Burd, David Q.

Gamble, Lucy

Kirk, Paul L.

English

Northwestern University Illinois, School of Law: Scholarly Commons

Journal of Criminal Law and CriminologyVolume 33 | Issue 5 Article 101943Glass Fragments as Evidence: A ComparativeStudy of Physical PropertiesLucy GambleDavid Q. BurdPaul L. KirkFollow this and additional works at: https://scholarlycommons.law.northwestern.edu/jclcPart of the Criminal Law Commons, Criminology Commons, and the Criminology and CriminalJustice CommonsThis Criminology is brought to you for free and open access by Northwestern University School of Law Scholarly Commons. It has been accepted forinclusion in Journal of Criminal Law and Criminology by an authorized editor of Northwestern University School of Law Scholarly Commons.Recommended CitationLucy Gamble, David Q. Burd, Paul L. Kirk, Glass Fragments as Evidence: A Comparative Study of Physical Properties, 33 J. Crim. L. &Criminology 416 (1942-1943)GLASS FRAGMENTS AS EVIDENCEA Comparative Study of Physical Properties*Lucy Gamble, David Q. Burd, and Paul L. Kirk-Bits of broken glass are so commonly encountered in the in-vestigation of crime as to be considered of major importance asevidence. Not only is this true in traffic accidents, hit-run offensesand so forth, in which headlight and spectacles lenses and perhapswindshields or windows may be broken, but also in a large pro-portion of burglaries and other similar offenses in which entrymay be gained by the breaking of windows. Due to the almost in-variable widespread distribution of minute fragments formed atthe moment of breaking, it is usually unavoidable that the personbreaking a window will collect and retain in his clothing a consid-erable number of these fragments. Less common, but still import-ant, is the collection of glass on shoes and pants cuffs from walkingover areas where considerable glass is lying.Some investigators prefer to subject. bits of glass to spectro-graphic examination for comparison purposes. This is undoubtedlyan effective procedure for determining qualitative and approximatequantitative similarities or differences in glass samples, in the*hands of a properly skilled spectrographer. However, it requiresa considerable amount of expensive equipment, and is subject togreater uncertainty than is the examination of many other types ofevidence, due to the inherent difficulties of generating a proper andreproducible light emission from a small glass fragment. In con-trast to the spectrographic examination, the comparison of physicalproperties is not materially slower; requires much less experience;no elaborate or particularly costly equipment; and is, we believe, ofequal validity. This follows from the examination of tables ofconstants of silicates in such publications as that of Winchell (1)and Wright (2) who have shown exact correlation between com-position of the glass and certain of its physical properties.The excellent discussion of glass fragments by Tryhorn (3) con-siders the physical properties of' glass as well as the effects ofweathering, color, hardness, form, fractures, and other propertieswhich may be examined. As he points out, only an exact fit oftwo pieces of glass is conclusive proof of a common origin. He doesnot stress the fact that identity of physical properties is virtualproof of chemical identity, which follows from the data compiledby Winchell, showing unequivocably that glasses of qualitativelydifferent composition usually fall in completely different ranges ofrefractive index, specific gravity, and dispersion, and that withina series of glasses of the same qualitative type, variations in the* Aided by a grant from the Research Board of the University of Califdrnia.t From the Division of Biochemistry, University of California MedicalSchool, Berkeley.GLASS FRAGMENTS AS EVIDENCEratios of the constituents give rise to corresponding variations inthese properties. Composition changes can be balanced in such away as to reproduce any particular physical property, but in sodoing, other properties must be thrown out of balance, and com-plete identity is not found.Tryhorn correctly states that identity of glass as based on itsproperties is inherently a matter of probability rather than con-clusive proof, but he does not present actual data to establishthe magnitude of the probability factors concerned with any of thespecific types of examination. Morris (4) has reported that of 65samples of glass examined, all were distinguishable on the basis ofrefractive index, specific gravity, and appearance under polarizedlight. He also presented no data in the publication, nor did Dono-van (5) who published limited studies on identification of glasssplinters.Without collecting very extensive or complete data on manufac-turing practices, batch variations, and distributions of the glassmanufactured to each specific formula, it cannot be decided fromavailable figures what is the probability of two glasses from dif-ferent sources being identical with respect to any or all of thesephysical properties (and corresponding chemical composition).In this paper is reported a study of a simpler type designed toyield an approximate answer to this question, and to give somequantitative idea of the value of evidence based on the determina-tion of physical properties of a sample of glass involved as evidencein a criminal trial. At the present time it is not practical to at-tempt the absolute determination of dispersion of small and ir-regular fragments of glass. For this reason, this study is confinedto determination and comparison of the refractive index andspecific gravity of 100 samples of glass, collected at random, takingcare that none of them are accidentally duplicated