Forensic Science International, 57 (1992) 73 - 80 Elsevier Scientific Publishers Ireland Ltd.

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BREAKING OF FLAT GLASS - PART 3: SURFACE PARTICLES FROM WINDOWS

JOHN LOCKE and JOHN K. SCRANAGE

Central Research and Support Establishment, Home Office Foren.& Science Sewice, Aldmmaston, Reading, Berkshire, RG7 qPN (UK)

(Received June 12th, 1990) (Revision received August 3rd, 1992) (Accepted September 8th, 1992)

Summary

The debris recovered after smashing of flat glass windows has been examined to determine the percentage of surface bearing particles. Four ranges of particle size were considered and a distinc- tion was made between particles originating from the front surface, from the rear surface and from the centre of a pane. The factors that influence the proportion of fragments from the two original surfaces versus the proportion from the bulk glass are discussed.

Key words: Criminalistics; Glass; Windows; Fragmentation

Introduction

Research is in progress at this Establishment to establish the number, the physical appearance, the size distribution and the spatial distribution of particles arising from instances where glass is broken. Of greatest interest are windows because these occur much more frequently in casework than other glass types. The reproducibility of pane breaking was first examined [l] and this was follow- ed by studies on the effect of altering pane parameters [Z]. An earlier study by Pounds and Smalldon [3] on the breaking of glass windows also dealt with the number of particles, their size and the distance travelled. Subsequently Zoro [4] examined the debris from a pane to assess the extent to which surface particles were involved. The study was limited by the fact that only the distribution of sur- face particles derived from the front of the pane was studied; the experimental design did not allow an assessment of rear surface involvement. Since the work reported here was completed, window breaking experiments which describe the incidence of surface particles has appeared [5].

Correspondence to: John Locke, Central Research and Support Establishment, Home Office Forensic Science Service, Aldermaston, Reading, Berkshire, RG7 4PN, UK.

0379-0738/92/$05.00 0 1992 Elsevier Scientific Publishers Ireland Ltd. Printed and Published in Ireland

This paper reports the proportion of surface particles from different types of window pane where an appreciable volume of the pane disintegrated and was subsequently dispersed. In order to identify surface particles all panes were coated in coloured ink before breaking; a preliminary experiment was therefore conducted to check whether or not the presence of ink influenced the results.

Experimental

Effect of ink on breaking A 1.00 m x 1.00 m x 4 mm plain window pane was inked with a chequer

board design on both sides, the length of the side of each square being 5 cm. The side closest to the breaker had the solid squares of the pattern inked in black, the remaining squares remaining untreated. A similar design on the rear side used red ink. In this way a twenty by twenty pattern of squares was established on each side, alternate squares being either inked or ink free. Both sides were therefore equally divided by area into inked and ink-free regions. The pane was then smashed by the impact of a sharp object using a rig of the type previously described [l] and the debris subsequently examined. The basis of the experiment was that, whatever the arrangement of cracks on the broken pane, the debris would be derived equally from inked and ink-free areas. If the ink had no effect on the fragmentation process then the debris from each area would be essentially indistinguishable.

Particles bearing coloured ink were easily identified as derived from the inked surfaces. For clear particles, close examination, using a stereomicroscope ( x 10 to x20) and incident light, was necessary to establish whether they were from the bulk or whether they bore a flat surface and were therefore from an ink-free region of the original surface. One hundred particles, selected at random from the collecting trays, were counted for each of two size ranges. Comparison was then made between the total number of inked and ink-free surface bearing particles.

Collection and counting of window o%ris Breaking and counting operations have already been described for window

panes of various sizes, thicknesses and types [1,2]. For each breaking operation a photographic record was taken of the broken pane and the general distribution of the debris in the counting trays.

The debris, from a selection of window panes, was analysed for the presence of surface particles. For these experiments two thicknesses of pane (6 mm and 10 mm), of size 1.0 m x 1.0 m, had been used as duplicate breakings to study reproducibility. Debris from four other panes was selected to study the effect of various parameters (e.g. size, type), making a total of eight window panes.

Results and Discussion

Effect of ink on fiagmmatatim A preliminary experiment was designed to establish whether or not the coating

of ink, used to identify surface particles, had any effect on the fragmentation

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TABLE 1

EFFECT OF INKING OF PANE ON DEBRIS

100 Particles in each size range, taken from a tray at a distance 0.0-0.5 m from point of breaking.

Size range (mm) 0) (2) (8) (4) (5)

Bulk Inked Inked front rear surface surfm3

Total parti- cles with inked surfaces (21 and (3)

Total parti- cks with ink-free surface

1.0-0.5 0.5-0.25 Combined results for

‘casework size’ particles

22 27 16 43 35 32 19 14 33 35 54 46 30 76 70

process. As indicated previously the experiment relied on breaking two equal areas of glass, one inked and one not inked, under identical conditions. The proportions of bulk and surface particles in the resulting debris are shown in Table 1.

