Two dimensional impressions can be recovered from virtually any surface. For wet or residue impressions the main recovery tool is going to be a black gelatin lifter. Locate area of impression, photograph with scale and light as described above. Gently peel back acetate cover and wait a few moments for gel to return to natural size, starting at one end of the impression lay gel down, gently roll out air bubbles, peel up and re-photograph with oblique light and scale. Cover gel with acetate sheet and submit to lab.
In very dry conditions an electrostatic lifting apparatus (ESLA) often outperforms a gel lift. Cut an area of ESLA foil to fit the area of the impression. Lay foil over impression black side down. Place electrostatic lifter with ground points on grounding plate and charging point on film. Turn on and roll out using a wooden or rubber roller. Make sure not to touch foil or surrounding area while charging. Turn off ELSA. Leave for a few moments for charge to dissipate. Peel up foil and examine using oblique light. Photograph with scale and tape into clean flat box. Submit to lab. NB ESLA lifts are very fragile, exposure to dusty environments, physical contact and packaging in static inducing containers (plastic) while destroy the lift.
Impressions in suspected blood can be enhanced using a variety of chemicals depending on the surface they are left on. Please contact the lab to discuss enhancement/recovery. Please note that the most visible impressions are often not the best for analysis. Fainter impressions often reveal much more detail upon chemical enhancement.