Every paper has additives. These additives are in the form of starch, protein, waxes, oils, and other fillers and pigments that can be used by the forensic document examiner to trace down specific sheets.
All these fillers and additives help in shaping the desirable final paper.
And in this post, you’re going to know more about additives that are used for making paper, and how they can be identified in real-life forensic paper analysis.
Note: Most of the tests in the articles are destructive. Hence, the test should be conducted over/with a smaller area of questioned documents.
Common Additives of Paper
|Sizing Materials||Improve strength, flexibility, increase printing ability, and decrease the absorption of liquid/oil. Eg: alkyl ketene dimer, gelatin, wheat starch, other proteins.|
(Process of adding Filler to pulp)
|Inorganic, non-fibrous material added to the paper pulp prior to papermaking. They improve optical, physical, and aesthetic properties (pigments).Eg: clay, TiO2, talcs, calcium carbonate, calcium sulfate, zinc, barium|
|Coating Materials||makes surface less absorbent, glossy, and photographic. Eg: polyethylene, silicone, wax|
|Calcium Carbonate||for making acid-free paper, resisting shrinkage, smoothing the surface, optical brightness agent|
|Pigments and Dyes||coloration|
|Cationic Retentions||For binding dyes and pigments with pulp fiber|
|China clay||More opaque, smooth and reduce shrinkage|
|Titanium dioxide||Stronger white pigments, smoother surface, tinting colored pulps|
|Soda Ash||Dissolving non-cellulose part of husk and barks in papermaking|
Identification of Starch
Starch is the third most commonly used component in the production of paper after cellulose fiber and mineral pigments. There are two types of starch being used; one is refined (most common) and the second is unmodified.
Refined starch is in the form of fine powders that are modified by the process of controlled hydrolysis, oxidation, or derivatization. Unmodified starch is not generally used in papermaking but is used as corrugating and laminating binders.[Source]
They are primarily used in the paper as a component of:
- Surface sizing
- Bonding agent
- Binder for coatings
- Corrugated adhesive
- Laminated glues
Sources: corn, potato, waxy maize, wheat, and tapioca.
Principle of Identification of Starch: Characteristics blue color (majorly) in iodine potassium iodide solution
- Dissolve 2.6g of potassium iodide in 5ml of distilled water
- Add 0.13g of Iodine and mix well.
- Finally, add 100 ml of distilled water and make a complete solution.
- Take a small cut-out of a square piece 1″x1″. Or, directly test it on a small portion of the sheet if it is a questioned document.
- Using a dropper, a drop is placed right over the sheet.
- If the color does not appear or fainter, tear a 0.5g piece of paper and boil them in 10 ml of water.
- Filter the supernatant and make it cool naturally to room temperature.
- Add one drop of the iodine reagent.
Indication of starch: Blue color within a few seconds.
- Amylose component of starch: deep blue color with iodine.
- Amylopectin of starch: less intense blue and reddish shade with iodine.
- Dextrins (modified starch by hydrolysis): produce red-violet to red colors.
- Regenerated cellulosic, hemicelluloses, and highly mechanically beaten fibers. But, can be omitted by using water extract from the defibered pulp (step 3 and 4 is the defibering process).
- Polyvinyl alcohol
Identification of Protein
Proteins (usually the plant ones) are used as the sizing materials. One most common one is the soy protein that is used as the bulk binder in papermaking.
For the identification of protein, the Ninhydrin test is employed. The test is highly sensitive thus if the paper is handled many times, it may give false-positive results.
- Solution 1: Dissolve 0.14g of sodium hydroxide and 0.42g of citric acid (0.43 g) in 50 ml of distilled water.
- Solution 2: 0.5g of ninhydrin in 50g of 2-Methoxyethanol.
- Solution 1 and 2 are added followed by the addition of 0.57g of alkyl naphthalene sodium sulfonate and properly mixed.
Indication: deep blue color
Identification of Waxes and Oils as Paper Additives
To make paper impregnating, water-resisting, laminating, or non-sticky, paper manufacturers use certain waxes and oils. They can also be used as additives to paper sizing.
They can be examined either physically (when the considerable amount is on paper) or by chemical solution (Dunlop test).
