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HomeCRIMEGlass, paint, soil & questioned documents analysis

Glass, paint, soil & questioned documents analysis

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Forensics: exact science or dramatised fiction? #15

No two soil samples are alike. Soil samples collected from different places are bound to be different

Forensics has an incurable habit of getting enamoured of everything for which we find different types. For example, glasses used in windows, cars etc. are manufactured by scores of companies. Paints used on cars, walls, etc. are also manufactured by very many companies. As for soil, it is different at different places. Those made the ignoramuses think that these could be used for identification.

Glass, paint and soil

The study of broken fragments of glass, soil and paint suffers from the same weaknesses as that in hair and fibre we discussed earlier. Yes, they are different but it is not possible to prove that such-and-such piece of glass could have come only from a such-and-such window or that, in a traffic accident case, the pain found on a bike hit by a car could have come only from that car and no other car. Once again, the fundamental problem is that we can speak, at most, in terms of ‘consistent with’, ‘not dissimilar’, etc. There are no definitive databases with which to determine the frequency of any stated ‘match’ occurring in the general population.

In respect of glass, the most commonly used comparison analyses utilize a combination of physical and chemical properties, such as refraction indexes, dispersion staining, density, chemical components, mineral content, and colour. In People v. Columbo, the deceased was found lying on his back in the living room, surrounded by broken glass with a torn and bloody lamp shade nearby, and also had a 2-inch slash across his throat. The forensic expert had submitted that, by using the refraction method, they had concluded that two of the fragments, one from the broken lamp base found on Columbo’s living room floor and one found in his car had the same degree of tolerance and, thus, could have originated from the same source. However, in cross-examination at the Illinois Supreme Court, the forensic expert was forced to admit that the so-called matched glass fragment recovered from the car could have come from any of thousands of pieces of glass with the same optical properties as the lamp base.

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Paint analysis is used mainly in hit-and-run and vehicular homicide cases with respect to accident dynamics or identification of the vehicles involved. It is also tried in burglary cases where paint residues are found on burglary tools or other devices used to gain entry to someplace. However, specimens are eventually compared side by side under a stereoscopic microscope for colour, surface texture, and colour layer sequence. That, by itself, makes it subjective like other techniques. Chemical analysis of the paints is a very complex process. And, even if it is done, could only prove that certain paint is the one manufactured by the such-and-such company. Once again, the company makes the paint in enormously large quantities and millions of people would have used that paint somewhere. It does not prove that it could have come, for example, only from a certain car. The National Academy of Sciences Committee noted that the community has not defined precise criteria for determining whether two samples come from a common source class.

Soil analysis is a relic of a bygone era. The chemical analysis of the composition of the soil was used in an earlier era in cases like kidnapping by vehicle and the disposing of bodies in rural or semi-rural areas or a wide variety of burial sites. If a suspect had soil from the scene of the crime on his shoes or clothes, they argued that he should have a convincing explanation for that if he had to claim innocence. This was pure atrocity. He could have loaned his car to someone for half a day, and he could have used the car for the offence. I call it a relic of a bygone era because, in this age, a suspect could wash or destroy his clothes and shoes, etc. after a crime. A vehicle involved in the crime could be got professionally washed after the crime.

Comparison of soils is so childish that it does not really warrant a comment. A person might have gone to someplace for a legitimate reason. The soil of that place might have got stuck onto his shoes. Suppose he is falsely implicated in a murder case for a murder that might have been committed at that place. Now if the forensics were to produce his shoes as evidence that he is the culprit, would you believe in it. In the first place, thousands of other people would have also gone to that place and would have the soil of that place on their shoes. Second, there is no such chemical test invented so far, which could prove that the soil found on his shoes could have come only from that place and not from any other place. For your information, the exact chemical composition of the soil at someplace, even if determined with some accuracy, would differ from that of the soil in that very locality a few yards apart. It all depends on what material has been dumped on the ground there. If people have been dumping vegetable waste in their kitchen there, for example, the composition of the topsoil would be one; if industrial effluent has been flowing over it, the composition would be different even if both places are in the same geological location. 

In any case, in the absence of other corroborative evidence, the mere presence of soil from the scene of the crime does not prove a crime. It should also be kept in mind that as soil particles and other earth-related materials, such as plant chips, diatoms, pollen and spores, and concrete or brick fragments are one of the major components of airborne dust, they can be frequently transferred on to an innocent person if they happen to touch the dusty surface of items such as a door, a windowsill, etc. They could also be transferred by a suspect touching some surface, such as sitting in some car.

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Tool marks

The basic premise of this childish idea is that no two tools make exactly the same mark on softer material either because of the manufacturing process or because of the subsequent use or misuse of the tool. This includes striation marking made in wood, putty, and other media that must be forced to gain entry to the property, or, in rare cases, used to cause blunt trauma to an assault or homicide victim. Pry bars, screwdrivers, knives, pliers, crowbars, wire cutters, bolt cutters, and a host of other tools may leave striation marking in building media. Building materials such as paint, brick, or glass may also attach themselves to the tool itself and thereby provide a possibility of linkage.

