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Paper Fiber and Pulp Analysis: How They Impact Questioned Documents Examination?

Paper Fiber and Pulp Analysis How They Impact Questioned Documents Examination

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:

  1. To identify the fiber and pulp types
  2. To know more about the composition % of different fibers used to make that paper
  3. Pulping process— conversion of fiber into pulp
  4. Identify wood and non-wood species
  5. 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 FiberMajorly Used for Manufacturing
1.Abacathin and strong sheets
2.Bagassefibrous (sugarcane) add softness to paper
3.Bamboopackaging bands, insulation, and cement bag
4.Espartohigh-quality paper for book manufacturing
5.Flaxpaper-thin and strong
6.Jutenumerous application
7.Wheat Strawwide 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

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

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.

  1. Grasses (Gramineae) fiber
  2. Bast fibers
  3. Leaf fiber
  4. Fruit fiber

The following tables state common examples of nonwood fibers with scientific names that are used for paper pulp making.

Non-Wood TypeCommon Name Scientific Name
Grasses (Gramineae)BambooDendrocalamus strictus
CornZea mays
EspartoStipa tenacissima
RiceOryza sativa
SabaiEulaliopsis binata
Sugar caneSaccharum officinarum
WheatTriticum sativum
Blast Fibers (stem)FlaxLinum usitatissimum
HempCannabis sativa
JuteCorchorus capsularis
Leaf FibersAbacaMusa textilis
SisalAgave sisalana
Fruit FibersCotton bollCotton Gossypium
KapokCeiba pentandra

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

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.

Read More:

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.

  1. Torn a small part of a sheet into small pieces.
  2. Soak them in distilled water in a beaker.
  3. Heat the mixture until it begins to boil, then allow it to cool.
  4. Transfer the residual fiber to a separate beaker after decanting the water.
  5. 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.

  1. Paper is torn into pieces and soaked in 1% sodium hydroxide.
  2. Heat the soaked sample to boil and set it aside to cool.
  3. Decant the NaOH and put the residual sample into a new beaker.
  4. Pour 0.05N hydrochloric acid into a beaker.
  5. The sample is made to sit for 10 minutes.
  6. Decant the HCL and residual sample is washed with distilled water.
  7. To make sure no acidic component residue is left, rewash the sample 2-3 times.

Part 1: Morphological Examination of Paper Fiber and Pulp

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:

  1. Shape and Structure of cells
  2. Cross marking on the surface
  3. Dimension of cells and their components

What Will You Need?

  1. Polarized light microscope
  2. Abbe condenser— control the light that passes to specimens
  3. Magnification Power: up to 400x (400x is best for microstructure such as vessel elements) 
  4. Slides and cover glasses

Procedure:

  1. From the prepared sample, take 0.5 ml of uniform fiber suspension over a glass slide.
  2. Allow the liquid suspension to evaporate on a glass slide, leaving behind fibers.
  3. As soon as liquid dries, transfer them to a hot plate to make fibers dry completely.
  4. 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. 

FeatureSoftwood FiberHardwood Fiber
OriginGymnosperms’ ConifersAngiosperms
Major fibersTracheids fibersLibriform fibers
Fiber pulp typeLong fibered pulpShort fiber pulp
Avg. fiber length range1.5–5.6 mm0.7–1.5 mm
Avg. diameter range30–75 μm10–50 µm
SuperiorityHigh tearing and wet strengthHigher density than softwoods
Ideal forPaper sacks and bagsPrinting and writing paper
ExamplesCedar, fir, hemlock, larch, pine, spruceAspen, alder, birch, eucalyptus, oaks, maples

Softwood Pulp and Fiber Characteristics

  1. Coniferous trees: Long, thin-walled fibers that are irregularly spaced and have smaller bordered pits.
  2. Douglas Fir: Tracheids with spiral thickening on the inner cell adjacent to lumen.
  3. Groundwood: Tracheids with right angles with undisturbed groups of cells.
Common NameScientific NameFiber length (mm)Fiber width (μm)
Silver FirAbies alba1.6–5.718–58
Balsam firAbies balsamea1.9–5.630–40
Parana PineAraucaria angustifolia5.6–9.019–60
Common LarchLarix decidua1.4–6.224–52
Western redcedarThuja plicata 1.4-5.930-40
Eastern hemlockTsuga canadensis 3.028-40
Western hemlockTsuga heterophylla1.8-6.030-40

