Fiber and pulp analysis is not commonly used by a Forensic Document Examiner (FDE). Why? Because it is a destructive technique.
For definition: Pulp fiber analysis is a destructive technique that first breaks the paper into individual fibrous and non-fibrous cellular components, and then analyzes them based on morphological characteristics, chemical staining color, and weight compositions of different fibers.
Nevertheless, they are highly useful in determining the origin of paper manufacture by evaluating the morphology and chemistry of the pulp and fiber.
Importance of Fiber and Pulp Analysis in Paper Examination
The main purpose of doing fiber and pulp analysis by the forensic examiner is:
- To identify the fiber and pulp types
- To know more about the composition % of different fibers used to make that paper
- Pulping process— conversion of fiber into pulp
- Identify wood and non-wood species
- And lastly, the origin of the fiber.
All these analyses and results help an examiner to depict whether the questioned document is morphologically the same as a standard sample or not.
Common Fibers used in Paper Manufacturing
The most commons fibers used for paper manufacturing are:
- Cotton— non-wood fiber
- Wood— hardwood, and softwood
Less commonly used fibers for paper manufacturing
|S.No.||Paper Fiber||Majorly Used for Manufacturing|
|1.||Abaca||thin and strong sheets|
|2.||Bagasse||fibrous (sugarcane) add softness to paper|
|3.||Bamboo||packaging bands, insulation, and cement bag|
|4.||Esparto||high-quality paper for book manufacturing|
|5.||Flax||paper-thin and strong|
|7.||Wheat Straw||wide range of paper and tissues|
Types of Pulp Fibers and Their Origin
Based on the source of origin, there are two types of fibers; one is wood fiber and the other is non-wood fibers.
A. Wood Fibers
There are two types of wood fibers: Hardwood and softwood.
Hardwood fibers are obtained from angiosperm— a flowering plant/tree family and can be deciduous and evergreen. They are commonly used to produce most of the office printed paper. Common origin plants for hardwood fibers are aspen, alder, birch, eucalyptus, oaks, and maples.
Softwood fibers originate from gymnosperms sub-family— coniferous. As per status, 85% of U.S. paper is made of softwood fiber. These fibers are also called tracheids— provides mechanical support and conduction of fluids
They are added in smaller amounts to office paper to increase wet strength and also to speed up the production process. Common origin plants for softwood fibers are cedar, fir, hemlock, larch, pine, and spruce.
B. Non-Wood Fibers
As per the names, these are non-woody plants, usually grasses and shrubs. They also belong to the angiosperms family. Their stems and seed coats are usually used for pulp production.
For papermaking, nonwood fibers can be categorized into four major groups.
- Grasses (Gramineae) fiber
- Bast fibers
- Leaf fiber
- Fruit fiber
The following tables state common examples of nonwood fibers with scientific names that are used for paper pulp making.
|Non-Wood Type||Common Name||Scientific Name|
|Grasses (Gramineae)||Bamboo||Dendrocalamus strictus|
|Sugar cane||Saccharum officinarum|
|Blast Fibers (stem)||Flax||Linum usitatissimum|
|Leaf Fibers||Abaca||Musa textilis|
|Fruit Fibers||Cotton boll||Cotton Gossypium|
Quick Fact: Cotton BOLL refers to dense roundish fur clumps that grow on the stems of cotton plants. Cotton BALL is fine, processed, cleaned cotton that has been shaped into spherical balls for household use.
Types of Pulping Process in Paper Manufacturing
There are a total of six types of pulps (or pulping processes) that are used by paper manufacturers.
1. Sulfite Pulping
- It is an acidic pulping process also called “cooking liquor”.
- Produce a very light-colored pulp with high opacity and low strength.
- Working solution: Mix of calcium or magnesium bisulfite, sulfur dioxide (SO₂), and water.
- During the pulping process, sulfur dioxide reacts with water to form sulfurous acid (that is why acidic pulping).
