the third model, it was concluded that it works only in about 25% of cases. While investigating this model, two phenomena were observed: ridge color inversion and valley color inversion. In conclusion the color of ridges can not be the only and ultimate indicator to the question of “which came first”, the fingermark or the blood stain.