Ridge Detail at a Crime Scene
by Mike Byrd
http://www.crime-scene-investigator.net/prints.html
Miami-Dade Police Department
Crime Scene Investigations

Many very good articles and books have been written on fingerprint technology. For the crime scene investigator or evidence recovery technician assigned the task of recovering fingerprint impressions at the crime scene, it is important that they have a good solid foundation of the basic knowledge. The knowledge should include the basic terminology, the equipment that is available , safety standards for the products being used and the basic skills that are being applied in the field.
Fingerprint identification
In criminal investigations fingerprints are one of the oldest and most common types of physical evidence found at a crime scene. One of the primary goals of the investigation deals with identification. Whether the identification is that of an unknown victim (body found at the side of the road) or that of the perpetrator of a crime. The ridge detail developed and recovered at a crime scene and later identified by a fingerprint examiner becomes an investigative lead (starting point) for the detectives assigned to the investigation. A fingerprint is simply defined as friction ridge detail of the hands and the feet. The friction ridges serve two basic purposes. First they allow us to grip and hold on to various surfaces. In forensics, they serve as a method of individual identification.
Fingerprints and The Crime Scene Investigator/Technician
Departments and agencies throughout the country vary as to whom is assigned the duties of processing the crime scene and recovery of the latent impressions.
The goal of the crime scene investigator or evidence recovery technician is recognition and recovery of friction ridge detail that might be deposited on the many different surfaces at a crime scene. The crime scene investigator/technician must have an understanding of the types of ridge detail that may be deposited on the scene, the types of surfaces that would retain the detail, and the methods used in recovering those deposits. The biggest challenge for technology today is adopting to keep up with the ever-changing surface materials.
Categories of prints
Different regions of the country have different phrases used to describe the terminology and techniques in ridge detail deposit and recovery. There are three basic categories or types of impressions that may be deposited, detected, developed if need be and recovered at a crime scene.
1. Latent
The skin has deposits of oil and perspiration that normally coat the surface. When the hand touches the surface, some of the moisture is transferred from the hand to the object, leaving an impression of the friction ridge detail. These are referred to as latent impressions. On most surfaces the latent impressions are not readily visible. The word latent is defined as not visible. That does not mean that it is actually invisible. Using available or oblique lighting on most surfaces will reveal the impression. The impression must be enhanced or developed to be seen fully and collected.
2. Patent
A patent impression occurs as the result of transferring a foreign material coating the skin of the fingers. Examples of the foreign material would be substances like paint, tar, grease, blood, or ink. The best example of a patent impression would be the ink standards or elimination's that we as investigators collect from individuals. After the ink is applied it becomes the foreign material coating the hands of the individual. A card is used as a transfer medium, thus becoming the object touched and a transferring of the patent impression takes place. The word patent means obvious or evident. Patent impressions are visible and usually need no enhancement. They are simply photographed and the item that they are deposited on is collected if necessary. The photograph and or object actually becomes the investigators lift.
3. Plastic or Molded
A plastic or molded impression is deposited when the hands, fingers, or feet is pressed into a soft rubbery type material that will retain the impression of the ridge detail. Where a latent impression is deposited on the surface, a plastic or molded impression would be deposited into the surface. Examples of the materials where a plastic impression would be deposited are clay, wet paint , blood, or tar. Plastic or molded impressions are visible and usually need no enhancement. They are simply photographed and if necessary the item is collected and may have the ability to be caste.
As a further example; If we were to ask the question, "Is a bloody print the same as a print in blood" ? The answer to this question would be, "NO". The print in blood would be an example of depositing a plastic or molded impression into the surface of blood. The bloody print would be an example of having a foreign substance, such as the blood, coating the hand and depositing an impression from the blood coating to another surface.
Discovery
The discovery process or visual search for a latent impression starts at a point of entry and works the way into and surrounding the crime scene. The search is to discover items that have been handled, moved or anything that appears out of place. The most suitable surfaces are smooth, hard, non-absorbent and non-porous surfaces. Patience, common sense, and experience are needed to assure that all latent impressions are collected. Continued practice will give the confidence needed to collect all types of impressions from different surfaces.