- Working temperature: 170°C
- Pressure: more than 8 atm
- Ideal for: writing and office papers
2. Kraft Pulping
- Other names: alkaline pulping process or sulfate process
- Produces very strong pulps (‘kraft’ is a german word that means strength)
- Working solution: Sodium hydroxide (NaOH) and Sodium sulfide (Na2S) in water
- Working temperature: 170°C
- Pressure: about 8 atm
- Ideal for producing packaging paper
- Even softer paper like office paper can be produce using kraft pulps by a multistage bleaching process.
More Explanation: Multistage bleaching process is used to dissolve brown-colored lignin that is produced by the Kraft process. Common bleaching agents are chlorine, ozone, chlorine dioxide (elemental chlorine-free—ECF), and hydrogen peroxide (totally chlorine-free—TCF).
3. Semi-chemical Hardwood Pulping
- They are cheaper to produce than other major pulping processes.
- It uses a lesser amount of chemicals with application of mechanical and heat energy.
- Lower quantity of sodium sulfite (at low pH of 7 to 9) or sodium hydroxide (at pH 12) with mechanical and heat applications.
- Ideal for producing low-quality paper or tissues.
4. Thermo-Mechanical Pulps (TMP)
- Produces very low-strength pulps using softwood chips.
- Chips are steamed and then mechanically processed.
- Ideal for producing papers for newspapers, junk mails, etc.
- Turns yellow when exposed to UV light or strong sunlight.
5. Soda Pulping
- Used sodium hydroxide in water as a cooking chemical.
- Majorly used for processing non-woody fibers such as wheat, bagasse, rice, etc.
- Brownish shade of lignin is removed by treating it at high temperature and pressure.
- Temperatures are adjusted as per the use of non-woody fibers.
- Requires less bleaching chemicals and thus required less optical brightness agents— OBAs.
6. Groundwood Pulps
- Also called mechanical pulping.
- No chemicals are used for separating fibers.
- Produced by grinding logs of wood against a rotating stone.
- Uses water for screening, and cleaning during the process.
- Can be easily distinguished from other pulps because of the high level of impurities.
- Low brightness values because of high lignin in pulp.
- Ideal for producing low-quality paper for telephone directories, rough notebooks, printing paper, and newspapers.
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Sample Preparation For Fiber And Pulp Analysis
For analysis, paper is washed with water to separate it from binders (defibering). In general, 0.2 grams of pulp sample is more than enough for analysis.
Please Note: ‘Defibering’ is a destructive technique. So, try to save as much of QD as possible. In addition, paper is taken from the area where there is no writing, typing or printing is present.
As per ISO laboratory standards, should be maintained with a relative humidity of 50 ± 2%, and a temperature: 23 ± 1°C for any kind of paper analysis.
A. Using Distilled Water
Ideal for normal paper such as stationary and printed ones.
- Torn a small part of a sheet into small pieces.
- Soak them in distilled water in a beaker.
- Heat the mixture until it begins to boil, then allow it to cool.
- Transfer the residual fiber to a separate beaker after decanting the water.
- Add some water and shake it vigorously until fibers get separated to individual fibers.
B. Using Acid/Base Action
For papers not get defibered by simple water digestion method.
- Paper is torn into pieces and soaked in 1% sodium hydroxide.
- Heat the soaked sample to boil and set it aside to cool.
- Decant the NaOH and put the residual sample into a new beaker.
- Pour 0.05N hydrochloric acid into a beaker.
- The sample is made to sit for 10 minutes.
- Decant the HCL and residual sample is washed with distilled water.
- To make sure no acidic component residue is left, rewash the sample 2-3 times.
Part 1: Morphological Examination of Paper Fiber and Pulp
The morphological examination helps an FDE to identify the species of origin of the fiber. For that, a polarizing light microscope is used. Why?
Because of two polarizing filters— polarizer and analyzer— that are ‘crossed’ (right angle to each other). This creates two wave components with respect to paper fiber; one along and the other across the fiber. And the velocities of these waves give a contrasting image of the paper fibered sample.
Common morphological characteristics that are looked for:
- Shape and Structure of cells
- Cross marking on the surface
- Dimension of cells and their components
What Will You Need?
