Chapter 6
Materials Used to Create Documents

Documents are increasingly being viewed electronically on computers and are moved around in cyberspace. But documents continue to exist in a real world of physical entities and it is necessary for the forensic document examiner to have a good knowledge of the materials from which documents are constructed. In this chapter, the materials and processes from which these physical objects are made will be described. However, the various analytical methods available for examining these materials will be considered in Chapter 7 and the casework context of alterations to documents will be described in Chapter 8.

Document examiners come from a variety of educational backgrounds, usually, but not always, with a strong scientific content. The examination of the materials present in a document can be carried out at a number of levels reflecting factors such as time available for the examination, costs, equipment availability and the expertise of the expert or their colleagues. It is to be expected that as the scientific complexity of an examination increases, the feasibility of carrying out these examinations decreases. For example, comparing two sheets of paper can be done at a straightforward level (such as size, colour, ruled markings), a more complex level (such as thickness measured using a micrometer to take an average of readings across a sheet or examination under differing ultraviolet wavelengths), or at an even more complex level (such as determining the plant material from which the paper is made by high power microscopy, requiring an associated high level of botanical knowledge).

In this chapter, the physical elements from which documents are made will be described and the general approach to their examination will be shown. Increasingly complex methods may simply be unavailable to the expert and, in any event, may be unnecessary if other, simpler techniques are sufficient. In general, the examination strategy tends to go from simple to complex and at the same time from non-destructive to destructive, from cheap to expensive and requiring increasingly specialist equipment and knowledge. In addition, showing that two physical objects – be they paper, ink or some other component of a document – are different is generally much easier than showing they are similar since in the latter situation there is always the possibility that ‘just one more (complex and expensive) test’ might show a difference. Further, since two objects cannot logically be one and the same object, there will always be some differences between any two ‘similar’ objects and hence the degree of similarity will always need to be determined when explaining its significance.

Examination strategies of the physical components of a document will therefore vary from case to case and from laboratory to laboratory. ASTM (formerly the American Society for Testing and Materials) in the USA has published standard approaches to examining the materials from which documents are created and these will be given in the relevant sections below. These standards are recommendations of good practice but in a given case they may need to be interpreted flexibly depending on the materials and resources available.

6.1 Paper

By far the most common substrate for a document is traditional paper. However, handwriting can occur on an almost limitless number of other materials such as graffiti on all manner of buildings, on fabric (such as the inside of a sports bag), on CDs and DVDs and so on.

Historically, handwriting was often written on other paper-like materials. One such is parchment made from animal skins such as goat or sheep or, when made from calf skin, known as vellum.

Paper is usually made from plant material, the main constituent of which is cellulose fibres. (Papyrus is made from parts of particular plants and its method of manufacture is generally different to that described below for modern paper.) The source of the fibres can vary from trees (by far the most commonly encountered) to grasses (may be used in some countries) and other materials such as cotton (specifically the hairs on the seeds for high quality papers) and bast fibres (certain botanical elements of plants such as hemp and jute). Individual fibres are typically about 3–5 mm in length and about 50 µ in diameter (1000 µ being the same as 1 mm). The basic requirement is that the fibres are processed in such a way as to produce a thin mat of overlapping fibres capable of receiving ink in a stable and long-term manner.

6.1.1 Manufacture of paper

In order to make paper, it is first necessary to create a pulp consisting mostly of water (it is for this reason that paper mills are situated near large sources of water) into which has been added the fibre material after it has been removed from the trees (or other vegetable matter) by a process of grinding down and chemical treatment. The latter treatment involves cooking the fibres in the presence of some strong chemicals that help to eliminate impurities from the pulp mixture, particularly lignin which tends to give paper a yellowish colour if left in the mix.

Paper of course has many uses and it depends on the intended use of the product as to how the basic paper formulation is treated. For example, paper that is intended for wrapping or paper bags needs to be reasonably physically strong but does not need to have a particularly pleasant appearance, whereas writing paper needs to be capable of taking ink onto its surface and it might be desirable that it be very white, while paper for currency needs to be very strong indeed to stay in circulation despite continuous handling and also needs to be secure against counterfeiting. For the various uses to which paper can be put, additives (see Section 6.1.2) will be put into the mix during manufacture to impart the desired properties.

The slurry-like pulp material forms the basic paper sheet through the removal the excess water leaving behind the paper fibres. There are two main processes used on an industrial scale to achieve this. They are known as the Fourdrinier process and the cylinder mould process.

In the Fourdrinier process, the pulp passes over a mesh, the so-called wire (today made out of plastic but historically from a metal wire), through which much of the water drains, leaving the fibres caught as a mat on the wire surface. The paper is still very moist at this stage but physically quite robust such that a watermark can be produced using a raised design on a cylinder (known as a dandy roll), which is pressed into the paper causing some fibres to be displaced in the area of the raised design and hence making the paper more translucent at that point. However, such watermarks tend not to be as good as those produced by the cylinder mould process (see below).