Part 1: Presumptive Test For Waxes and Oils
A. Physical Examination: It is helpful when waxes and oils can easily be seen over the surface such as paper with waterproofing, laminating, greasy texture, and fractures line over folded or creased areas.
B. Under Ultra Violet: if the paper is heavily waxed, the surface can be scraped off and observed under UV lamps. Any sign of white or light blue fluorescence is an indication of waxes (probably paraffin waxes) or oils.
C. Dunlop Test: The test is particularly a quantitative marker for the determination of wax in sizing materials.
The principle of the test is based on the fact that the hydrocarbons from waxes or oils are soluble in hot acetic anhydride while insoluble and precipitate out on cooling.
Procedure: 0.1 g of paper is defibered by boiling with distilled water. Take a small amount of defibered pulp and add an equal amount of acetic anhydride and again start boiling. The supernatant is poured into a clear 50ml beaker and rest to cool at room temperature.
Observation: Waxes appear as flakes while mineral oils form droplets.
Part 2: Confirmatory Test For Waxes and Oils
Pyrolysis gas chromatography and infrared spectrometry is a type of confirmatory test of presumptive tests.
For GC, Carbowax 20M and SE-30 columns are easily used to identify certain waxes with similar peaks to those of paraffin wax. The arrangement can also be used to detect polyethylene.
Identification of Fillers of Paper
Fillers are additives added while paper making (mainly after pulping) to improve optical, physical, and aesthetic properties (pigments).
Common Filler Used in Paper
|Precipitated Calcium Carbonate||Ground limestone, ground chalk||CaCO3|
|Precipitated aluminum silicate||Hydrous kaolinite (clay)||Al2O3.2SiO2.2H2O|
|Titanium dioxide||ilmenite, rutile, or titanium slag||TiO2|
And for the identification purpose, there are majorly four analysis methods such as chemical microscopy, x-ray diffraction, FTIR, and electron microscopy.
A. Chemical Microscopic Examination
You required a compound microscope with a magnification of at least x300. Most of the tests are done by means of chemical microscopic examinations.
|Clay||flat plate-like structure of kaolinite with some large plates of mica|
|Talc||larger chunks with irregular edges and fibrous tales|
|Calcium||Few drops of dil. HCl into ash-> dry the sample by heating-> add 1 drop of water + heating= needle-like crystals|
|Satin white papers||Add a few drops of dil. HCl directly to the original paper. Diamond-shaped crystals of calcium iodate|
|Zinc||Ash + 1 drop of nitric acid + let it dry + add 1 drop of water= feathery colorless crystals|
|Barium||Ash + 2 small drops of sulfuric acid+ dry over burner till white fumes start) = Barium sulfate white feathery structure.|
|Titanium Pigment||Ash+ 1 drop of sulfuric acid+ dry over a burner until white fumes appear + 1 drop of hydrogen peroxide= yellow-to-orange color|
In addition, a full-scale cationic and anionic analysis should be performed. However, they are time-consuming but can detect impurities that are present in fillers used during pulp, or other sources of impurities such as from water processing. You can check a full guided procedure on TAPPI Standard T421 and in ASTM D686.
B. Staging Test for Paper Fillers
With staining tests, paper ashes with certain fillers can be determined.
- Take a small amount of ash on a china dish.
- Add an equal mixture of methylene blue and azo acid red and mix it well.
- Remove the extra dye by tilting one side and soaked it with filter paper.
- Blue color: kaolinite, talc, and asbestine (pure fibrous magnesium silicate)
- Red: Calcium or barium sulfate
C. X-ray Diffraction Techniques
It is a rapid, and non-destructive method of testing. With X-ray diffraction, fillers and white pigments can be easily distinguished based on the characteristic crystalline structure.
Fillers such as kaolin, titanium dioxide, talc, clay (quartz), mica, calcium carbonate, and calcium sulfate can easily be identified even when their concentrations are less than 2% of the sheet’s weight.
- Not viable when the paper has several pigments.
- Organic resins not analyzed.
- Paper sheets usually have cellulose content that interferes with X-ray diffraction analysis because cellulose reflections hinder the observing ability.