- Polarized light microscope
- Abbe condenser— control the light that passes to specimens
- Magnification Power: up to 400x (400x is best for microstructure such as vessel elements)
- Slides and cover glasses
- From the prepared sample, take 0.5 ml of uniform fiber suspension over a glass slide.
- Allow the liquid suspension to evaporate on a glass slide, leaving behind fibers.
- As soon as liquid dries, transfer them to a hot plate to make fibers dry completely.
- Cover the glass slide with a coverslip and observe it under a polarized microscope.
A. Morphological Characteristics of Softwood and Hardwood
Following are the morphological differences between hardwood and softwood fibers that are used by paper manufacturers.
|Feature||Softwood Fiber||Hardwood Fiber|
|Major fibers||Tracheids fibers||Libriform fibers|
|Fiber pulp type||Long fibered pulp||Short fiber pulp|
|Avg. fiber length range||1.5–5.6 mm||0.7–1.5 mm|
|Avg. diameter range||30–75 μm||10–50 µm|
|Superiority||High tearing and wet strength||Higher density than softwoods|
|Ideal for||Paper sacks and bags||Printing and writing paper|
|Examples||Cedar, fir, hemlock, larch, pine, spruce||Aspen, alder, birch, eucalyptus, oaks, maples|
Softwood Pulp and Fiber Characteristics
- Coniferous trees: Long, thin-walled fibers that are irregularly spaced and have smaller bordered pits.
- Douglas Fir: Tracheids with spiral thickening on the inner cell adjacent to lumen.
- Groundwood: Tracheids with right angles with undisturbed groups of cells.
|Common Name||Scientific Name||Fiber length (mm)||Fiber width (μm)|
|Silver Fir||Abies alba||1.6–5.7||18–58|
|Balsam fir||Abies balsamea||1.9–5.6||30–40|
|Parana Pine||Araucaria angustifolia||5.6–9.0||19–60|
|Common Larch||Larix decidua||1.4–6.2||24–52|
|Western redcedar||Thuja plicata||1.4-5.9||30-40|
|Eastern hemlock||Tsuga canadensis||3.0||28-40|
|Western hemlock||Tsuga heterophylla||1.8-6.0||30-40|
Hardwood Pulp and Fiber Characteristics
The key differentiating point for hardwood species is by evaluating:
- Cell size
- Pits sizes
- Perforation detailings
Unlike softwoods and non-woods, vessel members in hardwoods generally don’t appear in microscopic observation. This is because they get lost during the pulping process.
Some common hardwoods are used for papermaking.
|Common Name||Scientific name||Avg. Length (mm)||Avg. Width (μm)|
|Alder (Black)||Alnus glutinosa||0.6 -1.01||16-40|
|Paper birch||Betula papyrifera||1.3||25|
|Southern blue gum||Eucalyptus globulus||0.3-1.5||10-28|
|American white oak||Quercus alba||1.4||14-22|
|English oak||Quercus robur||0.5-1.6||14-30|
|Red maple||Acer rubrum||0.3-1.1||16-30|
|Sugar maple||Acer platanoides||0.3-1.3||16- 30|
B. Morphological Characteristics of Non-Wood Pulp and Fiber
Following are the microscopic features that can help forensic document/ fiber analysts to identify the type of pulp used by paper manufacturers.
