Paint evidence

Ryland, S. G., Infrared Microspectroscopy of Forensic Paint Evidence , in Practical Guide to Infrared Microspectroscopy, Humecki, H. J. (Ed.), Marcel Dekker, New York, 1995, pp. 163-243. 

Rgure 2 Comparative analytical sequence for paint evidence with its control paint known source. Such sequence depends on the quality and quantity of paint evidential material recovered, on the equipment of the laboratory and on the personal choice of the forensic scientist. At every stage of the sequence, if the questioned and the known samples result undifferentiated, one continues with the following examinations. If the trace and its control are differentiated with a given technique and if the differentiation cannot be explained, a common origin can be excluded. 

Paint evidence is most commonly encountered with regard to automotive paints due to the frequency of transfer in cases involving collisions with automobiles. However, paint recovered from other settings can also provide a wealth of information (see the case description in the final section of this article). Figure 1 shows a cross-sectional image of... 

Paint evidence is usually submitted to the forensic scientist for comparison with paint from a known soimce. Automotive, architectural, art, marine, aircraft, tool, and other types of paints may become evidence in a variety of crime scenarios (43). Within the United States, most of these examinations involve automotive finishes as related to antomotive accidents and hit-and-run cases. The question frequently posed is, what kind of vehicle could have contributed the paint flakes recovered from the scene of a hit-and-run homicide case Additionally, house paints may be examined in connection with biu-glaries, and paint from tools in cases involving industrial sabotage. Rarely is it necessary to determine the manufacturer of a particular paint. 

Paint is one of the more common types of physical evidence that is submitted to the crime laboratory. Paint evidence may originate from a number of different... 

Searching a crime scene is a complex process (25), involving poHce, crime scene technicians, and forensic scientists. The procedure requires careful documentation, collection, and preservation of the evidence. Trace evidence (26) in criminal investigations typically consists of hairs (27,28) both natural and synthetic fibers (qv) (29,30), fabrics glass (qv) (31,32) plastics (33) sod plant material budding material such as cement (qv), paint (qv), stucco, wood (qv), etc (34), flammable fluid residues (35,36), eg, in arson investigations explosive residues, eg, from bombings (37,38) (see Explosives and propellents), and so on. 

Paint is one of the most common and widely used materials in home and building constmction and decoration (see Building materials). Its broad use comes from its abiHty to provide not only improved appearance and decoration but also protection of a substrate to which it is appHed. Evidence of the historical uses of paint goes back over 25,000 years to cave paintings found in Europe. The Bible describes pitch being used to coat and protect Noah s Ark. Over 10,000 years ago in the Middle East, various minerals and metals such as lime, siHca, copper and iron oxides, and chalk were mixed and reacted to produce many colors. Resins from plant sap and casein were also used. Over 2000 years ago in Asia, resins refined from insect secretions and sap from trees were used to make clear lacquers and varnishes (2). 

Tin [7440-31 -5] is one of the world s most ancient metals. When and where it was discovered is uncertain, but evidence points to tin being used in 3200—3500 BC. Ancient bron2e weapons and tools found in Ur contained 10—15 wt % tin. In 79 ad, Pliny described an alloy of tin and lead now commonly called solder (see Solders and brazing alloys). The Romans used tinned copper vessels, but tinned iron vessels did not appear until the fourteenth century in Bohemia. Tinned sheet for metal containers and tole (painted) ware made its appearance in England and Saxony about the middle of the seventeenth century. Although tinplate was not manufactured in the United States until the early nineteenth century, production increased rapidly and soon outstripped that in all other countries (1). 

After suitable surface preparation, e.g. blast cleaning, low-alloy steels can be coated by paints, sprayed metal coatings, etc. and there is some evidence that such coatings last longer than on mild steel under similar conditions of exposure. ... 

Flame Cleaning Now little used as a preparatory method, flame cleaning is a process whereby an intensely hot oxyacetylene flame is played on the surface of the steel. In theory, differential expansion causes millscale to detach. In practice, there is evidence that the treatment may not remove thin, tightly adhering millscale. Also, steel less them 5 mm thick can buckle. Finally, the process can burn in chemicals deposited on the surface, causing premature paint failure. 

Aluminium coatings exposed as sprayed show remarkable protective properties and in many cases the only evidence of age is the appearance of small hard nodules of aluminium oxide, and these do not appear to be deleterious. However, the dead white of the sprayed surface soon becomes discoloured by dirt and so a paint covering is usual. A chromate inhibitor is not essential and two coats of a simple vinyl-based sealing paint have given remarkable results in the tests of the American Welding Society over periods of 12 and 15 years. 

The outstanding virtue of zinc-rich paints is simplicity in application. No special equipment is required and the operation can, of course, be carried out on site, large or small structures being equally suitable for treatment. While there is some evidence that the zinc-rich paints will reduce iron oxides remaining on the steel surface, proper surface preparation is as important here as with traditional paints if the best results are to be achieved. The main use of zinc-rich paints is to protect structural steel-work, ships hulls, and vulnerable parts of car bodies, and to repair damage to other zinc coatings. 

When paint films are immersed in water or solutions of electrolytes they acquire a charge. The existence of this charge is based on the following evidence. In a junction between two solutions of potassium chloride, 0 -1 N and 0 01 N, there will be no diffusion potential, because the transport numbers of both the and the Cl" ions are almost 0-5. If the solutions are separated by a membrane equally permeable to both ions, there will still be no diffusion potential, but if the membrane is more permeable to one ion than to the other a diffusion potential will arise it can be calculated from the Nernst equation that when the membrane is permeable to only one ion, the potential will have the value of 56 mV. 

For painted structures it is essential that an additional paint coating be applied as soon as there is evidence of paint breakdown. The Protective Coating Sub-committee of BISRA recommend painting of steel surfaces when 0-2-0-5% of the surface area shows evidence of rust. Delay in repainting may be a false economy, as if rusting is extensive it may be necessary to clean down to bare metal before paint can be applied. 

The most important uses for phosphate coatings entail sealing with oil or paint and it is therefore of interest to study absorption values. Table 15.9 compares the absorption of diacetone alcohol into coatings of widely differing thicknesses and composition despite these differences, values of 10-8-12-9g/m are obtained throughout. It is therefore evident that absorption is predominantly a surface effect and not appreciably influenced by coating thickness. 

Although the boll weevil, a very serious cotton pest in the South, feeds voraciously on cotton squares and bolls in response to an at-tractant or attractants contained therein, these same cotton parts (49) contain also a boll weevil repellent that shows its effect once the at-tractant has volatilized completely. Department of Agriculture scientists are attempting to isolate and identify this repellent, which presumably could be applied to ward off weevil attack. Cotton seedlings painted with an aqueous emulsion of the material effectively repelled 100% of the weevils for 5 hours and only medium damage was evident after 12 hours, whereas control seedlings were completely destroyed after only 2 hours. 

Bryan, G.W., Gibbs, P.E., Hummerstone, L.G., and Burt, G.R. (1986). The decline of the gastropod Nucella lapillus around Southwest England—Evidence for the effect of tributyl-tin from antifouling paints. Journal of the Marine Biological Association of the United Kingdom 66, 611-640. 

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