D. Electron Microscopy
X-ray diffraction is majorly done for pigments with crystalline structures. Thus, amorphous pigments such as calcined clays can’t be detected. But, non-crystalline structure— irrespective of crystalline structure— can be profoundly characterized using x-ray with scanning electron microscopy.
Thus, with the use of x-ray diffraction and Scanning Electron Microscopy (SEM), the morphological characteristics and composition of even small-sized (~0.3 μm) pigments and fillers can be observed.
E. Fourier Transform Infrared Spectroscopy
Identifying filler using infrared is based on the analysis of the sample before and after calcination at 600° C. Frequently, FTIR has been used to determinant the kaolin as paper additives in the form of four distinctive hydroxyls stretching region (3695, 3670, 3650, and 3620 cm−1), with bands around 3695 and 3620 cm−1 being specific to kaolin minerals. [source]
Instrumental methods such as Neutron activation analysis (NAA) and Atomic Absorption (AA) are quite viable for trace amounts of fillers for quantitative analysis. And one of the classic methods for separating cations from filler compounds is thin-layer chromatography.
Identification of Paper Coating Adhesives
Identifying common adhesives can be chemically identified based on certain chemical tests.
Common Adhesives: Certain protein, scratch, polyvinyl resin, polyethylene casein, and rubber.
A. Test for Protein-Based Adhesives
|S.No.||Test for Protein Adhesive||Observation|
|1.||Millon’s Reagent||Brick red|
|2.||Tetrabromophenolphthalein ethyl ester||Blue|
|3.||Schmidt’s reagent||Heavy white precipitate|
|4.||Ninhydrin||Blue to violet to brownish|
I. Millon’s Test
- Millon’s Reagent: Acidified mercuric nitrate and sulphate dissolved in distilled water.
- Few drops of reagent were added to the defibered sample and heated gently
- Color: brick red
II. Ammonium Molybdate (Schmidt’s) Test
- Defibered paper sample is extracted with 0.5% NaOH with a pH of 8, followed by the addition of Schmidt’s reagent.
- Color: Heavy white precipitate
III. Tetrabromophenolphthalein ethyl ester
- Reagent: Tetrabromophenolphthalein ethyl ester in 0.1% of benzene.
- One drop of reagent is placed over the paper
- Color: blue
B. Test for PolyVinyl Based Adhesives
|Poly Vinyl Types||Test||Observation|
|Vinyl Acetate||Bromine water||Yellow precipitate|
|Vinyl Acetate||Iodine-boric acid||blue|
|Vinyl alcohol||Iodine-boric Acid test||Blue lake|
|Vinylidene chloride||Copper Wire test||Sharp gree|
|Vinyl resins||Storch-Morawski||Light greenish blue to brown|
C. Other Non-Common Adhesives and Test
|Natural and Butyl rubber||Mercuric Sulfate||Yellow to brown|
|Butyl rubber||Tetrabromophenolphthalein ethyl ester||Brownish-yellow|
|Casein||Conc. H2SO4 + glacial acetic acid||Red violet color|
|Styrene-butadiene||Storch-Morawski||Blue to brownish-gray|
|Carboxymethylcellulose||Dihydroxynaphthalene + conc. H2SO4||Violet|
D. Infrared Examination (FTIR) of Coating
The infrared spectral examination is based on comparing the coating spectra with the source including plasticizers, solvents, and inorganic pigments. Plus, you don’t need to separate the mixture into components in order to identify them.
The principle of FTIR (Fourier transform infrared) is based on IR reflectance spectra that are comparable to transmission spectra obtained by the KBr pellet technique. As papered sample (usually pulp) is in liquid spectra, it should be first evaporated on a thallium bromide-iodine plate.
|Coating||IR Examination Result|
|Starch, gum arabic, poly (vinyl alcohol)||Pigment: Adhesive ratio= 5.5: 1|
|CaCO3 band||Absorption bands at 1,435 cm-1|
|BaSO4||Absorption band at 632 cm-1|
|Acrylic copolymers||At 1,735 cm-1|
|Casein||At 1,640 and 1,520 cm-1|
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