SCIENTIFIC & TECHNICAL

A systematic study of the lifting technique for determining the writing sequence of intersecting

ball pen strokes SC LEUNG and YM LEUNG

Forensic Science Division, Government Laboratory, 88 Chung Hau Street, Ho Man Tin, Kowloon, Hong Kong

Science & Justice 1997; 37: 197-206

Received 8 February 1996; accepted 8 January 1997

The determination of stroke sequence of intersecting pen strokes by the Kromekote technique was studied and a theory is proposed to explain the lifting of ball pen ink and its application in the method. The effects of the writing pressure of the intersecting strokes and of the rubbing force on the success rate of the technique are described. Various factors, including writing pressure, which affect the stroke width are considered and it was concluded that the method is valid provided that the intersecting strokes are similar in width and that controlled rubbing is used on the Kromekote paper.

La dttermination de la sequence d'apposition de traits B leur croisement par la technique Kromekote a CtC CtudiCe et une thCorie est proposCe en vue d'expliquer le transfert de l'encre de stylo B bille et son application pour cette mCthode. Les effets de la pression de 1'Ccriture et de la force de frottement, sur le taux de rkussite ont CtC dkcrits. Plusieurs facteurs qui affectent la largeur du trait, y compris la pression de l'kcriture, ont CtC considCrCs, et il a CtC conclu que la mCthode est valide B condition que les traits se croisant soient de largeurs similaires et que le frottement sur le papier Kromekote soit contr6lt.

Die Bestimmung der Strichfolge sich iiberschneidender Kugelschreibeerlinien mit hilfe der Kromekote Technik wurde untersucht und eine Theorie zur Erklmng des Abnehmevorgangs der Kugelschreiberpaste und der Anwendung der Methode vorgeschlagen. Die Einflusse des Schreibdruccks der sich uberkrenzenden Linien sowie der Anriebskraft auf die Erfolgsrate der Technik wird beschrieben. Es werden verschiedene Parameter betrachtet, einschlieBlich des Schreibdruckes, welcher die Strichbreite beeinflufit, und geschlossen, dafi die Methode zuverlassig ist, vorausgesetzt die Strichuberkreuzungen sind gleich in ihrer Strichbreite und das Kromekote Papier wird mit kontrolliertem Druck aufgerieben.

Se estudia la determinacidn de la secuencia de 10s trazos en 10s trazos transversales de una pluma por la tCcnica de Kromekote y se propone una teorfa para explicar el levantamiento de la bola de la pluma y su aplicacidn en el mbtodo. Se describen efectos de la presidn a1 escribir 10s trazos transversales y 10s de la fuerza de raspado. Se consideran varios factores que incluyen la presidn de escritura que afectan a1 ancho del trazo y se concluye que el mCtodo es valid0 con tal de que 10s trazos transversales Sean de anchura similar y que se use un raspado controlado en el papel Kromekote.

Key Words: Forensic science; Document examination; Lifting technique; Kromekote paper method; Intersecting strokes; Sequence of writing.

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Study of lifting technique for determining w mriting sequence of intersecting ball pen strokes

Introduction Document examiners are often required to determine the sequence of strokes with the aim of verifying either the writing habit, or any alterations to the original entries. The scanning electron microscope is adequate for the examina- tion of stroke crossings produced by heterogeneous writing instruments (e.g., pencil over or under ballpoint pen, type- writing over or under ballpoint pen, etc.), but stroke cross- ings produced by homogeneous writing instruments do not yield satisfactory results [I].

Various techniques have been developed for the determina- tion of the writing sequence of two intersecting ballpoint pen strokes [ 2 4 ] . Igoe and Reynolds reported a lifting technique which they claimed, achieved a 100% accuracy [4], and several modifications of the original procedure have been suggested [5-71. Despite general acceptance of the lifting process, Moore questioned the validity of the technique on the grounds that the results of the lift were determined by writing pressure variation rather than the actual sequence of drawn lines [8]. In view of the contro- versy and the lack of a general theory for the methodology, the authors of this paper felt that a more detailed inquiry into the lifting technique was required.

