Advances in Fingerprint Technology.pdf

Water and Acid Pretreatments
The water pretreatment involves distilled water and is used to remove any
dirt or debris that may be present on the evidence. It will also remove
ninhydrin-developed prints and stains as well as water-soluble inks. The acid
pretreatment is absolutely necessary for basic papers because these usually
contain (alkaline) calcium carbonate.
Silver Physical Development
This part of the process produces the visible prints, with their color ranging
from gray to black. It should be performed in subdued light because silver
nitrate can photo-reduce to silver and increase background development; it
also reduces the activity of the developer. Prints begin to appear within 5 to
10 min and processing should stop once sufficient contrast is achieved. Processing
time ranges from 10 to 30 min.
Water and Hypochlorite Post-Treatments
Once the development is complete, the evidence is washed in running tap
water. Some recommend distilled water because tap water produces colloidal
size silver chloride particles that can become lodged in the paper and darken
with time. We have found that using running water for about 5 min gets rid
of these particles and chemicals from the Ag-PD (e.g., ferric, ferrous, and
silver ions). To enhance the darkness of the print and lighten the background
(assuming the background is not high in silver physical development), Phillips
et al. 2 found that treating the evidence with a 50% solution of household
bleach (hypochlorite) for approximately 2 to 3 min gave this result. If the
background is high in silver physical development, then it too will darken.
The reaction occurring is the conversion of silver to silver oxide. After this,
the evidence is washed again in running tap water and dried; drying can be
done with a photodryer. The evidence is photographed using film. The resulting
photograph can be digitally scanned for digital enhancement.
Examples
This chapter section provides casework examples of additional prints developed
by the Ag-PD process, ninhydrin prints enhanced by the Ag-PD, Ag-PD prints
enhanced by hypochlorite, and Ag-PD prints enhanced by digital processing.
Figure 7.3 compares a ninhydrin palm print on a letter and the same letter
after Ag-PD treatment. The Ag-PD process brought out an additional fingerprint
in the middle of the palm print. Figure 7.4 shows how the Ag-PD process
enhanced a ninhydrin print on a forged United States Treasury check. Figure 7.5
shows two things: the enhancement of a ninhydrin print on a counterfeit $20
note and the removal by the Ag-PD treatment of the jet ink that printed the
counterfeit. Figure 7.6 shows an Ag-PD print before and after enhancement with