in the collection.The type of source is known in most cases, e.g., window, bottle, etc.,but not the absolute source, the manufacturer, or the formula. Thisprocedure is valid because the similar collection by a burglar, orfrom street accidents would have exactly the same degree ofrandomness.The data of this paper are designed to supplement the other-wise excellent studies of Tryhorn and of Morris, and to determineapproximately the significance of the refractive index and specificgravity comparisons in establishing identity of glass fragments.Similar studies on a larger scale might profitably be made on theseproperties and on others listed by Tryhorn.EXPERIMENTALRefractive index. The well known immersion method (Beckeline) described by Winchell (1), Chamot and Mason (6), and otherswas followed with certain variations. The immersion media were418 LUCY GAMBLE, DAVID Q. BURD, AND PAUL L. KIRKmade from selected quantities of methylene iodide, cc-chlornaphtha-lene, "cellosolve", and nitrobenzene. A large number of standardswas prepared, each of which varied from the adjoining membersof the series by as small an amount as was practical, i.e., about0.001. Each standard was then carefully calibrated with an Abb6type refractometer which measured to 4 significant decimals.All glass samples were immersed in appropriate media and therefractive indices determined with respect to the closest immersionmedium. It was thus possible to determine a relative index toabout ± 0.0005, it being understood that variations in tempera-ture, evaporation during measurement, and similar factors pre-vent an absolute accuracy to this value. Some of the measurementswere made with a sodium vapor lamp and some in white light.-It was found that comparative results were as accurate in theone case as in the other, though it is recognized that the expressionof absolute values would require a definition of the wave lengthused, due to dispersion effects.Having arranged all glasses in a series with respect to theirrefractive indices, it remained to check comparatively .all of thosepairs or groups of glass samples which were close together andtherefore either identical or subject to confusion. Comparisonswere carried out by immersing one of the questioned fragmentsin the closest standard medium, observing its shape and appearancecarefully, then adding to the same drop a fragment of the otherglass being compared. The pieces were manipulated so that theiredges actually overlapped at some one point. Thus, the movementof the Becke lines could be compared up to the actual point ofoverlapping, ensuring against any effect from local variations inthe medium. The refractive index of the immersion drop wasthen varied up and down with respect to that of the fragment byadding small amounts of media of slightly lower or higher index.After each such addition the comparative behavior of the Beckelines was observed. If any detectable differences existed betweenthe two samples, it could be ascertained with a degree of certaintyat least ten times as great as an absolute index value could be de-.termined.Specifie gravity. The determination of absolute and relativespecific gravity of the various glass samples was made by thetechnique described by Kirk and Russel. (7) Methylene iodide andchloroform were mixed in a small test tube or shell vial to give amedium in which a glass sample would neither settle nor rise to thetop. The specific gravity of the liquid was then determined byweighing a sample of it in a micropycnometer. After grading allglass samples in a similar manner to that employed for absoluterefractive index values, all of the samples which had values closeto each other were-carefully compared by immersing two frag-ments from different sources in a larger quantity of the same liquidGLASS FRAGMENTS AS EVIDENCE1.535-A BOTTLE AA WINDOW1.530 0 AUTOMOBILEa EYEGLASS0 LIGHTSL525 0 HOUSEHOLD A* MIRRORS • NX UNKNOWN * ±4,L1.520 AL.515 AX Wohi * 4 oz * NId< .505LL. ft.2.44 2.4 8 2.50 2.52 2.54 25 Z60 2.52SPECIFIC GRAVITYFigure 1and ascertaining whether any possible mixture would allow one torise while the other one sank. Use of larger volumes ensuredsmaller changes after a small addition. Thus, the relative specificgravities could be determined with far greater accuracy thanthe absolute value could be measured.RESULTSThe results of the study of the refractive index and specificgravity of the 100 samples of glass are shown as a chart in Fig. 1.This chart is made in a manner similar to a graph, not in order torelate the properties listed along the coordinates, but rather toshow the differences between the glasses studied. Each point givesthe values for refractive index and specific gravity of a single glasssample. If any two glasses were identical, the two points wouldcoincide. It is immediately apparent that each glass studied wasdistinguishable from all others, even though there is a definiteparallelism between the two properties. Thus, two glasses havingdifferent values of refractive index would be expected on theaverage, to show differences of the same sign in specific gravity.This is only true in the statistical sense, since the sign of the changemay be reversed, and there is no proportionality between thevalues in any instance.The refractive index values found ranged from a minimumof 1.469 to a maximum of 1.566. This range is decidedly more nar-420 LUCY GAMBLE, DAVID Q. BURD, AND PAUL L. KIRKrow than the over-all range given by Winchell, as would be expectedbecause of the omission of unusual types of glass, such as the highindex optical glasses and the natural glasses. The lower limit ofthe range was somewhat -under that of Winchell, presumably be-cause of alterations in methods