Vacuum Metal Deposition
This blog is about VMD , yes getting right its Vacuum metal deposition.
Some forensic students think that its a new technique but actually its an old one.
Tolansky in 1964 stated that Vacuum system can develop the fingerprints using
silver metal deposition method.
Hambley in 1972 produced a thesis in which described about developing
fingerprints using gold or silver followed by another layer of cadmium or
zinc. In old system the maintenance cost was high and it was very expensive
, so only few labs utilized it.
Now come to VMD - Vacuum metal deposition is the most powerful and
sensitive technology to develop latent fingerprints by the process of metal
deposition on non-porous and semi-porous exhibit such as
Plastic bottles
Flexible plastic packaging
Glass
Fabrics
Firearms
Waxed paper
Glossy paper
Polymer and paper Bank notes
Wood
Cell phones
IED's
Latex gloves etc
Vacuum metal deposition Process - In the deposition process on exhibit(in Vacuum chamber) , Gold is evaporated and gold nuclei (very small) form a fine layer on the surface but these layers are discontinuous , to maintain even coating low pressure is used in the VMD chamber for coating. Gold nuclei not only disperse over the surface but they also diffused in to the fatty acids present in the ridges. Gold used in cylindrical wire form usually 0.5mm of diameter. 5mm of length wire used, gold particle boiled at 1600 degrees Celsius .
Now its time to apply zinc. Zinc has a melting point of around 420 degree Celsius. Zinc evaporates directly to gas. The zinc particles agitates very fast when the boiling point of zinc reached .The layer of zinc covers the whole surface leaving the friction ridges of the fingerprint. More air pressure in to the chamber serve the purpose. This results in a negative print of the fingerprint. Where the zinc region looks grey .
Other metals such as silver , sterling silver , aluminium and copper also led to the expension of the VMD technique. Cadmium can be used but due to its toxicity it is avoided. When Vacuum metal deposition is used to develop latent fingerprints on polymer surfaces, a Au layer is used first then applied the zinc layer. Fingerprints developed on fabrics by using this technique found to be more effective than using cynoacrylate.
Speciality of Vacuum metal deposition -
1) Developed fingerprints more than 20 years old.
2) Results on those exhibits which are submerged in water or buried underground are remarkable.
3) Fingermarks developed by Vacuum metal deposition are of higher definition, often to third level detail.
4) Developed prints has superior contrast than prints developed by cynoacrylate fuming technique.
5) Vacuum metal deposition technique is very rapid typically take less than 10 minutes.
6) Used for items where fingerprints residue may have been removed.
7) very small amount of metal is used , for Ex only 4 mg of silver wire (0.25 mm diameter)is used .
8) Used for items exposed to high temperature.
The factors which affected the process in Vacuum metal deposition chamber.
1) Dirt and oily substances on the surface like grease affect the development phase.
2) Local difference in chemical composition.
3) Abrasion on the surface.
4) local difference in oxidation
These factors resulted in over-development (when zinc cover the entire area even over the friction ridges) and reverse development (zinc deposited more rapidly on latent finger ridges than the surface). vacuum metal deposition is a good technique for development of latent prints , some forensic persons consider it is highly priced but it is not expensive .
Thats it's. 🙏
MR. DEEPAK PAWAR has been working as Finger Print Expert / Police Officer in Finger Print Bureau , Delhi Police at New Delhi since 2001 . He is the senior most Finger Print Expert , attending / visited several scene of crime and solved many crime cases .He has passed M.Sc (Chemistry) and he is the Member of International Association of Identification . He is the Member of Delhi Police welfare association . He is supervising the Forensic , New Delhi District , Mobile Crime Team . He is one of the Expert in the Special Investigation Team held for investigation of important cases . He has been giving fingerprint proficient training to Delhi Police personnel's . He has been Presented / giving lectures on topics related to forensic science . He is also making examination paper of Finger Print proficient course . He is awarded with Police Medal by MHA and Delhi Police.