|Nonwood Fibers||Length (mm)||Width (μm)||Morphological Features|
|Bamboo||1.5-4.4||7-27||(1) Fiber: narrow, blunt, or tapering ends|
(2) Very wide vessel elements
(3) P. cells: small, thick-walled, barrel-shaped
|Corn||0.5-2.9||14-24||(1) Fiber: narrow, thick-walled, blunt/ pointed ends|
(2) Epid. cells: small, thick-walled, irregular shaped
(3) P. cells: large, round, saclike
|Esparto||0.2-3.3||6-22||(1) Fibers: short, thick-walled, pointed ends|
(2) small, comma-shaped hairs
(3) Parenchyma cells: small, narrow, few in numbers
|Sugar Cane||0.8-2.8||10-34||(1) Fiber: thick-wall, blunt ends|
(2) Very large thin-wall vessel elements
(3) Parenchyma cells: large, broken, barrel-shaped
|Sabai||0.5-4.9||4-28||(1) Fibers: long, narrow, blunt, or pointed end|
(2) narrow vessel elements
(3) Parenchyma cells: small, rounded, rectangular, or rodlike
|Wheat||0.4-3.2||8-34||(1) Fibers: narrow, blunt, or pointed ends|
(2) Epidermal cells: large, regular-shaped
(3) Parenchyma cells: large saclike, rounded or barrel-shaped, abundant
|Rice||0.4-3.4||4-16||(1) Fibers: short, very narrow, mostly pointed ends|
(2) Parenchyma cells: relatively small, abundant
(3) Epidermal cells: narrow with conical protuberances
|Flax (linen)||9-70||5-38||(1) Fiber: long, narrow, pointed ends|
(2) if intact: ribbon like miniature bamboo
(3) walls are longitudinal striations and swelling
|Hemp||5-55||10-51||(1) Fiber: long, thick-walled, wider than flax|
(2) Fiber wall: prominent, dislocations, swell
|Jute||2-5||10-25||(1) Fiber: short, too narrow, thick-walled, spoon-shaped ends|
(2) Fiber wall: faint dislocations
(3) weIl defined lumen with varying length
|Abaca||5-12||16-32||(1) Fiber: relatively short, tapering pointed ends|
(2) wide well-defined lumen
(3) Fiber wall: very faint
|Sisal||1-8||8-41||(1) Fibers: short, narrow, thick-walled, tapering blunt ends|
(2) Fiber wall: very faint
(3) annular, spiral, or netlike vessel elements, and separated spirals
|Cotton||10-40||12-38||ribbon-like twists, also seen in a rag (cotton paper), lint fibers|
|Kapok||8-30||10-30||(1) Fibers: smooth, transparent, structureless, tubelike, thin-walled|
(2) Wide lumen enclosed with air bubbles
Part 2: Chemical Staining Method for Pulp and Fiber Analysis
The staining method is an effective technique to determine the pulp type (pulping process) and fibers. Different paper pulp and fibers react with certain chemicals to give respective stained characteristics as their identification marks.
What will you need?
- Binocular compound light microscope with Abbe condenser
- Magnification: up to x100
- One eyepiece fitted with the crosshair
- Glass slides and coverslips
- Take 0.5 ml of 5% of uniform mix from the prepared sample (listed above) in a test tube.
- Using a dropper, place a few drops of sample on the glass slide and let it dry.
- Stained it with 1 drop of the chemical stain and cover it with glass.
- Allow the arrangement to be at rest for a few minutes.
- Remove surplus stain solution by tilting the slide.
- Observe under a binocular compound light microscope.
A. Stain Graff C Method
C. Stain method is one of the common methods for defining different types of pulps or even species present in a questioned document papered sample.
Stain Graff C Solution Preparation
- Aluminum chloride solution: 4g of AlCl₃ + 10 ml of distilled water
- Calcium chloride solution: 10g of CaCl2 + 15 ml of water
- Zinc chloride solution: 2.5 ml of water is added to 5g of dry and fresh ZnCl₂
- Iodine-Iodine solution: 0.1g of dry KI + 0.06g of dry Iodine + 5 ml of water
Stain C. Solution: Aluminum, calcium, and zinc chloride solution is added as 2ml, 1ml, and 1ml respectively. The reagent is mixed, and 1.25ml of iodine-iodine solution is added. The solution is made to settle for 24 hours and then pipette out a clear solution that can be used as Stain C reagent.
Storage: Stain C. should not be used that are prepared for more than one week unless they are stored in a dark bottle with some iodine pellets.
Observations With Stain Graff C Reagent
|2.||Unbleached sulfite softwood||bright yellow|
|3.||Bleached sulfite softwood||purplish gray to weak reddish purple|
|4.||Unbleached kraft softwood||pale brown to brownish-yellow|
|5.||Bleached kraft softwood||blue to gray|
|6.||Thermomechanical softwood||vivid yellow|
|7.||Unbleached sulfite hardwood||navy blue to pink|
|8.||Bleached sulfite hardwood||purplish blue to purplish gray|
|9.||Unbleached soda hardwood||blue-green to reddish gray|
|10.||Bleached soda hardwood||dusky blue to dusky purple|
|11.||Cotton (rag)||moderate reddish orange|
|12.||Unbleached Jute||Bright yellowish orange|
|13.||Bleached Jute||Light yellow green|
|14.||Cereal straw||Greenish yellow to blue|
B. Wilson Stain
Another common method to determine the pulp and fibered species on paper. And following is the composition of the working solution for the Wilson stain.