The wet mat of fibres needs to have almost all of the remaining water removed by pressing and heating it. In order to compact and make the paper surface smoother, it is then passed through heavy rollers (a process called calendering).

Cylinder mould paper machines are more rarely encountered and they are often used to produce security paper (for documents that are made more difficult to counterfeit). Paper is made on a revolving wire-covered cylinder that is partly immersed in a vat of paper pulp. A relief pattern on the mesh will impart a watermark to the paper as the fibres are less densely accumulated in these areas. The watermark produced can have a multi-tonal (shades of grey) appearance of great clarity and for this reason it is the manufacturing process of choice for many types of security documents as the watermark remains one of the most difficult properties of a sheet of paper to simulate.

6.1.2 Additives used in papermaking

The use for which particular paper products are made will determine the detailed manufacturing process used, including what additives are needed to give the paper its desired properties. The colour of paper is an obvious property and for many uses white paper is needed. Wood pulp tends to produce paper of an off-white appearance, so in order to improve its whiteness optical brighteners are often added. These chemicals have the property of absorbing ultraviolet light and re-emitting it in the blue part of the visible spectrum (see Box 8.1 in Chapter 8). This makes the paper appear brighter and whiter. When viewed under ultraviolet light, such papers are extremely bright whereas paper that does not contain optical brighteners will appear a dull violet colour.

Paper made from fibres only (no additives) will tend to have poor ink receptivity. In other words, an ink applied to the paper will tend to bleed into the fibres with unsatisfactory results. The particular ink that is to be used may be for writing purposes or printing purposes and the quality of the final product will be of importance to ensure satisfactory results. In order to make the paper more receptive to ink it is treated with a material called a sizing agent (Biermann, 1996). There are a number of such agents that can either be added to the paper while it is still wet (and hence the agent is dispersed throughout the sheet) or once the sheet has been dried and the agent is present on the paper surface only.

Fillers are added to the paper to make it smoother and to improve its optical qualities (since translucent paper is not usually ideal), effectively filling in the gaps between the paper fibres. Examples of fillers are clay and titanium dioxide, which are added at the pulp stage so that the fillers are present through the whole sheet.

Paper can be coated with a variety of further chemicals to impart various properties to it, such as a glossy finish, again depending on its intended use in, for example, a high quality brochure or catalogue.

6.1.3 Paper for security documents

The majority of security documents are produced on UV-dull paper as this is much less encountered in the wider commercial marketplace, and it is often made using the cylinder mould process with a high quality, tonal watermark. The production of such paper is expensive and tightly controlled to make it unavailable to unauthorised use. Additionally, coloured or ultraviolet fluorescent fibres can be added randomly to the paper as can planchettes (small, usually circular small pieces of coloured paper) to impart uniqueness to each sheet of paper produced.

Simulating security paper should be as difficult as possible for the counterfeiter as the value of the original document is undermined by easy copying. Simulating UV-dull paper may be done by printing a suitable material onto the surface of a piece of UV-bright paper, and likewise the appearance of a watermark can be attempted by printing onto the surface of a piece of paper in a pale cream ink to give the appearance of the correct watermark. Fibres and planchettes might also be printed on (but almost certainly the same fibre pattern will then be apparent on each simulated sheet) and it is not unknown for forgers to draw in fibres by hand!

6.1.4 Paper products

Paper can be the basis for other products apart from sheets of paper. The paper is the main constituent of the product but it is processed in various ways to create something more than just a piece of paper.

6.1.4.1 Envelopes

Envelopes are nearly always made from paper although some may be made from plastics when there is a need for exceptional physical strength against tearing and damage. The paper from which an envelope is made can be examined in just the same way as for paper itself, but in addition the dimensions of the finished envelope, the method of construction, the adhesive used and any printing matter on the envelope (both on the outside surface and more commonly on the inside surface) can be used to differentiate between envelopes. It is common practice for envelope manufacturers to print various batch details or even dates of production on an inaccessible inner corner of the envelope, which can also help in either comparing or dating envelopes.

6.1.4.2 Passports

Paper for security documents that consist of a single sheet of paper, such as currency or driving licences, has already been described. However, passports are somewhat different in that they require construction of a booklet from constituent pieces of paper and indeed other materials such as the cover and the thread used to sew the pages into the booklet. All aspects of the design of security documents are constantly being updated in line with technological changes. Even the thread used to sew up passports has become increasingly sophisticated, with several colours intertwined (Figure 6.1) and some fluorescing under ultraviolet light, for example. The material used to produce the covers also will be tightly controlled so as not to be accessible to unauthorised users. Tampering with genuine passports can involve undoing the stitching and re-stitching to reform the passport. The stitching holes in the paper will often show evidence of widening as the thread is manipulated more than normal causing damage to the original stitch holes.