Once again, this idea originated in the era of handmade tools. Handmade tools were likely to differ slightly from each other. Now, except for the most elementary tools, most of the modern tools are made on automatic precision machines. This means that for all practical purposes, they are identical.

For both tool marks and weapons analysis, the National Academy of Sciences Committee notes that because not enough is known about the variabilities among individual tools and guns, we cannot specify how many points of similarity are necessary for a given level of confidence in the result. In the USA, the AFTE (Association of Firearms and Tool Mark Examiners) has adopted a theory of identification, but it does not provide a specific protocol. However, the NAS Committee points out that this AFTE document, which is the best guidance available for the field of tool mark identification, does not even consider, let alone address, questions regarding variability, reliability, repeatability, or the number of correlations needed to achieve a given degree of confidence. Thus, tool mark ‘matching’ is a pseudo-scientific fraud.

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Explosive evidence and fire debris

Explosives evidence encompasses a wide range of materials from unburned, unconsumed powders, liquids, and slurries, to fragments of an explosive device, to objects in the immediate vicinity of an explosion thought to contain residue from the explosive. High explosives that detonate, as against conflagration in case of low explosives like gunpowder, do not leave any residue if packed properly. Results, under the best of circumstances, can be stated only in terms of ‘the residue present was consistent with an explosive material’ to ‘the residue is only indicative of an explosive’ to ‘no explosive residues were present. And, as readers must have noticed; any terrorist explosion anywhere in the country and the self-styled forensic experts declare by just looking at the site that RDX was used!

The examination of fire debris not associated with explosions often aims to determine whether an accelerant was used. They believe that its accelerant was used; it would suggest that the fire was deliberately started by an arsonist—otherwise, it could be accidental too. They do not realize that an arsonist could very well commit the crime without an accelerant too.

To assess the effects of an accelerant, one will have to design an experiment, under a range of conditions (for example, wind speed, temperature, presence/absence of other chemicals) with two groups: one in which materials are burned in the presence of an accelerant (‘treatment’) and one with no accelerant (‘control’). Then the damage patterns, for example, depth of char and size of bubbles on surfaces have to be compared with the damage observed at the scene of the crime.

In the first place, hardly anybody undertakes such a difficult exercise. Second, they do not realize that there is a considerable natural variability of burn patterns and damage characteristics. The National Academy of Sciences Committee determined that many of the rules of thumb that are typically assumed to indicate that an accelerant was used (for example, ‘alligatoring’ of wood, specific char patterns) have been shown not to be true.

The ridiculous business of examination of questioned documents

Questioned document examination involves the comparison and analysis of documents and printing and writing instruments in order to identify or eliminate persons as the source of the handwriting; to reveal alterations, additions, or deletions; or to identify or eliminate the source of typewriting or other impression marks. Questions about documents arise in business, finance, and civil and criminal trials, and in any matter affected by the integrity of written communications and records.

The whole business of examination of questioned documents does not have even that much support as the dubious techniques of hair and fibre examination have. All that it is possible to say from a comparison of the two-dimensional images of two signatures is that whether they are sufficiently similar or not. It would still not tell us anything about the identity of the persons who made those two signatures—after all; there have been perfect forgeries too.

I hope readers understand that two signatures made by the same person are never exactly alike. Significant variations in a signature are necessarily introduced by the type of writing instrument used; paper used; the surface or backing on which the paper was kept while signing; the height of the table if there was one with respect to that of the chair; whether the person signing was standing or was seated; his mental condition in terms of stress; the presence of intoxicants in the body; whether the writing surface was stable or shaking as is the case in writing in a moving vehicle; and a host of other factors.

A scientific problem with the analysis of two-dimensional images is that they cannot tell you about the pressure, which one habitually and characteristically applies at different places on individual letters and in cursive writing.

In view of the above, it is not really possible to categorically state whether, even with a given extent of similarities or dissimilarities, whether something is written by a certain person or not. Moreover, they suffer from significant human errors too, as demonstrated by M. Kam et al.

In any case, these days, why would someone bother to write something—he could simply print it. As such, the examination of questioned documents is a relic of the past and must be consigned to the dustbin of history.

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Dr N C Asthana IPS (Retd)
Dr N C Asthana IPS (Retd)
Dr. N. C. Asthana, IPS (Retd) is a former DGP of Kerala and ADG BSF/CRPF. Of the 51 books that he has authored, 20 are on terrorism, counter-terrorism, defense, strategic studies, military science, and internal security, etc. They have been reviewed at very high levels in the world and are regularly cited for authority in the research works at some of the most prestigious professional institutions of the world such as the US Army Command & General Staff College and Frunze Military Academy, Russia. The views expressed are his own.


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