Recovery
In recovering latent impressions there are numerous products available for the task. The techniques can be broken down into four simple areas. The conventional methods consist of mechanical and chemical development. The more modern techniques applied today in recovering latent impressions are florescent die stains or powders (aided with some type laser or light source), and superglue fuming. It should be noted that latent impressions on some surfaces may be fragile.
Mechanical Development
Mechanical development is the use of a brush and powder to physically dust the surface. This technique allows the particles of the powder to adhere to the contaminates which causes the ridge deposits. The mechanical development is for nonporous items and surfaces.
There are three types of brushes manufactured for the use of mechanical ridge detail development:
1.	Fiberglass 
2.	Camel hair 
3.	Feather brush 
There are an assortment of powders, lifters and tapes that are specially formulated and manufactured for the use of impression recovery.
Chemical
For protection and safety reasons these methods are applied in the laboratory under well ventilated conditions. A variety of chemicals are produced to react to certain components that make up the ridge deposits. More than a general knowledge of the chemicals as well as the contaminates in the deposits are needed to perform these tasks. The chemical processing techniques are for porous, absorbent items such as paper, untreated wood, and matte, semi-porous finished items. 
Superglue Fuming
Cyanoacrylate ester is the active ingredient in superglue. It creates a vapor that polymerizes (creates a white residue) on contaminates such as latent impression deposits. Superglue is sold in a liquid or pouch form and comes in a variety of viscosity's. As documented by the US Army Crime Lab, superglue fumes are strongly irritating to the eyes and respiratory system. Personal Safety and caution are the keys to this development technique. The primary purpose of superglue in the field is to fix the impression on items where the latent impression may be fragile, so that it is less destructible during processing or transport. It does not make the impression indestructible. Without the proper caution the impression can still be wiped away or destroyed.
Contributing factors and conditions for deposit & recovery
These conditions contribute not only to the depositing of the ridge detail but also to the development and recovery stages.
•	Nature and cleanliness of surface 
•	Environment 
•	Weather conditions 
•	Amount of contaminant 
•	Pressure applied 
•	Movement during transfer 
•	Condition of the friction ridge detail 
Documentation
After developing the latent impression it is lifted with a clear tape or lifting medium and placed on a backing card with a contrasting background. The area where the lift was taken from is documented on the back of the card along with the case number designating it to a particular investigation, the name and identification number of the investigator lifting the impression, and the date of the lift. The integrity of the lift is maintained by the investigator's/technician's initials, a seal is created by the initials placed half on the tape that contains the lift and half on the card itself. To show no tampering has occurred.
Many Departments have a policy that each latent impression be photographed before being recovered. This is done mainly as an orientation to show the item the latent impression was recovered from, and as a safeguard just incase the latent impression is lost or destroyed during the recovery stages. The investigator/technician should follow his/her departments particular policies in this action.
Safety and Do's and Don'ts
Remove any excess powder from the surface of the developed impression before applying the lifting tape. This can be done by blowing on the surface, using a small can of compressed air or brushing lightly in the opposite direction over the area . This assures a clean lift with no air pockets (fish eye). Air pockets are the tiny bubbles of air that hinder the tape from being smoothly applied over the surface. The air pockets are caused by excess powder and other debris on top of the surface.
Wear a respiratory (dust) mask, gloves and eye protection when working with powders of any kind for a prolonged periods. It is important that the investigator/technician be aware of the tools that they are using in the field as well as in the lab. The Manufacturer Safety Data sheet (MSDS) is available with all products and should be read on any items used by the technician/investigator. This will assure that the investigator is aware of any potential safety hazard that might exist with the products being used. Safety should be the top priority for the investigator.
Never hold the bristle part of the brush. The brush should be free of any oil or contaminates. This assures the technician/investigator does not have a brush with hardened bristles that will rub out or destroy the print.
Never reach up to apply powder to an item above you. Use a ladder or step stool to keep the processing procedure below eye level. Small powder particles may cause irritation and/or infection to the eyes.
I have facilitated more than 100 8-hour workshops in crime scene recognition and latent impressions recovery since 1988. During each workshop I have acquired some new knowledge from my students. There is a tremendous pleasure in knowing that experienced students have walked away from those workshops with refreshed ideas along with enhancing the skills which they had already developed. First time students have walked away with the solid foundation of basic skills concepts needed for success and confidence.