The lifting technique and the railway effect theory A ballpoint pen consists of a small sphere of freely rotating hard metal in a closely fitted cage located at one end of a tube of ink. Such an arrangement enables the rotating ball to draw out a continuous supply of ink which is deposited onto a paper surface [9]. Ballpoint pen inks are typically slow-drying and viscous so that they do not normally exude out of the pen by gravitational force or by capillary action. When a stroke is being written, the ball of the pen rotates

along an axis that is perpendicular to the general direction of the stroke (Figure 1). Since the 'equator' of the rotating ball has the largest circumference, it follows that any point at the equator (such as 'E') rotates with the highest speed whereas those nearest the 'polar' regions and away from the median (such as 'PI' and 'P2') rotate with the slowest speed.

As ink is continuously drawn by the rolling action of the ball, it accumulates at the polar regions that do not come into contact with the paper surface. However, as a result of its viscosity and the gliding movement of the ball along the paper surface, the ink present immediately adjacent to the rim of the stroke will be drawn and incorporated into the stroke. In addition, during writing, the very tip of the ball exerts the highest pressure on the surface. On a smooth, coated paper, the slow-drying and viscous ink, being not immediately absorbed into the paper, is squeezed towards the flanking edges (R1 and R2 in Figure lb) of the stroke. Heavier deposits of ink will be present at both edges with less ink present at the centre, thus creating the appearance of two parallel streaks of ink demarcating the outline of the stroke, similar to the tracks of a railway.

In the lifting technique, the glossy side of a piece of Kromekote paper is placed over the questioned intersection and is rubbed with moderate pressure with a blunt instru- ment. Observation of the lift under low magnification dis- closes a pattern of edge markings, with those from the last written line passing through those from the previous stroke at the intersection.

The authors of this paper believe that the transfer of ball- point pen strokes to the Kromekote paper is directly related

writing direction

ink stroke I ball pen

pen pressure

FIGURE 1 Diagrammatic explanation of the 'Railway Effect'. (a) The ball rotates along an axis perpendicular to the direction of the stroke, unused ink accumulates at P1 and P2. (b) The tip of the ball exerts the highest pressure on the paper surface and

ink that has not been absorbed is squeezed towards the outer boundaries of the stroke (R1 and R2). - - -

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3 "=-j 4

FIGURE 2 (a) Part of a ball-pen stroke written on a white card showing conspicuous 'Railway Effect'. (b) Chronological sequence, (1) and (2)' of intersecting ball pen strokes written on a white card.

to the railway effect. In Figure 2, the effect is particularly evident in a ballpoint pen stroke written on a glossy coated paper. When the stroke is cut through by a subsequent stroke, the original ink tracks of the first stroke are smudged by the passing ball and are replaced by the newly laid tracks of the second stroke. As a result, the chronological sequence of two intersecting strokes written on a non-absorbent paper surface is clearly visible.

Although the railway effect is generally not distinctly per- ceivable on ordinary paper which has a greater affinity for ball pen ink, it is postulated that the tracks of ink are nonetheless present; for ordinary paper surfaces, the many tiny crevices and depressions that exist between the paper fibres can accommodate much of the ballpoint pen ink which, under the writing force, is compressed and compact- ed into them, and so less ink is expected to be exuded to either side to form easily discernible railway tracks of the stroke. A ballpoint pen stroke written on ordinary paper therefore consists of a central column of more tightly packed ink that is sandwiched at both sides by loosely laid streaks of ink.

As is shown in Figure 3, when the Kromekote paper is rubbed with an appropriate force against the intersection, it comes into close contact with the tracks of ink situated at the top'of the depression made by the strokes, but not with the ink at the bottom of the depression, more of which is absorbed'by, or has penetrated deeper into, the constituents of the paper. Therefore, if the rubbing pressure is correct, only the railway tracks of ink will be lifted. At the intersec- tion, where the rails of ink of the first stroke have been replaced by the newly laid rails of the second stroke, a suit- able rubbing pressure will selectively transfer the tracks of ink to the Kromekote paper, thereby facilitating the deter- mination of writing sequence.