Hardwood Pulp and Fiber Characteristics

The key differentiating point for hardwood species is by evaluating:

  • Cell size 
  • Shapes
  • 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 NameScientific nameAvg. Length (mm)Avg. Width (μm)
AspenPopulus tremuloides0.4-1.910-27
Alder (Black)Alnus glutinosa0.6 -1.0116-40
Paper birchBetula papyrifera1.325
Southern blue gumEucalyptus globulus0.3-1.510-28
American white oakQuercus alba1.414-22
English oakQuercus robur0.5-1.614-30
Red mapleAcer rubrum0.3-1.116-30
Sugar mapleAcer platanoides0.3-1.316- 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 FibersLength (mm)Width (μm)Morphological Features
Bamboo1.5-4.47-27(1) Fiber: narrow, blunt, or tapering ends
(2) Very wide vessel elements
(3) P. cells: small, thick-walled, barrel-shaped
Corn0.5-2.914-24(1) Fiber: narrow, thick-walled, blunt/ pointed ends
(2) Epid. cells: small, thick-walled, irregular shaped
(3) P. cells: large, round, saclike
Esparto0.2-3.36-22(1) Fibers: short, thick-walled, pointed ends
(2) small, comma-shaped hairs
(3) Parenchyma cells: small, narrow, few in numbers
Sugar Cane0.8-2.810-34(1) Fiber: thick-wall, blunt ends
(2) Very large thin-wall vessel elements
(3) Parenchyma cells: large, broken, barrel-shaped
Sabai0.5-4.94-28(1) Fibers: long, narrow, blunt, or pointed end
(2) narrow vessel elements
(3) Parenchyma cells: small, rounded, rectangular, or rodlike
Wheat0.4-3.28-34(1) Fibers: narrow, blunt, or pointed ends
(2) Epidermal cells: large, regular-shaped
(3) Parenchyma cells: large saclike, rounded or barrel-shaped, abundant
Rice0.4-3.44-16(1) Fibers: short, very narrow, mostly pointed ends
(2) Parenchyma cells: relatively small, abundant
(3) Epidermal cells: narrow with conical protuberances
Flax (linen)9-705-38(1) Fiber: long, narrow, pointed ends
(2) if intact: ribbon like miniature bamboo
(3) walls are longitudinal striations and swelling
Hemp5-5510-51(1) Fiber: long, thick-walled, wider than flax
(2) Fiber wall: prominent, dislocations, swell
Jute2-510-25(1) Fiber: short, too narrow, thick-walled, spoon-shaped ends
(2) Fiber wall: faint dislocations
(3) weIl defined lumen with varying length
Abaca5-1216-32(1) Fiber: relatively short, tapering pointed ends
(2) wide well-defined lumen
(3) Fiber wall: very faint
Sisal1-88-41(1) Fibers: short, narrow, thick-walled, tapering blunt ends
(2) Fiber wall: very faint
(3) annular, spiral, or netlike vessel elements, and separated spirals
Cotton10-4012-38ribbon-like twists, also seen in a rag (cotton paper), lint fibers
Kapok8-3010-30(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

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?

  1. Binocular compound light microscope with Abbe condenser
  2. Magnification: up to x100
  3. One eyepiece fitted with the crosshair
  4. Glass slides and coverslips

Procedure:

  1. Take 0.5 ml of 5% of uniform mix from the prepared sample (listed above) in a test tube.
  2. Using a dropper, place a few drops of sample on the glass slide and let it dry.
  3. Stained it with 1 drop of the chemical stain and cover it with glass.
  4. Allow the arrangement to be at rest for a few minutes.
  5. Remove surplus stain solution by tilting the slide.
  6. 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 

  1. Aluminum chloride solution: 4g of AlCl₃ + 10 ml of distilled water
  2. Calcium chloride solution: 10g of CaCl2 + 15 ml of water
  3. Zinc chloride solution: 2.5 ml of water is added to 5g of dry and fresh ZnCl₂
  4. 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