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Figure 6.1 Multi-coloured thread for stitching (x25 approx.).

Passport numbering has also become more involved with, for example, perforation of the whole passport with its unique reference number through all pages (Figure 6.2). This can be achieved mechanically or by using a laser beam. The end product in a genuine passport has a very clean appearance around the holes, whereas counterfeiters do not have access to the necessary equipment and so produce an inferior simulation.

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Figure 6.2 Perforated pages of a passport (x3 approx.).

6.1.4.3 Laminates

Laminates are plastic (polymer) based materials and as such are not, of course, paper products. However, they are widely used in security documents such as passports and identity cards. The personalising of passports and other security documents is usually done using digital technology as described in Chapter 4 (typically a laser printer), but the protection of that information from tampering has become more effective with the use of polymer laminates such as polycarbonates. These have become increasingly sophisticated, often made up of several layers, some of which, for example, can be engraved by a laser, some of which can contain yet more security devices such as holograms. Because of the importance of documents of this type, international standards are set to ensure that security documents can meet an adequate level of security that enables counterfeits to be identified with confidence.

6.2 Ink

Inks come in a variety of formulations depending on their use. The most commonly encountered in forensic document examination are pen inks, and in particular ballpoint pen inks, although other types, such as gel pen inks, are becoming more popular. Printing inks are used in many documents, but those of forensic interest tend to be security documents, where ink comparison per se is not the issue so much as determining whether the correct type of ink is present or whether an incorrect ink has been used as part of a simulation.

6.2.1 Pen inks

The methods of production and in particular the chemical composition of inks is often sensitive commercial information. However, in order to be satisfactory products, inks must comply with the needs for which they are made. Writing inks require a number of properties to be effective, such as the need to be colour-fast (that is they do not fade under normal circumstances, although many inks subjected to strong sunlight will fade given enough time), dry quickly on the page, and be delivered by the pen type concerned (such as a ballpoint pen or a fountain pen). As a result, inks contain a number of components that impart particular properties to them (Brunelle & Crawford, 2003). The all-important coloured component is made up from dyes (soluble colour) and pigments (insoluble colour).

A solvent is required to act as a vehicle for the other components of the ink. The solvent can be either water (for example in fountain pen inks) or organic (for example ballpoint pen inks). Ink has to flow from the writing implement onto the paper and this requires that the ink has the appropriate thickness (or viscosity), this is controlled by the presence of resins in the ink. Other components of the ink may be needed to control its surface properties once it is on the paper and emulsifiers can be added to ensure that any aqueous and non-aqueous components mix properly. Once the ink is on the paper it is important that it does not fade or react with the environment, so to reduce this the ink may contain antioxidants.

6.2.2 Printing inks

The mainstream, everyday printing of material such as magazines and packaging rarely requires special inks to be used. The inks used will have the necessary properties for the product being made.

Specialist inks are available for use in the production of security documents. The manufacture and distribution of such inks are likely to be more tightly controlled to avoid them getting into the hands of the counterfeiter. Examples of such inks are:

  • Pearlescent inks, which have a shiny pearl-like appearance derived from the presence of flakes of refractive and reflective material suspended in the ink.
  • Thermochromic inks that change colour with a change in temperature.
  • Iridescent (optically variable) inks that change colour depending upon the angle at which they are viewed.

6.3 Staples

Occasionally a multipage document will be held together by means of staples, and if the source of such documents is called into question then it may be necessary to determine whether staples recovered from a suspect match those in question.

Staples are of two main kinds: those that are available commercially in pre-packed form; and those that are often used in the printing industry, where a length of metal is cut from a roll of wire and bent into a staple before being inserted into the document. Pre-packed staples come in a variety of different sizes and colours (some are silver coloured others more bronze coloured for example). The thickness of the wire from which the staple is made is an important factor in determining how many sheets of paper can be stapled. A staple made from a very thin wire would buckle if it was used to staple together too many sheets. On the other hand, a thicker, heavy duty staple used to staple together just a couple of sheets will often cause the ends of the staple to overlap on the reverse side of the document.

Pre-packed staples are held together in ribbon-like strips and inserted into a stapler. The individual staples are held in the ribbon by an adhesive layer. In contrast, individual staples that are cut from a reel of wire will not have the adhesive and the cutting of the individual staple is done by a sharp blade. If there are any imperfections in the cutting blade, these might show up at the endings of the individual staples which would have a distinctive appearance that could, in principle, provide an evidential link between a staple and a stapling machine. Of course, it is still necessary to compare the physical properties of the wire, such as its colour and thickness.

