Formic Science International, 51 (1991) 251- 262 Elsevier Scientific Publishers Ireland Ltd.

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BREAKING OF FLAT GLASS - PART 1: SIZE AND DISTRIBUTION OF PARTICLES FROM PLAIN GLASS WINDOWS

JOHN LOCKE and JOHN A. UNIKOWSKI

Central Research and Support Establishment, Home Office Forensic Scime Service, Alo!ermaston, Reading, Berkshire, RG7 4PN (U.K.)

(Received June 12th, 1991) (Revision received August 12th, 1991) (Accepted August 23rd, 1991)

Summary

An experimental rig was constructed, for the systematic breaking of windows, where a steel ball struck the pane in a controlled manner. For plain window glass the distance travelled by particles from the resulting debris was determined according to fragment size. The general form of the results agreed well with earlier findings. Replicate breakings were then performed to establish the reproducibility of the method. By combining the data for replicates for particular size ranges, smooth relationships were demonstrated between the number of particles and the distance travelled. The distribution of the glass particles radially, which was size dependent, was also studied.

Key words: Criminalistics; Glass; Windows; Fragmentation

Introduction

Glass derived from windows is commonly encountered on the clothing of suspects in cases of breaking and entering or malicious damage to premises. In order to account for the numbers and sizes of glass particles found on clothing, Pounds and Smalldon [l] simulated the conditions which exist in a typical case of window breaking. The distance travelled by glass particles backwards towards the breaker was studied for various size ranges. Since then there has been a greater realisation of the importance of surface flaking as a mechanism for generating casework-sized particles. Zoro [2] described an experiment where a single pane was broken and the particles were examined to establish the propor- tion that bore some of the original window surface.

Apart from these two papers there is little systematic information on the number, size and surface involvement of particles generated when plain window glass is broken. Although the effect of pane thickness had not been investigated, Pounds and Smalldon [l] did give some indication of the effect of size of pane. In order to establish a more complete picture of the way in which windows break, a systematic series of breaking experiments was carried out. This paper describes the first stage of the work which was to construct and test a suitable

Printed and Published in Ireland

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breaking rig and then to smash a number of panes to assess the reproducibility of the breaking process.

Experimental

The breaking rig A large, clean room was selected in which to construct the breaking rig (Fig.

1). This construction consisted of a booth to hold the pane and an area in front of it, covered with trays, to catch the debris flying backwards towards the breaker. A pendulum device was used to strike the pane. This consisted of a rigid steel arm which pivoted from a roof beam at its upper end and had a heavy steel ball (6.25 kg) fixed to its lower end. The booth was constructed of wood, reinforc- ed with angle iron to make it as rigid as possible, The rear of the booth was covered in stout polythene sheet to retain all unwanted debris ejected into it. In a later modification a chute was constructed to catch the large pieces of glass which fell vertically downwards and to divert them into the booth. The mouth of the chute projected 150 mm outwards in front of the booth. Without such an arrangement large shards of glass fell onto the trays, smashed and gave rise to

steel pendulun a

Fig. 1. The breaking rig.

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i centre I lme

4.0 2.0 1.5 1.0 0.5 0 0.5 1.0 1.5 2.0 4.0 metres centre of pane

Fig. 2. Arrangement of trays for collecting debris.

secondary breaking. Hardboard wings were added to the sides of the booth to simulate a window in a wall.

The trays were made from mild steel, had raised edges 25 mm high and were painted medium blue to help the investigators to see the smaller glass particles. The trays were constructed with straight rather than curved sides because this made for simplicity of manufacture and ensured they butted together exactly. Figure 2 shows the geometrical arrangement of the trays. For the outermost trays the areas involved were very large and so each tray was made in several parts for ease of handling.

When carrying out a breaking experiment the downward movement of the ball was initiated by a simple release mechanism. The pivot arrangement was set so that the ball struck the window symmetrically with maximum energy at its cen- tral point. A light rope was attached to the arm, just above the ball, in such a way that it could be used to restrain the ball after it had struck the window. This prevented the ball, after rebounding, from hitting the pane a second time which would have caused further breaking. The point of impact, in the centre of each pane, was maintained at 1.5 m above ground level.

Studies were restricted to breaking panes of equal size and thickness, the panes being 4 mm thick and 1.00 m by 1.00 m square. A preliminary run established that the rig and the experimental procedures were operating as an- ticipated. This was followed by eight replicate experimental breakings. Before breaking, all panes were coated with ink of the type used in fibre tipped pens, red being used for the inside and black for the outside surface. No account was

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taken as to whether a float glass surface (if present) was towards or away from the breaking mechanism. At least 2 days were allowed for the ink to dry and then each pane was mounted in the booth so that its edge rested against wooden strips arranged along the whole perimeter of the pane. Wooden beading was then nail- ed into the framework to retain the window by clamping its edges against the wooden strips. No cushioning or putty-like material was used.