Influence of writing pressure, rubbing pressure and frequency It can be assumed that the width of a ballpoint pen stroke is related to the pressure imposed on the pen during writing. The relative widths of two crossed strokes affect the result of the lift; document examiners have observed that strokes with the same width normally give better results than those with different widths. It was also acknowledged that a large difference in the pressure of the two lines, especially when the first line was drawn with a larger pressure than the sec- ond, led to erroneous results [4,8].

Whilst Igoe and Reynolds [4] had stated that the back of the Kromekote paper should be rubbed 'with an even, moder- ately heavy pressure with a blunt instrument (such as the

rubbing pressure

79/ surface

- ink stroke +

FIGURE 3 The lifting process whereby appropriate rubbing pressure causes the lifting of only the ink 'railway tracks' (R1 and R2) at the edges of the stroke but not that at the centre

of it.

Science & Justice 1997; 37(3): 197-206

Study of lifting technique for determining wl viting sequence of intersecting ball pen strokes

TABLE 1 Loading weights for first and second strokes.

FIGURE 4 The pen pressure gauge.

cap of a felt tip pen)', the exact specifications were not given. The lifting technique is a manual process and, as such, relies heavily on the craftsmanship of the document examiner carrying out the test. Since the best result is nor- mally obtained with the first rubbing and the nature of the method allows only a limited number of repeated trials to be conducted on the questioned intersection, inevitably the method is susceptible to human error. Accuracy of the lift- ing technique would be improved, and human inconsisten- cy would be reduced or eliminated if the rubbing pressure could be accurately controlled.

The lifting technique was therefore critically reviewed with respect to the influence of writing pressure on the result of the lift and the optimal rubbing pressure, if any, to be applied.

Experimental A pen pressure gauge was devised, as shown in Figure 4, consisting of a plastic cylinder with holes at the centre of both circular ends that could just allow a ballpoint pen to pass through freely. The top of the pen was firmly stuck to a circular disk which accommodated the loading of weights. Strokes could be written with fairly accurate pen pressure by using this simple set-up.

Sample Loading weights (gms) for First stroke Second stroke

Equal pressures 80 80 100 100 150 150 200 200 250 250 300 300 350 350

Different pressures

One-year 100 20,40,60,80,150, 200,250,300

200 20,40,60,80,100, 150,250,300,350

One-week 300 20,40,80,120,150, 180,220,250

In order to control the rubbing pressure, a rubbing pressure gauge was invented by replacing the ballpoint pen in the pen pressure gauge by a piece of metal rod with a spherical end of 6 mm diameter.

Using the pen pressure gauge, approximately 2000 crosses were drawn, in known sequence on A4 size, 80 gsm white paper so that the two strokes of each cross were roughly at right angles to one another. About half of the samples were approximately one year old while the other half were fresh- ly made and allowed to stand for one week before any tests were conducted. Some of the crossed strokes were written with equal pen pressure generated by loading weights of 80, 100, 150, 200, 250, 300 and 350 grammes. Other crosses, written with different pen pressure, were also prepared (Table 1).

These crosses were subjected to the lifting technique for stroke sequence determination in conjunction with the rub- bing pressure gauge which was loaded with weights of 100, 200, 300 and 400 grammes. The number of movements of the rubbing instrument on the Kromekote paper past the intersections varied from 20 to 400.

The number of tests conducted on different combinations of pen pressure and rubbing pressure varied; if a clear-cut result was obtained at the first instance, further trials were not carried out. However, if a result was ambiguous, more tests were conducted until the operator was satisfied that the result was accurate. For a single set of conditions, up to 13 repeated trials were made. In the rare event of ambiguous or contradictory results that were not due to human error, the scoring was represented by the results with the highest frcqucncy of occurrence.

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SC LEUNG and YM LEUNG

2.00 J 1

0 500 1000 1500 2000 2500 3000 3500 pen pressure (loading weight in grarnrnes)

FIGURE 5 The relationship between stroke width and pen pressure. (a) Lamy ball pen refill - fine. (b) Kippy-727 ball pen - medium. (c) Lamy ball pen refill - coarse.

