The Detection of Mastic on Plastic
By: Erich J. Speckin
The detection of mastic on many different surfaces is usually not a difficult
task. Most of the time when some residue of the mastic is left, with the
use of ultra violet or infrared luminescence, the mastic will be made
clearly visible. However, when the mastic has been chemically washed and
no residue is present, the detection of a previous sticker becomes more
difficult. A laser can aid in this detection.
Key words: Laser, mastic, infrared, ultra-violet, PVC, plastic.
In 1979 when lasers were first introduced, my partner and father, Leonard
Speckin, was involved in a case where a coincidental finding was noted.
A vinyl notebook was examined using a laser and the luminescent image
of a price sticker was noticed on the inside cover. However, no residue
of mastic was apparent. The image under the laser light was so clear that
the handwritten amount was visible. This was not important to the case
at hand but was filed away in my father's mental notations only to be
used fifteen years later.
Our firm was contacted by an attorney to determine if a warning sticker
had been removed from a product or never placed on it. A known type of
the same product was obtained to get a reaction to the mastic at a known
wavelength to compare to the questioned product. Ultra-violet light disclosed
strong fluorescence. It was also detectable using infrared luminescence.
However, when the questioned product was examined under the same light
sources, no residue from the mastic could be detected. A short difference
was noted in the area of the product that should have contained the mastic,
as opposed to the surrounding areas. It was then brought to my attention
that before a conclusion is drawn that a sticker had never been placed
on the product, it must be examined by using laser illumination. Anargon
laser was used at 514nm and 3 watts. The known showed a reaction to the
laser at this wavelength. When the questioned was viewed under the same
wavelength, the mastic was visible. It was so clear that the size could
be measured with calipers and matched exactly to the known sticker,as
well as the radiused corners. This is probably because the ultra-violet
will detect the mastic and the laser detects the interaction between the
mastic and plastic that couldn't be removed even when the mastic had been
physically removed and the surface had been chemically washed.
Whenever a question arises whether a sticker has ever been on a surface,especially
plastic of PVC, it should be examined with the aid of a laser before a
conclusion is rendered as to the absence of a sticker.
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