Wilson Stain Solution Preparation
- 1.5g of iodine + 70 g of cadmium iodide + 100 ml of distilled water.
- Heat the mixture to 43°C and continue stirring to make a complete solution.
- In another beaker, 37% 15ml formaldehyde, 140g of calcium nitrate, 40g of cadmium chloride in 180ml of distilled water.
- Pour the beaker’s content into a preheated solution.
- Heat the final solution at 43°C minutes— usually required when the solution is too strong.
Storage: The solution should be cooled and stored in an amber black-brown bottle. Titration should be performed before each test to check the strength of the Wilson stain. If it appears too weak, the stain solution should be made fresh.
Strength testing solution:
- Titrate 10 ml of Wilson stain against 0.01 N sodium thiosulfate.
- Indicator: starch
- 10 ml of stain = 12 ± 2 ml
- If too strong, it should be heated (step 5)
- If too weak, discard and prepare a new one (due to storage)
Pulp Observations With Wilson Stain
|2.||Bleached groundwood||bright yellow|
|3.||Unbleached sulfite softwood||light brown|
|4.||Bleached sulfite softwood||pinkish lavender|
|5.||Unbleached kraft softwood||brown|
|6.||Bleached kraft softwood||bluish grey|
|7.||Thermomechanical softwood||golden yellow|
|8.||Unbleached sulfite hardwood||very pale yellow|
|9.||Bleached sulfite hardwood||lavender|
|10.||Unbleached soda hardwood||greenish gray|
|11.||Bleached soda hardwood||pale purple|
|14.||Cereal straw||green to blue|
C. Herzberg Stain
The staining solution is not commonly used but used as a confirmatory test to distinguish among rag, pulping process, and groundwood pulps.
Herzberg Stain Solution Preparation
- Iodide-iodine solution: 0.25g of Iodine + 5.25g potassium iodide + 12.5 ml of distilled water
- Zinc chloride solution: 50 g of Zinc chloride dissolved in 25 ml of distilled water
- 25 ml of Iodide-iodine solution is added to the prepared zinc chloride solution
- Allow the sample to rest for 24 hours.
- Collect the supernatant solution as a working Herzberg solution.
Storage: Herzberg’s solution should be stored in an airtight amber bottle with iodine pellets.
Observation with Herzberg Stain
|2.||Unbleached softwood||bluish gray to blue|
|3.||Bleached softwood||purplish gray to light reddish-purple|
|4.||Hardwood (bleached/unbleached)||purplish gray to deep reddish-purple|
|3.||Cotton (rag)||purplish pink to bright red-purple|
|4.||Unbleached Jute||moderate yellowish-orange|
|5.||Bleached Jute||strong greenish-yellow|
|6.||Cereal straw||bluish gray to pale purplish blue|
D. Selleger Stain
The Selleger stain can be used as a confirmatory secondary test to distinguish:
- Coniferous vs hardwood
- Bleached and coniferous sulfite and sulfate pulped fibers.
Disadvantage: Solution preparation required 7 days of stand time. Thus, it should be prepared weekly.
Selleger Stain Solution Preparations
- 100g of calcium nitrate dissolved in 50 ml of distilled water.
- In another beaker, 3 ml of potassium iodide was added to 90 ml of water.
- Pour the KI solution into the calcium nitrate solution.
- Add 1g of iodine and stand it for one week.
Observation with Selleger Stain
|2.||Unbleached sulfite softwood||yellow|
|3.||Bleached sulfite softwood||red|
|4.||Unbleached sulfate softwood||yellow|
|5.||Beached sulfate softwood||blue-gray|
|6.||Bleached sulfite hardwood||bluish red|
|7.||Un/bleached soda, sulfate hardwood||blue|
Part 3: Finding Pulp Fiber Weighting and Composition
Weighing paper fibers is the first metric that is needed to calculate in order to find the individual composition of each fiber type.