Inking of the panes was carried out to permit ready identification of fragments that bore original surfaces. Details of the counting of surface material is left to a subsequent report. Experiments which established that inking of the panes did not affect the fragmentation process will also be described.

After each breaking experiment the debris from each tray was transferred to paper packets using a soft paint brush. The paint brush was selected for sweep- ing up the debris in view of the large areas to be covered. Great care was taken however to use a routine which minimised the carry over of particles from one tray to another. During brushing and subsequent operations every effort was made to minimise further fragmentation of the debris.

The separation of glass particles into various size fractions was carried out using 38 mm diameter test sieves with mesh sizes of 1000,500 and 250 pm. This generated material of nominal size fractions 1 - 0.5 mm and 0.5 - 0.25 mm. Gen- tle agitation and tapping of the sieve was applied rather than vigorous shaking to minimise further fragmentation. Large particles retained by the coarsest sieve were classified into size fraction >20 mm, 20- 5.0 mm and 5.0- 1.0 mm by direct measurement with a ruler.

The number of particles of each size was determined by counting using a stereomicroscope (x 20). To facilitate the counting of the smallest particles, a suction device was used to remove each particle as soon as it had been counted. The pickup nozzle of this device consisted of a Pasteur pipette, widened at its narrow end by cutting off the fine tip. This was connected by a short length of plastic tubing to a stainless steel filter tube of diameter 13 mm containing a fine mesh grid. The filter tube was made of stainless steel as this material was found to produce less problems with static electricity than glass or plastic. The filter body could be rapidly opened and the contents emptied. Gentle suction was provided from a low powered air pump of the type used in vacuum tweezer devices.

Results and Discussion

Smashing of panes and collection of debris The breaking rig was sturdily constructed and every effort had been made to

ensure that the impact on the pane was symmetrical. Putty was not used to re- tain a pane because it could not be dried and hardened to represent the aged material normally present in window frames. Firm clamping using wooden strips was considered adequate to simulate the rigid mounting of a typical window.

Considerable energy was needed to break the pane and this may have been because the impacting object was smooth and rounded. Another contributory factor was the high symmetry of the experimental arrangement which helped to

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absorb energy: an asymmetrical arrangement would probably have encouraged crack formation. From the weight of the steel ball (effectively 8.2 kg when the weight of the arm was included) and from the distance through which it fell (1.5 m) it was estimated that the energy imparted to the pane was 123 J. As a result about one sixth of the area of the pane disintegrated and was dispersed.

The choice of sizes for fractions was somewhat arbitrary with greater atten- tion being given to the casework size range which was taken to be a combination of the two fractions 1.0 - 0.5 mm and 0.5 - 0.25 mm. Of the eight replicate runs, large fragments (> 5 mm) were recorded for only the last three. These runs also employed the chute mechanism to minimise secondary breaking.

It must be appreciated that long thin particles with the longest dimension (1) will pass through a mesh size smaller than (1). Thus where a particular size range is described it relates to the fraction that passed through this mesh size.

Reproducibility and symmetry A major aim of the research was to explore the effect of window size and the

effect of the thickness of the pane. However, a preliminary goal was to establish the reproducibility of the data between breakings when all the physical condi- tions were maintained as constant as possible. Clearly if the reproducibility was poor under such conditions then the effect of varying certain parameters such as size and thickness would be very difficult to assess.

Eight replicate breakings were carried out and the particle counts from one of the runs (run 8) are reproduced in Fig. 3. The data from this run show good

size fraction in m Key:

El 20-S Cl.0

la-O.5 0.5-0.25

sectorr 1*2

Fig. 3. Particle counts for various size fractions for run 8.

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overall symmetry in that the particles are almost equally distributed between the left hand side and the right hand side of the figure. Certainly for the smallest size fractions, where the number of particles is large, the difference between one side and the other is only a few percent. When the distribution is more closely inspected and the number of particles within individual trays are considered then the pattern of results becomes less uniform. The number of particles of a particu-

200

Number of fragments

150

100

50

0

0

I

Run

Key: x 1 0 2 A 3 &4 .5 v 6 0 7 . 6

curve fit to _____._ mean results

range _-__-

I.