------0------ stroke written on paper placed on Formica surface ------A------ stroke written on paper placed on a piece of paper (0.11 mm thick) ------.------ stroke written on paper placed on a pile of paper (16 mm thick)

The results were interpreted by comparison with three stan- too low (e.g., 100 g loading for both one-week and one-year dards consisting of ICromekote paper lifts with increasing samples), the transfer of ink to the Kromekote paper was intensity of the transferred outline of crossed strokes. not sufficient to enable a conclusive determination of stroke

Results sequence to be made. If the number of rubbing movements

The results of the Kromekote paper tests on strokes drawn Was too high (e.g., in the One-Year 80 g180 g samples: 400

with equal pen pressure indicated that irrespective of the times for 200 g rubbing pressure, 160 times for 400 g rub-

number of rubbing movements, if the rubbing pressure was bing pressure), the result was ambiguous. Inconclusive

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Study of lifting technique for determining writing sequence of intersecting ball pen strokes

results were also obtained if the number of rubbing move- ments on the Kromekote paper was too small (e.g., in the one-year 80 g/80 g samples: 20 times for 300 g rubbing pressure). In addition, it was generally observed that better results were derived from the more recent (1 week) samples than from older (1 year) writing.

The results of the Kromekote paper tests on strokes drawn with different pen pressures indicated that incorrect results would be obtained if the writing pressure for the first stroke was much larger than that for the second stroke; this hap- pened when the loading on the pen pressure gauge for the writing of the first stroke was approximately 2.5 times that for the writing of the second stroke. The same did not apply, however, if the writing pressure for the second stroke was much higher than that of the first stroke.

Although the results were not very clear-cut, they neverthe- less showed that there is an optimum condition in respect of the number of rubbing movements and the pressure applied to the rubbing instrument on the Kromekote paper. The rub- bing pressure must be high enough to produce a transfer of ink from the crossed strokes to the lifting medium, while at the same time the number of rubbing movements should be reasonably small so as to reduce the chance of a shift in rel- ative position between the document bearing the questioned intersection and the Kromekote paper. In the present set-up, 40 rubbing movements with a loading weight of 300 g on the rubbing pressure gauge were considered to be most suit- able and, in subsequent investigations, these conditions were used.

The relationship between stroke width and writing pressure In order to secure a more reliable determination of the sequence of crossed strokes with different widths, the rela- tionship between the width of ballpoint pen strokes and the applied pen pressure was investigated.

calliper gauge with a measurement range of 0-150 mm and a resolution of 0.01 mm.

Results The results of stroke width measurements are presented graphically in Figure 5. It is clear that stroke width was not linearly related to pen pressure and that a stroke produced by a large pen pressure was only slightly wider than that generated by a much smaller pen pressure. As expected, stroke width increased as the loading on the pen was increased. The increase in stroke width with the addition of loading weights was larger at the beginning, gradually levelling off at a pen pressure loading of about 500 g.

The surface of the substrate on which the piece of paper was placed also affected stroke width. A single sheet of paper placed on a formica bench top gave thinner strokes as com- pared with strokes written on the paper placed on top of another sheet of paper and strokes written on the paper on top of a pile of paper. It was found that strokes written on the top page of a pile of paper were the thickest.

A comparison between manual rubbing and optimal pressure rubbing In a practical situation, success in carrying out the Kromekote paper test would be better ensured by first con- ducting a trial run on an intersection of known sequence in the same writing and similar to the one of interest. By doing so, the examiner will become familiar with the amount of pressure and rubbing frequency that will give a correct result.