The figures of particular interest are those for the inked area of surface (front and rear combined), column no. 4 and those for the ink-free area, column no. 5. If these totals are equal, then inking has no measurable effect on the proportion of various fragments generated. In fact, for both size ranges, the totals in col- umns 4 and 5 are comparable, bearing in mind the natural variation in results that will arise from statistical considerations. A separate experiment, similar to that described here, established that the distance to which a fragment was scat- tered was also independent of the ink treatment. It was deduced therefore, that the presence of the ink would not introduce appreciable bias into subsequent breaking experiments.

For completeness, the proportion of bulk particles is shown in column 1 in Table 1.

Proportion of particles derived from original surfaces In this report four size ranges of glass fragments are considered, the largest

ranging between 20 mm and 5 mm in maximum dimension. Pieces larger than 20 mm are infrequently encountered in casework but, where present, frequently bear both original surfaces and so provide valuable points of comparison by way of thickness and surface characteristics. The two smallest ranges combined cor- respond to what may loosely be termed ‘casework size’, i.e. 1.0-0.25 mm.

The percentage of fragments bearing some portion of an original surface is shown, for a variety of window pane types, by the bar charts in Fig. 1. Particles originating from the front surface (i.e. facing the breaker) are distinguished from those from the rear surface (facing away from the breaker). Subtracting the sum of these two from one hundred percent gives the percentage of bulk particles.

a bed B

abed

plain , 1.0 m il.0 m x 10 mm plain . 1.0 m x 1.0 m x 6 mm

100

%

50

0 III 1 E abed

plain ,

l.Omx l.Omx4mm

Key : Percentage Particles

100 1

%

__ kl Percentage bulk particles

I Percentage particles 3U

Isi

from rear surface

Percentage particles 0 from front surface

100

50

0 1 F

abed plain ,

0.25 m x 0.25 m x 4 mm

100

50

0

100

50

0

a

b H

abed

patterned .

l.Omx l.Om

a bed wired ,

l.Omx l.Om

Size m

a)20-5mm

b) 5 - 1.0 mm

c) l.O-05mm

dIl05-025mm

Fig. 1. Percentage of bulk and surface particles from various types of pane. Four ranges of particle size are shown.

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The relative numbers of particles of each size are not shown here but, for these pane types, full details have already appeared [2].

It is useful, initially, to consider all of the surface particles together without distinguishing from which side of the pane they originated. Inspection of the bar charts reveals, for all of the panes, a marked trend for the percentage of surface particles to increase as the particle size becomes larger. This is intuitively reasonable because, in the limit of very large pieces of glass, the fraction that bears at least one surface will approach unity. For the largest size range, (20 - 5.0 mm) the percentage of surface bearing particles was generally between 75% and 90%. For the smallest size range, (0.5 - 0.25 mm), the percentage was between 35% and 70%. For the ‘casework size’ range (1.0-0.25 mm), which is of greatest interest, the percentage of surface particles, when averaged over all of the panes was very close to 50%. The proportion of particles bearing an original surface may seem high but it must be remembered that some surface particles will only present a small facet of original surface. In casework, of course, the surfaces are free of ink and a very small facet may be difficult to locate unless the particle is examined in detail under suitable lighting conditions.

The bar charts in Fig. 1 distinguish between fragments coming from the front of the pane and those from the rear. The ratio of front particles to rear particles is seen to vary markedly between panes. This held true even for duplicate panes where conditions were held as constant as possible between breakings. Thus the duplicate pairs A with B and C with D exhibit high variability in front/rear sur- face involvement. As a result it was not possible to detect a relationship between the ratio of front to rear particles and the dependence on pane size, thickness or type. This again illustrates the problem of predicting completely the behaviour of panes during breaking. However, when considering the breaking of one par- ticular pane, it can be seen from Fig. 1 that the ratio of the percentage of par- ticles originating from front and rear surfaces is roughly constant over the four ranges of particle size.

Considering all types of pane the ratio of front to rear particles can vary from 3:l to 3O:l (bar charts D and B respectively). Large areas of these two panes completely disintegrated thus seemingly providing equal opportunities for debris from the rear of the pane to be projected forwards. With the wired glass, (bar chart H), the wire restrained pane disintegration and thus the proportion of material from the rear of the pane was understandably small.

A bar chart was also prepared to establish the relationship between the propor- tion of surface particles and the distance travelled. Figure 2 shows the mean results for the eight experimental breakings for each of the five trays. The percentage of rear surface particles increased steadily with distance from the pane. The innermost trays, A and B, contained only a few percent of rear surface particles, whereas this figure rose to 23% for the outermost tray E. This trend was general and not confined to any one range of particle size.