I ‘. I ! (1 zero result) I \ 9 ’ \ I

’ ‘\ ’ I 1

0 \ . I ’ I a ’ I

: \ v (4 zero results) (4 zero results)

“q- - _.-.-._._._._.~_._._

-ILL:: I, I

O- 0.5- l.O- 1.5- 2.0- 4.0m 0.5m 1 .Om 1.5m 2.0m Distance range of tray in metres

Fig. 4. Variation of particle count between runs as a function of distance from point of breaking. The example is for sector 3 and the size range 1 - 0.5 mm. Dashed vertical line indicates range of results for eight breaking runs.

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TABLE I

NUMBER OF PARTICLES RECOVERED FROM EACH SECTOR

Run Size range (mm)

>20 20-5 5-1 l-O.5 0.5 - 0.25

Total particles in sector 1 1 2 _ 3 4 5

6 (29) 7 Nil 8 Nil

Mean* Nil

Total particles in sector 2 1 2 - 3 4 _ 5 6 (Nil) 7 2 8 5

Mean’ 4

Total particles in sector 3 1 2 3 - 4 5 -

6 Nil 7 3 8 2

Mean* 2

Total particles in sector 4 1 2 _ 3 4 5 _

6 Nil 7 Nil 8 Nil

Mean* Nil

(106) 4 1

18 23 26 27 13

(271) 19 17

119 74 36

135

(1472:) 84 70

340 121 180

48 120

(2764) 196 84

3 20 78 156

_

-

(32) 40 11

62 56 71 46

(1::) 159

68

141 182 134 145 168 292 96 75

100 226 (363) (733) 428 519 137 196

26 73 172 234

_

_

23 48

7

85 170 215 62 97 177 80 113 150 31 76 34 74 152 270 41 149 166

184 409 671 60 148 188

26 77 164 234

23 130 347 28 121 104 2 15 17 6 11 3

31 91 344 14 42 107 22 59 83 9 40 118

17 64 140

*Mean calculated from runs 1 - 8 except for results in parentheses for run 6.

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lar size in one tray is not always closely reproduced on the opposite side of the figure. This behaviour was true also of the other seven runs. For most runs there was fair overall symmetry but corresponding trays on opposite sides of the col- lecting field gave numbers of particles that agreed only roughly with one another. Similarly, when a tray in a particular position was selected and com- parison made between the counts from all eight runs, the reproducibility was poor.

An example of the variation in results between runs is shown in Fig. 4. Essen- tially this plots the number of recovered particles as a function of distance from the point of breaking. Results are given for all eight runs but restricted, for the purposes of illustration, to a particular size range and to one sector. Plots for other sectors and size ranges behaved similarly. The general observation was that, despite the breaking conditions being held as constant as possible, the parti- cle counts between runs varied by up to a factor of four.

Run 8, which was used to illustrate the variation within a particular run, was much more symmetrical than certain other runs. It can be seen with runs 4 and 5 (Table 1) that about twice as much material was deposited on one side of the field compared with the other. The most extreme case was run 6 where second- ary breaking gave rise to considerable additional debris on one side of the collec- ting field. In this run, despite the presence of the chute, large shards of glass fell into certain collecting trays and further fragmented, the quantity of this second- ary debris far exceeding the primary material.

It is not easy to detect trends until the data from more than one run are com- bined. For example, the mean results from all eight runs show a particularly clear pattern (Table 1). If we compare sectors 1 and 4, which are geometrically equivalent by symmetry about the centre line of the debris collecting area, the mean numbers of particles recovered compare closely. In a similar manner there is a close agreement of results between the other two geometrically equivalent sectors, 2 and 3. This is true, however, only when one side of the field of run 6 is ignored. In this run large shards of glass missed the chute and fell into the col- lecting trays. The data shown in parentheses in Table 1, which resulted essential- ly from the secondary breaking mechanism, were not therefore used when calculating the mean results. This is not to say that secondary breaking was en- tirely absent in the remaining runs. However the presence of relatively few large pieces of glass (> 5.0 mm) in the trays suggested that no larger shards had been ejected towards the breaker and therefore secondary breaking was probably of minor importance.

Averaging of the data reveals a trend in the distribution of the material with particle size. When the particles are large (> 1 mm) then the number falling into the inner sectors 2 and 3 is some 4 - 5 times that falling into the outer sectors, 1 and 4. It is intuitively reasonable that the bulk of the material will be projected directly backwards towards the breaker rather than be scattered sideways. How- ever this is progressively less true as the particle size becomes smaller. Thus, for the smallest size of fragment, the ratio of material falling into inner sectors is only between one and two times that of the outer sectors.