Although the above tactic has been found to be useful, the authors of this paper believe that the lifting technique could be further improved if the rubbing were performed with the aid of the rubbing pressure gauge and in accordance with the optimal conditions of 300 grammes loading and 40 rub- bing movements. It was therefore considered worthwhile to investigate whether or not optimal pressure rubbing gave

Experimental more accurate results than manual rubbing. A Kippy-727 ball pen with a medium sized tip (I mm diameter) and Lamy ball pen refills with fine (0.7 mm diameter), and coarse (1.4 mm diameter) tips were installed consecutively in the pen pressure gauge loaded with a large variety of weights. Three strokes for each loading were drawn by using the pen pressure gauge on sheets of A4 size, 80 gsm paper placed on three different substrates: a formi- ca surface; on another piece of paper of 0.11 mm thick on the formica surface (simulating a direct tracing); and a pile of paper of 16 mm thick placed on top of a formica surface (simulating the use of a writing pad). The strokes were placed under a Nikon SMZ-10 stereo-zoom microscope connected to a Nikon HFX photomicrograph unit, and a grid with 0.1 mm graduations was overlaid on the strokes. Photographs of the strokes overlaid with the grid were taken and the widths were measured by a PAV 6511 electronic

202

Furthermore, in the case of alterations on a document, inter- secting lines may have been made at the same time or at dif- ferent times relative to one another. Therefore, simulated alterations which consisted of intersecting lines produced by different pen pressures and made at different periods were prepared; in each intersection, one stroke was drawn two years earlier than the other stroke.

Experimental Blind tests were conducted conventionally by two examin- ers in comparison with the optimal pressure rubbing method, on intersecting ballpoint pen lines written with dif- ferent pen pressure on pieces of paper placed on a pile (16 mm thick) of paper. Pen pressure loading ranged from 40 g to 350 g, while the ratio of the pressure associated with the writing of the two strokes varied from 1.0 to 3.8. All the

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SC LEUNG and YM LEUNG

TABLE 2 Results of blind tests for comparing optimal pressure rubbing and manual rubbing, the intersecting strokes being written with equal pen pressure.

Success Rate (%) Pen pressure ( g ) Result Optimal Pressure Manual Rubbing Manual Rubbing

Rubbing (DEl ) (DE2)

+ = positive result. x = incorrect result DE = document examiner

crosses were drawn with the pen pressure gauge on A4 size, For intersecting strokes simulating alterations at different 80 gsm white paper, three days before the tests were con- times, where the pen pressure for the writing of the first

ducted. stroke was much higher than that for the second stroke, opti-

Manual rubbing on Kromekote paper was carried out by two examiners on ten samples for each set of pen pressure combinations. The same number of intersections was tested with the rubbing pressure gauge using the favoured condi- tions of 300 g loading and 40 rubbing movements. Interpretation on all the results was made by only one examiner so as to ensure uniformity and consistency and give a reliable comparison between the accuracy achieved bv the two methods.

ma1 pressure rubbing did not seem to have an advantage over manual rubbing (Table 3). The observations confirm the inference that when the two strokes of an intersection have been written with vastly different pen pressure, the result might not be reliable. However, when the pen pressure associated with the writing of the crossed strokes was such that the ratio of the writing pressure of the thicker stroke to that of the thinner stroke was less than two, optimal pressure rubbing was seen to have given more accurate results.

Results The results of the simulated alterations and the blind tests are summarized in Tables 2 and 3. It is obvious from Table 2 that for intersecting strokes written with equal pen pres- sure, optimal pressure rubbing generated a higher percent- age of correct answers in comparison with manual rubbing; the average 'success rate' for optimal pressure rubbing was 67% with no false positive results, while those for manual rubbing conducted by two examiners were respectively 40% with nil error and 3 1 % with 1 % error.

Discussion The Kromekote paper lifting technique is a simple and effective method for the determination of the sequence of intersecting ballpoint pen strokes, but has some limitations particularly with regard to writing pressure [8 ] . Results have shown that when the first line is written with a pres- sure approximately 2.5 times or more than that used for the second stroke, the resultant lift may give a wrong interpre- tation of stroke sequence; the same does not apply when the second stroke is written with a much higher pen pressure than the first.

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Study of lifting technique for determining writing sequence of intersecting ball pen strokes

TABLE 3 Results of blind tests for comparing optimal pressure rubbing and manual rubbing, the intersecting strokes being written with different pen pressure.