of manufacture and compositionsince the data there compiled were determined. The differencebetween maximum and minimum values was 0.09.7, but 88 of theglass samples fell within the narrow range of only 0.0265. It is aninteresting and unexpected fact that refractive index comparisonalone was sufficient to distinguish every single glass samplestudied from every other one. In ten instances, pairs of sampleswere so close that only the most careful observations were suf-ficient to distinguish them. One set of three and one set of fourglasses were likewise very difficult to distinguish.Values for the specific gravity ranged from 2.2447 to 3.1278, adifference of 0.8831. This range was also much narrower than thatof Winchell, for the same reasons that apply to the refractiveindex. In spite of the rather broad over-all range, 96 samples fellin the much narrower range of 0.4877. Specific gravity was not asvaluable in distinguishing glass samples as was the refractiveindex. In all, five pairs of samples and the members of one set ofthree samples could not be distinguished while the samples intwo additional pairs, three sets of three and two sets of four wereonly distinguishable with difficulty. All of these glasses had differ-ent refractive indices, some of the differences being quite large (upto 0.028). The combination of properties gave very clean-cut anddefinite distinctions between all of the 100 glass samples, even inthose cases in which one or the other property was close or identical.It is a well known fact among those who are familiar withglass manufacture, that not only do successive batches made tothe same formula show variations due to imperfect measurementsand other variations, but also, different parts of the same batch mayvary in composition: and physical properties. These effects arisefrom a variety of causes:(a) Solution of material from the crucible by the melt. Thisfactor is a function of the age and previous history of the crucibleand its construction material.(b) Volatility of constituents of the melt. This tends to affectthe surface portion of the melt disproportionately.(c) Inadequacy of stirring and mixing of the melt due to itshigh temperature and viscocity.(d) Local variations in the crucible due to inadequate originalmixing of the constituents of the furnace charge.(e) Variations in heating or other conditions due to inadequatefactory control.Consideration of these factors, which are so important in precisemanufacture, such as that of optical glass, inevitably lead to theGLASS FRAGMENTS AS EVIDENCEconclusion that not only different batches would be expected to showdifferent physical properties, but that different portions of the samebatch might show such variations, and indeed that different portionsof the same glass object might show variations from point to point.In order to test this possibility, widely different portions of twentylarge pieces of glass (windows, large bottles, etc.) were compared.In one case only, there was found a slight but detectable differencein refractive index. This was of the same order as shown by thefew very close samples quoted earlier. It is tentatively concludedfrom this observation, that minute differences in refractive indexmay not always mean that the glass is from a different source.Comparison should then be made with other portions of the standard,and other physical properties checked whenever such small differ-ences are observed. It is desirable to determine as many physicalproperties as possible before drawing final conclusions about anytwo samples of glass which are very similar, as was pointed out byTryhorn. The implications of his statements, however, would tendto underrate the value of physical properties as a means of estab-lishing chemical identity or non-identity of glass fragments, asshown in this paper and that of Morris. It is not possible as yet toestablish an absolute probability factor in the use of either refractiveindex or specific gravity, or for that matter, any other physicalproperty. The indications are, however, that these two factorsalone are nearly always sufficient to establish lack of identity ofdifferent samples, and that identity can be established with a highdegree of probability, but not absolute certainty.SUMMARYOne hundred samples of glass, collected at random were com-pared as to their refractive index and specific gravity. All of themwere definitely distinguishable on the basis of these properties alone.All of the samples could be distinguished on the basis of refrac-tive index alone, but in a few cases the diffe-eices were not largerthan might occur at times as local variations due to manufacturingimperfections.Most, but not all samples could be distinguished from theothers on the basis of specific gravity comparisons. In no case wereboth the refractive index and specific gravity of two samples soclose as to lead to possible confusion.REFMEUMCES1. Winchell, A. N., The microscopic characters of artificial inorganic sub-stances or artificial minerals, 2nd Ed., (1931).2. Wright, F. E., U. S. Ord. Dept. Doe. 2037, (1921); J. Am. Ceram. Soc.III, 783 (1920).3. Tryhorn, F. G., J. Crim. Law Criminol. (Police Sci. Sec.) 20, 404(1939-40).4. Morris, N. A., Analyst, 59, 686 (1934).5. Donovan, W., Analyst, 59, 347 (1934).6. Chamot, E. M. and Mason, C. W. Handbook of chemical microscopy, volI, 2nd. Ed., (1939).7. Kirk, P. L. and Russel, R., J. Crim. Law Criminol. (Police Sci. Sec.) 82,118 (1941).

Glass Fragments as Evidence:  A Comparative Study of Physical Properties

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Glass Fragments as Evidence: A Comparative Study of Physical Properties

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