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Number of fragments per unit area Further analysis of the results was carried out in a similar manner to that of

Pounds and Smalldon 111. Plots for each size range of glass particles were con- structed in which the density of glass fragments was plotted on the vertical axis

1000

Number of fragments per meWe

100

10

1

0.1

Size Range

> 5mm

2+3

I I I I 0 1 2 3 4

1000 -I

100

10

1

0.1

Size Range

2+3

outer sectors I+4 2’

I I I 1 0 1 2 3 4

Distance from window in metres

Fig. 5. Number of fragments per metre squared plotted against distance from pane for larger size ranges. Mean of 8 panes; ( ??) combined data from outer sectors 1 and 4, ( 0 ) combined data from inner sectors 2 and 3.

260

using a logarithmic scale against, on the horizontal axis, the distance from the window, Figs. 5 and 6. The density of the glass fragments was the number of fragments per square metre and was calculated by taking the area of each tray into account. For the purposes of constructing a plot the distance of the particles

1000 -

Number of lragrnents per metre*

100 -

10

1

0.1

2+3

I I 1 I \ 0 1 2 3 4

1000

100

10

1

0.

0

I

I Size Range

I I I 1 1 2 3 4

Distance from window in metres

Fig. 6. Number of fragments per metre squared plotted>against distance from pane for smaller size ranges. Mean of 8 panes; ( ??) combined data from outer sectors 1 and 4, ( 0 ) combined data from inner sectors 2 and 3.

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from the window was taken as the radial distance at ground level from im- mediately below the point of breaking to the centre of the relevant tray. The points on each plot were fitted to a smooth curve. The advantage of the present work is that averaging of the results from eight windows enabled a much clearer picture to emerge than that obtained by previous workers.

The plots demonstrate, in a more precise manner, the effect already described above: small fragments of glass were seen to fall more uniformly over the whole field than the larger fragments. Considering first the larger size ranges (> 5 mm and 5- 1.0 mm), relatively more fragments fall into the inner sectors than the outer sectors (Fig. 5). For the largest size, > 5 mm, the difference is about a fac- tor of ten and for the range 5 - 1.0 mm it is about a factor of four. When the size of the particles decreases further (Fig. 6, 1.0-0.5 mm) this factor decreases slightly. For the smallest size (Fig. 6, 0.5 - 0.25 mm) the two curves are virtually coincident.

Additional information can be extracted from the plots by considering the slope of a curve at a particular distance from the pane. The slope indicates how rapidly the number of particles is diminishing with distance. Consider, for exam- ple, the smallest particle size range, 0.5 - 0.25 mm. Next consider the behaviour at a distance of 1.25 m from the pane, this distance being taken to represent an individual intimately involved with the deliberate breaking of a pane. By examin- ing the slope of the curve at this point it was calculated that for every 20 cm in- crement in distance, the number of particles fell to one half. This figure emphasises the rapid decline in the number of particles with distance and is in agreement with the observations of Pounds and Smalldon on similar plots.

Variation in size and thickness of panes Although breaking has resulted in an asymmetrical distribution of debris,

numerical counts from equivalent sectors can justifiably be combined to aid data analysis. Using this approach to consider results from the eight replicate runs showed that there was sufficient reproducibility between runs to warrant studies in which the size and thickness of the pane are varied. Further breakings were conducted in duplicate and the data will form the basis of a subsequent paper.

Transfer, persistence and casework relevance The investigations described here were aimed at establishing the nature of the

debris that results from the smashing of panes. A logical step in relating this to problems of casework interpretation would be to consider how much glass debris is transferred to a person standing nearby (i.e. transfer) and the rate at which this would subsequently be shed (persistence). One could then construct a simple model which assumes that ejected particles will travel along regular trajectories until they reach ground level unless interrupted by a person standing nearby and in this way the present data could be used to predict the extent of transference.

Conclusions

The experimental rig for breaking window panes performed well and yielded

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data in agreement with earlier reports. By combining data from a number of ex- perimental runs, a more precise picture has emerged concerning the manner in which fragments are scattered according to size.

Acknowledgements

The authors are indebted to the students and lecturers at the North War- wickshire College of Art and Technology at Nuneaton who assisted on a volun- tary basis. They are also indebted to Keith Clemo of the Motor Industries Research Association (MIRA) for engineering support and to Anne Welch and Catherine Collinson of the Metropolitan Police Forensic Science Laboratory, London, for their efforts as consultant operational forensic scientists.

References

1 CA. 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. Forensic Sci. Sot., 18 (1978) 197 - 203.

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