Textile Analysis for Forensic Testing

Textile Analysis for Forensic Testing

Textile as used in Forensic Evidence

Fibers represent one of the forensic evidences known as trace evidence.

The forensic examiners must handle a victim with care, to minimize fiber loss. Retrieving the victim's clothing as soon as possible is very necessary to prevent as much fiber loss as possible. Fibers are typically collected using adhesive tape.

The following details are particularly noted when analysing fibers. This helps to determine the source of the fibers.

1. Fiber Type: The presence of less common fibers at a crime scene or on the clothing of victim or suspect increases its significance.

2. Fiber Colors: It greatly influences the significance of fiber comparison. Fading and discoloration further adds significance to a fiber association.

3. Number of Fibers: The greater the number of fibers on the body of a suspect or victim, the more is the likely hood of a direct contact between individuals, however converse may not be true.

4. Fabric type: Loosley knit or woven fabric or new fabric shed more fibers.

Distinguishing Linen from Cotton

The Following are the basic differences on the basis of which we can distinguish Linen from cotton:


1. Linen is about 20% more heavy than cotton.

2. It has a leathery feeling that is absent in cotton .

3. Cotton feels warmer(about 15-30% warmer) and holds heat better than linen.

4. On holding linen against light, the threads and the fibers composing the threads appear uneven and streaked as it is not possible to make linen yarn as uniform as cotton yarn.

5. On burning a linen thread, the fibers lie in the same position as before with no change except the scorched appearance. Burning a cotton thread causes the fibers to spread like a tuft.

6. Linen absorbs oil much better than cotton. To distinguish Linen with cotton in a piece of fabric, first remove all the impurities by washing and boiling. Then when if the fabric is dipped in oil, the linen fibers look transparent if held against the light. The Cotton remains nearly opaque.

7. Linen stands the action of sulphuric acid better than the cotton. To check a blend, first remove all the impurities then dip in con. sulphuric acid for a minute or two. Wash in water and dry on a blotting paper. All that remains on the blotting paper is linen. The cotton almost immediately dissolves in acid.

 

Chemical Identification of Silk

Please see the following link for details on chemical identification of silk.

Fiber Identification - Burning Test- Man-made Fibers

Viscose Rayon

All viscose including High Wet Modulus scorch and ignite quickly when brought near the flame. Like cotton they burn quickly with yellow flame when in the flame. When removed from the flame they continue to burn. There is no afterglow unlike cotton. The smell is that of burning paper. They leave a light gray and feathery ash.

Acetate Rayon ( And Triacetate Rayon)

When brought near the flame, it fuses away from flame turning black. When in the flame, it flames quickly. The fabric puckers, sputters and melts. It drips like burning tar. When removed from the flame, it continues to burn and melt. It smells like vinegar. It leaves a brittle hard, irregular black ash which is difficult to crush.

Nylon

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Nylon fuses and shrinks away from the flame when brought near the flame. In flame, it burns slowly without melting. When removed from flame the flame diminishes and tends to die out. It has somewhat pungent odor. It leaves a hard, round, tough and gray bead.

Aramid ( Nomex)

When brought near the flame, it shrinks away from the flame. When in the flames it puckers and chars. When removed from flame, it extinguishes by itself. It has no smell and it leaves a hard black bead.

Polyester

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Polyester fuses and shrinks away from flame. When in flame, it burns slowly with melting. When removed from the flame, it burns with difficulty. It has slightly sweetish smell. It leaves a hard round brittle, black bead.

Acrylics

Orlon, Acrilan and Creslan and Zefran fuse and melt away from Flame when brought near the flame. When in flame Orlon flames rapidly. The fiber puckers, sputters and melts. Acrilan flames rapidly and melts. Creslan flames and melts and Zefran sputters slightly and flames. When removed flame all of acrylics continue to burn and melt. Orlon has a slightly burning meat-like smell. Acrilan has a buring steak smell. Creslan has sharp sweet smell and Zefran has a turmeric like smell. Orlon, Acrilan and Cresla have hard, brittle and irregular black bead. Zefran has irregular black ash that can be crushed easily.

Modacrylics

Verel and SEF fuse and shrink away from the flame when approached near a flame. When in flame, Verel burns very slowly with melting. SEF shrinks, melts and smolders. When removed from flames, all modacrylics are self extinguishing. Verel has a gunpower smell whereas SEF has a sharp sweet smell. Verel leaves a hard and irregular black bead whereas SEF leaves a hard and irregular black bead.

Spandax

Fuses but doesn’t shrinks away from the flame when approached near the flame. When in flame, it burns with melting. It has an acrid smell. It leaves a soft, fluffy black bead.

How to Identify Constituent Fibre Percentage in a Blend-1

Blend of Polyester/Cotton (viscose)

1. Take 0.5 to 1.0 gms of blend sample, carefully weighed, and put it in a flask.

2. Add 75% (w/w) Sulphuric Acid (M:L::1:200).

3. Put in a water bath for one hour at 50+-5 deg C.

4. Filter it, whatever is left is polyester.

5. Wash it thoroughly.

6. Neutralise it with Dilute solution of Ammonia

7. Dry at 110 deg C, cool and weigh to find the Percentage of Polyester and the other cellulosic component.

Blend of Cotton/Viscose

1. Take 0.5 to 1.0 gms carefully weighed sample and put it in 60% w/w Sulphuric Acid. Keep material to Liquor ratio as 1:100.

2. Stir this solution mechanically for 30 minutes. Viscose fibres will dissolve by this process and cotton fibres will be left.

3. Filter the cotton fibres and wash it in Sulphuric Acid.

4. Again wash it with water and neutralise it with a dilute solution of Ammonium Hydroxide.

5. Dry and Weigh. Note that cotton fibres lose weight by 5% in this process. Apply this correction factor in finding the blend percentage .

Blend of Polyester/Cotton/Viscose

1. Put the fibres in 60% w/w sulphuric acid (after weighing). Viscose will dissolve in 60% w/w sulphuric acid.


2. Dry and weigh carefully the rest of the fibres.

3. Put these fibres in 75% sulphuric acid. Cotton will dissolve.

4. The fibres left will be of polyester, which are weighed after thorough washing and drying.

Degree of polymerisation of some fibres

Fibers and DP ( Degrees of Polymerisation)

Nylon 6 -120
Nylon 6,6-200
Polyester (PET)- 100
Polyacrylonitrile > 2000
Viscose Rayon- 150-350
Polynosic- 700-1100
Cotton- 4000-10,000
Wool- 60,000-100,000

Specific Gravity of Fibres

Acetate= 1.33
Acrylic=1.17
Glass=2.50
Nylon=1.13
Polyester=1.38
Rayon=1.52
Alpaca=1.31
Angora=1.10
Camel hair=1.31
Cashmere=1.31
Cotton=1.54
Linen=1.50
Flax=1.50
Hemp=1.50
Jute=1.50
Mink=1.26
Mohair=1.31
Ramie=1.55
Silk Weighted= >1.60
Silk - B. Mori (raw)= 1.33
Asbestos=2.1
Silk Tussah=1.32
Wool= 1.31