For calculating the weight of each fiber, the weight factor is multiplied by the number of fibers of various fibers type and species.
Weight count of pulp/fiber of specific type = Number of Fibers x Weight factor
The number of fibers can be easily observed with a multistation tally counter. And weight factors for different types of pulp and species are given in the following table.
|S.No.||Pulp Fiber Types||Weight Factors|
|1.||Cotton (Rag), linen||1.0|
|2.||Jute or manila||0.6|
|5.||Unbleached sulfite/ kraft softwood||1.0|
|6.||Bleached sulfite/ kraft softwood||0.9|
|7.||Unbleached kraft/kraft softwood pulp||1.0|
|8.||Un/ Bleached kraft/ soda hardwood||0.4|
|9.||Semi chemical hardwood||0.6|
|12.||Bleached thermomechanical softwood||1.6|
By coming up with the final values of each fiber weight, the composition of the whole paper pulp/ fibers can be calculated using the following formulae.
Fiber composition (%) = (Weight count/ total weighted count) x 100
Now, let’s do the experiment part.
What will you need?
- Binocular compound light microscope with Abbe condenser
- Magnification: up to x400 (x40, x100, and x400)
- One eyepiece fitted with the crosshair
- Multistation tally course— counting fibers
- Glass slides and coverslips
- From 0.5ml of prepared sample, make it dry over slide.
- Start analyzing samples from the top corner and move horizontally.
- Each fiber type is counted (multistation tally counter) and features are noted that passed over by crosshair.
- As one line is completed, the sample stage is moved 5 mm vertically down, and again start analyzing the same.
- It is recommended to take at least 200 total fiber counts.
Note: For forensic purposes, a multistation tally counter should be used to count various types of fibers in a single run, which helps you in presenting your findings with more generalized results (quantitatively).
Forensic Rules for Using Multistation Tally Counter for Fibers and Pulp
- At Least 200 total fiber counts. Some FDU (forensic document units) standardized to check at least 800 total fiber counts.
- When a single fiber is long enough to pass through more than once crosshair center, it is counted only once.
- Very fine fragments should be ignored.
- Each fiber in a bundle is counted.
Let’s do the practice:
Following are the fiber count values of a paper that the examiner observed with respect to fiber/pulp types.
|Fiber Types||Fiber count||Multiplying Factor|
Step 1: Calculating Weight Count Values
- Weight count value for Bleached softwood= 245 x 0.9= 220.5
- Similarly, bleached hardwood value=124.4, and
- Mechanical softwood value= 97.2
Thus, total weight count value = 442.1
Step 2: Finding Fiber Composition Percentage
- Fiber % for Bleached softwood= (220.5/ 442.1) x100= 49.87%
- Fiber % of bleached hardwood= (124.4/ 442.1) x 100= 28.13%
- Fiber % of mechanical softwood= (97.2/ 442.1) x 100= 21.98%
After coming up with the specific fiber%, the examiner has to compare them with manufactured samples of known compositions.
In most cases, there might be a variation of % composition. As a result, the examiner should take this into consideration as well. Any variation of more than 3% is considered a different paper.
Different small paper manufacturers from a locality usually share relatively the same pulp composition and raw materials. Second, there is no ground rule for setting and using non-paper fibers. This leads to the addition of different kinds of fiber. Thus, the examiner should have the experience to detect these minor differences between two fibers species.
It is a collection of standard known fibers for comparison with the questioned documents. The record is maintained by the Institute of Paper Chemistry (est.1929), Appleton, Wisconsin (now moved to Atlanta, Georgia).
- Morphology of pulp fiber from hardwoods and influence on paper strength by Horn (source)
- Stains for the Determination of Paper Components and Paper Defects (source)
- Morphological characterization of pulps to control paper properties (source)
- Scientific Examination of Questioned Documents [book]
- The practical identification of Wood Pulp Fibers by Parham and Gray (source)