Ratio Ratio

of pen of stroke pressure width Success rate (%)

Pen (Heavier vs (Thinner vs Optimal Pressure Optimal Pressure Manual Rubbing Manual Rubbing pressure (g ) Lighter) Thicker) Result Rubbing* Rubbing? @El)? (DE2)f

300180 3.8 0.75 + 50 30 10 10 x 10 60 60 0

+ = positive result. x = incorrect result * 1st stroke: 2 years, 2nd stroke: 1 week t both strokes: 1 week DE = document examiner

This phenomenon can be explained by the Railway Effect appeared to be darker in colour and more uniform in inten- Theory. If the pen pressure associated with the writing of sity. On the contrary, in the Kromekote paper lift, the the first stroke is much larger than that of the second stroke, outline of a stroke written with a much heavier pen pressure the writing tip cannot completely obliterate the railway was represented by the railways of ink with very little ink tracks of ink of the first stroke and so the newly laid rails of present at the centre of the stroke. It is this difference in the ink of the second stroke cannot replace those of the first intensities of the lifts from the two categories of intersect- stroke, causing an ambiguous or erroneous interpretation. ing strokes that generates ambiguity of interpretation. The reverse situation will be easier to interpret, as the ink

Referring to the railway effect theory, it is believed that the tracks of the second stroke written with higher pen pressure

application of a force of 80g or larger on the pen results in can easily replace the lighter rails of ink of the first stroke.

an 'ironing' of the somewhat fluffy paper surface accompa- By studying Kromekote paper lifts of intersecting strokes nied with the squeezing of ink into the spaces between the with writing pressure ratios greater than ten, it was found paper fibres and the deposition of loose 'heaps' of ink at the that apart from exhibiting no railway effect, the whole body flanking edges of the stroke. For a stroke written with a very of a stroke written with a very low pen pressure invariably light pressure (e.g., 20-40 g), the force applied to the

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writing instrument is so small that the ink of the whole edges of the strokes written by a new ballpoint pen are stroke is gently laid on the surface of the paper. Hence the expected to be thin, thus furnishing a diminutive transfer of railway effect that is fundamental for sequence determina- ink onto the lifting medium and leading to higher than tion is non-existent. When the lifting technique is applied to expected inconclusive results in the blind tests. such crossings, the Kromekote paper picks up ink from the whole of the stroke written with a very small pen pressure. On the other hand, the same treatment to a stroke written with a large pen pressure can only effect the removal of the loose, flanking stacks of ink, thus resulting in the transfer of the outline of the stroke to the lifting medium. As a result, an accurate determination of the chronological sequence of such intersections becomes impossible.

Extreme caution has to be observed in the event of the detection of diagnostic appearance of lifts from strokes written with vastly different pen pressure, an appropriate interpretation for which would be inconclusive. It can be concluded that in most instances the lifting phenomenon is caused, not by a difference in the amount of ink in different parts of the stroke, but by the differential adherence of the ink to the paper.

The accuracy of the lifting technique can be improved if the inconsistency inevitably present in a manual process can be addressed. It is easy to understand that if the Kromekote paper is rubbed with too small a force, no transfer will be effected. On the other hand, if the rubbing pressure applied on the Kromekote paper is too large, the trough of the strokes will collapse and all the ink, including that at the bottom of the depression, will be pressed against the lifting agent, thus obscuring the railway tracks of ink that are instrumental for stroke sequence determination. This explains why ambiguous results will be obtained if the rub- bing pressure is too high.

The rubbing pressure gauge will help to ensure that a repro- ducible as well as an accurate rubbing pressure is imparted on the Kromekote paper. In addition, the discovered optimal conditions of 300 grammes loading and 40 times of rubbing will give the best result from the lift. One drawback of the rubbing pressure gauge is the necessity of moving the Kromekote paperlquestioned intersection composite under the rubbing instrument, instead of the more convenient way of moving the rubbing instrument on the Kromekote paper. However, it would not be difficult to modify the rubbing pressure gauge to conform with the latter, more convenient set-up.