One mechanism that would give rise to a large proportion of rear surface par- ticles is secondary breaking. In primary breaking (i.e. the immediate collapse of the pane on impact), front surface particles can be expected to predominate in the collecting field. During secondary breaking, when glass pieces further frag-

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100

Percentage Of

pattii 1

4 metres

tray identity 1 A B C D E

Key : PercentaQe Par-tic&

) Percentage

% 1 bulk particles

I Percentage particles

50 from rear surface

Percentage particles

0 from front surface

Fig. 2. Percentage of bulk and surface particles with distance from pane. Results are a mean of the eight experimental breakings shown in Fig. 1, all sizes of particle combined.

ment on striking a hard surface (i.e. the trays), equal numbers of front and rear surface particles can be expected. This secondary mechanism will tend therefore to raise the overall proportion of rear particles in the collecting field.

In the design of the breaking rig secondary breaking was minimised by employ- ing a chute to remove large pieces of glass that fell vertically downwards. A study of the photographs of all of the breaking experiments confirmed that, in the main, few large particles, of a size likely to fragment further, were projected onto the collecting trays.

When results from all of the eight windows were averaged, only a few percent of the surface particles were from the rear of panes. This can be seen by inspec- ting Fig. 2 bearing in mind that the majority of particles were recovered from trays A, B and C.

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Secondary breaking is the probable reason for the increase in the proportion of rear surface particles as one moves away from the pane (Fig. 2). Very large numbers of small particles were present in the innermost trays and, with these, the proportion of rear surface particles was only a few percent. For the outer- most trays however, where only a few sporadic particles were generally recovered, it would require only a small number of extraneous particles to radically alter the overall nature of the material present. Thus a very small degree of secondary breaking in the centre of the field, spilling, as a conse- quence, both front and rear particles outwards onto the outermost trays, would profoundly affect the data. The results suggest that, beyond a radius of 2 m, at least 40% and possibly as high as 80% of the particles arose from the secondary mechanism.

The results establish that, for the majority of the collecting field, the recovered glass particles were essentially derived from a primary breaking process. The distribution of debris therefore represents material that would be scattered over a person breaking a pane and can be related to casework circumstances.

At this point it is possible to compare the results with those of Zoro [4] where a bronze ‘Spectrafloat’ window was smashed. The nature of his experiment was such that the only surface particles that could be conveniently detected were those from the bronzed front surface of the pane. Generally the size ranges described in his work were slightly different from those reported here. However the size range 1.0-0.5 mm is directly comparable and Zoro reported that 47% of these particles were derived from the front surface. The comparable figure in the present work (mean of 8 panes) is 56%. The trend in the results for the other size ranges compares well between the two studies bearing in mind the dif- ferences in pane size and thickness and the manner in which the panes were broken.

Comparison can also be made with a study in which panes were smashed and the glass recovered from the clothing of individuals standing nearby [5]. Again the size ranges are not strictly comparable but, for the combined size range of the smaller particles (0.85 - 0.15 mm), the proportions were 52% from the front surface, 45% from the bulk and 3% from the rear surface. (These figures were calculated from the tabulated counts.) For the combined casework range (1.0-0.25 mm) described here the comparable figures are 48% (front), 49% (bulk) and 3% (rear). Agreement between the two laboratories is therefore good.

Conclusions

Earlier work [4] has been confirmed in that, following the breaking of a win- dow, many of the backward flying fragments bear some of the original window surface. For the size range normally encountered in casework, the proportion is in the region of one half of all particles. Furthermore, some of these surface fragments may originate from the rear surface of the pane, although the propor- tions of front, bulk and rear particles are highly variable. Even though in many instances, the actual area of original surface on a particle may be very small, its

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presence may still be of significance as it could give rise to anomalies during refractive index determinations.

References

1 J. Locke and J.A. Unikowski, Breaking of flat glass - part 1: size and distribution of particles from plain glass windows. For& Sci. Znt., 51 (1991) 251- 262.

2 J. Locke and J.A. Unikowski, Breaking of flat glass - part 2: effect of pane parameters on par- ticle distribution. Forensic Sci. Znt., 56 (1992) 95 - 106.

3 C.A. Pounds and K.W. Smalldon, The distribution of glass fragments in front of a broken win- dow and the transfer of fragments to individuals standing nearby. J. Forti Sci. Sot., 18 (1978) 197-203.

4 J.A. Zoro, Observations on the backward fragmentation and refractive index of float glass. Forensic Sci. Znt., 22 (1983) 213-219.

5 R.J.W. Lute, J.L.BuckIe and I. Mclnms, A study of the backward fragmentation of window glass and the transfer of glass fragments to individual’s clothing. J. Can. Sot. Forti Ski., 24 (1991) 79-89.