S.No.Pulp typeFeatures
1.Groundwoodbright yellowish-orange
2.Unbleached sulfite softwoodbright yellow
3.Bleached sulfite softwoodpurplish gray to weak reddish purple
4.Unbleached kraft softwoodpale brown to brownish-yellow
5.Bleached kraft softwoodblue to gray
6.Thermomechanical softwoodvivid yellow
7.Unbleached sulfite hardwoodnavy blue to pink
8.Bleached sulfite hardwoodpurplish blue to purplish gray
9.Unbleached soda hardwoodblue-green to reddish gray
10.Bleached soda hardwooddusky blue to dusky purple
11.Cotton (rag)moderate reddish orange
12.Unbleached JuteBright yellowish orange
13.Bleached JuteLight yellow green
14.Cereal strawGreenish 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. 1.5g of iodine + 70 g of cadmium iodide + 100 ml of distilled water.
  2. Heat the mixture to  43°C and continue stirring to make a complete solution.
  3. In another beaker, 37% 15ml formaldehyde, 140g of calcium nitrate, 40g of cadmium chloride in 180ml of distilled water.
  4. Pour the beaker’s content into a preheated solution.
  5. 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: 

  1. Titrate 10 ml of Wilson stain against 0.01 N sodium thiosulfate.
  2. Indicator: starch
  3. 10 ml of stain = 12 ± 2 ml
  4. If too strong, it should be heated (step 5)
  5. If too weak, discard and prepare a new one (due to storage)

Pulp Observations With Wilson Stain

S.No.Pulp typeFeatures
1.Unbleached groundwoodgreenish-yellow
2.Bleached groundwoodbright yellow
3.Unbleached sulfite softwoodlight brown
4.Bleached sulfite softwoodpinkish lavender
5.Unbleached kraft softwoodbrown
6.Bleached kraft softwoodbluish grey
7.Thermomechanical softwoodgolden yellow
8.Unbleached sulfite hardwoodvery pale yellow
9.Bleached sulfite hardwoodlavender
10.Unbleached soda hardwoodgreenish gray
11.Bleached soda hardwoodpale purple
12.Cotton (rag)red
13.Linenpink
14.Cereal strawgreen 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

  1. Iodide-iodine solution: 0.25g of Iodine + 5.25g potassium iodide + 12.5 ml of distilled water
  2. Zinc chloride solution: 50 g of Zinc chloride dissolved in 25 ml of distilled water
  3. 25 ml of Iodide-iodine solution is added to the prepared zinc chloride solution
  4. Allow the sample to rest for 24 hours.
  5. 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

S.No.Pulp typeFeatures
1.Groundwoodvivid yellow
2.Unbleached softwoodbluish gray to blue
3.Bleached softwoodpurplish 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 Jutemoderate yellowish-orange
5.Bleached Jutestrong greenish-yellow
6.Cereal strawbluish 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

  1. 100g of calcium nitrate dissolved in 50 ml of distilled water.
  2. In another beaker, 3 ml of potassium iodide was added to 90 ml of water.
  3. Pour the KI solution into the calcium nitrate solution. 
  4. Add 1g of iodine and stand it for one week.

Observation with Selleger Stain

S.No.Pulp typeFeatures
1.Groundwoodyellow
2.Unbleached sulfite softwoodyellow
3.Bleached sulfite softwoodred
4.Unbleached sulfate softwoodyellow
5.Beached sulfate softwoodblue-gray
6.Bleached sulfite hardwoodbluish red
7.Un/bleached soda, sulfate hardwoodblue
8.Cotton (rag)red
9.Espartoblue
10.Cereal strawblue

Part 3: Finding Pulp Fiber Weighting and Composition

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), linen1.0
2.Jute or manila0.6
3.Cereal straw0.4
4.Esparto0.5
5.Unbleached sulfite/ kraft softwood1.0
6.Bleached sulfite/ kraft softwood0.9
7.Unbleached kraft/kraft softwood pulp1.0
8.Un/ Bleached kraft/ soda hardwood0.4
9.Semi chemical hardwood 0.6
10.Thermomechanical softwood1.7
11.Mechanical softwood1.8
12.Bleached thermomechanical softwood1.6

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

Procedure:

  1. From 0.5ml of prepared sample, make it dry over slide.
  2. Start analyzing samples from the top corner and move horizontally.
  3. Each fiber type is counted (multistation tally counter) and features are noted that passed over by crosshair.
  4. As one line is completed, the sample stage is moved 5 mm vertically down, and again start analyzing the same.
  5. 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 TypesFiber countMultiplying  Factor
Bleached softwood2450.9
Bleached hardwood3110.4
Mechanical softwood541.8

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.

General FAQ

What are the limitations of fiber and pulp analysis in questioned documents?

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.

What is ‘Fibrary’ in forensic document examinations?

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).

References:

  • 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)

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