The Legal Investigator Vol. 37, Issue 3 Fall 2012 : Page 3

Proposition 2 A clean finger made contact with blood that was already on the object. These types of marks are dependent on the type of object, pressure of the finger and the rate at which the blood dries. Figures 2a and 2b demonstrate how fingerprints are formed when they come into contact with blood at various time intervals. Figure 2b shows that the optimum time for formation of a good mark was around fourteen minutes after the finger came into contact with the wet blood. These types of marks are less common but can often be considered by the defense counsel in an attempt to reduce the impact of the fingerprint evidence. This is because if the fingerprint was simply from sweat with very little sebaceous material, then there isn’t much interaction with blood. The majority of sweat is water soluble, so it dissolves away when it comes into contact with blood. ( Figure 4 ). Figure 4. There is no interaction with the blood and the fingerprint in sweat. However, as the grease content of the fingerprint increases, then there is interaction with the blood ( Figures 5 and 6 ). Figure 2a . As the finger lifts from the surface of the fresh wet blood, the blood gathers in the middle of the mark. Figure 5. Interaction with a greased fingerprint with blood. Figure 2b. Optimum time -fourteen minutes. Proposition 3 Blood has come into contact with an existing finger mark on the object. fall 2012 Figure 6. Blood migrating along the edges of the fingerprint. 5

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