Blind tests indicated that optimal pressure rubbing is prefer- able to manual rubbing although the success rate for inter- secting strokes written with equal pen pressure was far less than 100% because a brand new ballpoint pen had been

Where the difference in pen pressure between the strokes was high, optimal pressure did not appear to have an advan- tage over manual rubbing. The fact that chronological sequence of crossed strokes can be determined by the Kromekote paper method depends on the obliteration and the subsequent replacement of the railway tracks of ink of the first stroke by those of the second stroke. A crossing stroke written with very light pen pressure cannot wipe out and replace ink rails of the existing stroke written with very heavy pen pressure and so no matter what rubbing method is utilized, an inconclusive or even erroneous result ensues. However, when the ratio of the writing pressure of the thicker stroke to that of the thinner stroke was less than two, optimal pressure rubbing was seen to have given more accurate results. Undoubtedly document examiners must be cautious in the examination of intersecting ball pen lines with different widths. Several variables affect the width of strokes, including pen pressure, the size of the ball pen tip, and the surface on which the questioned document was placed when the writing took place. In a case situation, all these parameters are generally unknown. The investigation indicated that a correct interpretation would be obtained provided that the pen pressures of the two strokes differed by no more than a factor of 2.5; this from experiment, gave a width ratio of 0.85. The two intersecting strokes should ideally be equal in width, or the ratio of their widths should be greater than 0.85. In this regard, an important inference can be drawn from the blind test: frequencies of error were close to zero for intersecting strokes whose stroke width ratios were greater than 0.85. This ratio of 0.85 can there- fore be used as a reference datum for deciding whether or not to apply the lifting technique and for interpreting the result.

Apart from the above, there are other factors that affect the validity of the result, notable ones being, firstly the proper- ties of the ink and the amount of ink deposited onto the paper; a brand new ball pen usually does not write as flu- ently as a well-used one. The railway tracks of ink in a stroke written by a brand new ballpoint pen are expected to be much less than those in a stroke written by a used ball pen. Secondly, passage of time causes the ink to become dry and be absorbed into the paper and so provided that other factors are the same, lifts from the more recent intersections are usually more clearly defined. Freshness of the intersec- tion will thus contribute towards accuracy of the result.

used for writing the crosses. The ball of a new pen is fitted Thirdly, the surface of the paper also affects the quality of tightly in the ball housing so that the flow of ink is some- the lift. A glossy surface that does not absorb ink offers an what restricted when compared with that in an old ball pen. unequivocal visual display of the chronological sequence of It follows that the railway tracks of ink deposited at the intersecting ballpen strokes. A piece of paper with a rough

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Study of lifting technique for determining w ~riting sequence of intersecting ball pen strokes

surface provides plenty of crevices to be filled by the ink so that less of it will be forced towards the sides of the stroke and be incorporated into the railway tracks of ink.

Fourthly, the speed of the writing instrument to a certain degree controls the amount of ink deposited on the paper and the intensity of the railway tracks of ink in a stroke. It is also possible for a writing tip with a very high speed to 'fly' across the trough of the previous stroke, leaving no evidence of the cross-over.

In the case of alterations on a document, it is not impossi- ble for the two intersecting strokes to be written by differ- ent pens, so that the ink in the two intersecting strokes could have different properties; the writing tip of the pens may also have different sizes. It is therefore advisable to check the ink lines with a microscope and an instrument such as the VSC and take into consideration any difference in writ- ing instrument and/or ink in the two crossed strokes before conducting the Kromekote paper lift.

The authors are aware of the fact that actual writing activi- ties are much more dynamic and variable. The present study provides a simplified model which enables a better under- standing of the lifting technique.

Acknowledgements The authors would like to record here their gratitude to Dr BN Dailly, the Government Chemist of Hong Kong for his interest and encouragement in this work, and to Dr DG Clarke, Assistant Government Chemist for proofreading the draft and for his valuable suggestions. Thanks are due to Mr KW Tam who helped collect the data and to Mr YL Wong who helped in the preparation of the manuscript.

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