6.4 Adhesives

Adhesives can be used in a number of ways in a document. Pages in a pad are often held in place at one edge by a thin adhesive layer. Of course, adhesive can be applied as glue to a document or perhaps as a material from an adhesive tape. Adhesives are derived from a number of sources including natural biological products (which typically use a protein called collagen that is treated to produce glue) and hydrocarbons (which are products of the petroleum industry). Most modern adhesives fall into the latter category.

Various resins and plasticiser petroleum compounds have a tacky or sticky property that is ideal for adhesives and adhesive tapes. These compounds can be analysed using many of the techniques referred to in Chapter 7. For example:

Using such analytical methods, it is possible to create a database of available adhesive materials against which suspect materials can be compared. The value of different techniques in their ability to discriminate between adhesives can help decide which analytical methods are most appropriate (Maynard et al., 2001).

6.5 Miscellaneous materials

(Dry transfers are described in Chapter 5 as they are akin to a form of printing and are often used to simulate printed material.)

No list of components of documents can be exhaustive as anything can crop up, from lipstick used as a writing medium to a piece of tape stuck to a document. In such circumstances, the forensic examiner will need to use their own experience and quite possibly the expertise of colleagues to assist them with such examinations. It may even be necessary to conduct some small-scale research or experiments to gain background information to enable findings to be interpreted. The reporting of conclusions in such cases needs to be based on sound methodology and properly recorded details and results so that the conclusions can be reasonably challenged by others if necessary.

6.6 Case notes relating to the physical components of a document

In all document examination cases it is a good idea to note some basic details of what is present on a document, such as the colour of inks used or the type of pen used. However, the focus on the physical components of a document is usually most important in cases where alteration to a document has been alleged (discussed in more detail in Chapter 8). In many cases a careful visual examination, possibly with microscopy or other optical (non-destructive) methods, will suffice. The important point is that the findings relating to the paper and ink, and other components where relevant, are noted, the methods of examination are described clearly and the results are recorded, ideally photographically so as to aid the demonstration of findings if oral evidence is needed at a subsequent hearing. If photographs are taken and form part of the notes, then the conditions in which they were taken (such as with transmitted light when showing a watermark) and any magnification used should also be noted.

Measurements taken must be recorded (such as the thickness of a sheet of paper) and if specialist lighting is used the conditions must be recorded (such as the wavelength of the incident light and of any filters used).

In other words, the case notes should be such that another person can read them and be able to reconstruct what was done, what was observed and what conclusions were reached with regard to the inks, paper and other components present.

6.7 Reports relating to the physical components of a document

It is unusual for a document examiner to be asked to examine paper or ink or any other component of a document in isolation. Rather it is usually in the context of allegations of alterations to a document that these aspects need to be examined (see Chapter 8).

If, for example, an examination of some paper is carried out to establish its date of production, then the report will benefit from some background explanation of relevant aspects, such as a description of paper making and any supporting witness information (such as a statement from a manager of a paper mill saying when certain papers were made and using what materials). This will enable the reader to comprehend the significance of the expert's findings and conclusions in what otherwise is likely to be unfamiliar to them from their general knowledge.

The avoidance of technical terms may be impossible in such cases, but the use of a glossary or brief explanations in brackets will assist the reader when jargon is unavoidable. As with any complex report, a brief summary describing the conclusions in the absence of explanatory findings will make it clear to the reader what the expert's opinion is.

 

References

  1. Aziz, N., Greenwood, P. F., Grice, K., et al. (2008). Chemical fingerprinting of adhesive tapes by GCMS detection of petroleum hydrocarbon products. Journal of Forensic Sciences, 53(5), 1130–1137. doi:10.1111/j.1556-4029.2008.00798.x.
  2. Biermann, C. J. (1996). Handbook of Pulping and Papermaking. London: Academic Press.
  3. Brunelle, R. L., & Crawford, K. R. (2003). Advances in the Forensic Analysis and Dating of Writing Ink. Springfield, IL, USA: Charles C. Thomas.
  4. Canetta, E., & Adya, A. K. (2011). Atomic force microscopic investigation of commercial pressure sensitive adhesives for forensic analysis. Forensic Science International, 210(1–3), 16–25. doi:10.1016/j.forsciint.2011.01.029.
  5. Kumooka, Y. (2009). Hierarchical cluster analysis as a tool for preliminary discrimination of ATR-FT-IR spectra of OPP acrylic and rubber-based adhesives. Forensic Science International, 189(1–3), 104–110. doi:10.1016/j.forsciint.2009.04.025.
  6. Maynard, P., Gates, K., Roux, C., & Lennard, C. (2001). Adhesive tape analysis: Establishing the evidential value of specific techniques. Journal of Forensic Sciences, 46(2), 280–287.