ABSTRACT
Visible bloodstains and impressions present at crime scenes are often accompanied by the less obvious partial and latent impressions of footwear, palm and fingerprints, and other objects such as fabrics and weapons that may prove crucial to the investigation and scene reconstruction. Partial or latent impressions in blood may prove to be of superior quality for identification of class and individual characteristics after chemical enhancement than those impressions that are more heavily bloodstained. Therefore, a working knowledge of blood-enhancement techniques and their preparation, in conjunction with experience in their application in specific situations, is essential. There are numerous chemical enhancement procedures available for selective application, and the analyst should consider the following factors before choosing any enhancement procedure:
• The suitability of the technique for the specific crime scene with respect to the potential health hazards posed by the chemicals involved in the procedure that may dictate that the procedure be carried out under laboratory conditions
• The color of the enhancement relative to the contrast between the blood impression and the surface or substrate on which it exists
• The effectiveness of the technique on a porous versus nonporous surface • The possible chemical damage to the surface • The feasibility of lifting the enhanced impression from the surface or substrate • The feasibility of optimizing the visibility of the impression using an alternative
light source
The criteria by which the blood-enhancement techniques were selected for inclusion in this chapter are safety, availability of chemicals and equipment, ease of reagent preparation, and their stability and ease of application to the surface textures where blood impressions are commonly located. The following chemical enhancement techniques will be discussed in detail:
• Amido Black Water-based • Amido Black Methanol-based • Aqueous Leucocrystal Violet (ALCV) • Crowle’s Double Stain • Coomassie Brilliant Blue • Hungarian Red • Titanium dioxide
Prior to beginning a chemical enhancement technique at a crime scene or with an item of physical evidence in the laboratory there are recommended preliminary procedures that should be performed. The blood impression should be examined visually with and without the use of an alternative light source. After this visual examination the analyst should photograph the visible area of the impression. Initial photographic documentation prior to using an enhancement technique may serve as a record should the enhancement technique fail and destroy the original impression. The impression should be documented as to its exact position at the crime scene or on the object.
