Study Unit 2.6 Fibre Identification Flashcards
What method can be used to identify fibre?
The burning test is a good preliminary test for categorising fibres.
The burning test
Observation of burning provides information on behaviour in a flame, smoke generation, odour during burning and ash or residue.
It should never be used as the only method of identifying a fibre.
But it provides valuable information that may be used with other evidence to make a positive identification of an unknown fibre.
Blends of fibres are difficult to test as the reaction of the dominant fibre may mask the presence of a second fibre. Finishes, especially flame retardant finishes, can also give misleading results.
Safety precautions
The test is relatively easy to perform but it is necessary to observe certain safety precautions. Use a small flame in an area where there is no danger of igniting other materials. A candle in a stable base or a small alcohol lamp is preferable to a hand-held match. A non-flammable pad should be used under the burning material to provide protection from molten drip and smouldering ash. Do not touch the ash or tweezers while they are still hot.
Procedure
The sample to be tested should be in fibre form.
A single yarn from a woven or knitted fabric should be untwisted to produce a tuft of fibres for testing.
Use the instructions, and observe the reaction of the burning fibre very carefully.
Repeat the process several times to check results.
Instructions
- Hold the tuft of fibres with a pair of tweezers.
- Move the tuft close to the side of the flame; do not place the fibres above or below the flame. Observe carefully to see if the fibres melt, shrink or curl away from the flame.
- Move the fibre tuft slowly into the flame to observe its burning behaviour and any smoke produced.
- Remove the tuft slowly and carefully from the flame to observe the reaction once the flame source is no longer present.
Careful observation provides an answer to these questions:
(a) When the tuft is brought near the flame does it shrink, melt or curl away from the flame?
(b) When the tuft is introduced into the flame does it burn slowly or rapidly, or does it show no sign of ignition?
(c) Does the material melt?
(d) Does the material produce a spluttering or steady flame or no flame at all when it is in the flame?
(e) When the material is removed from the flame, does it continue to burn or does it self-extinguish?
(f) If it burns, does it continue to melt? Does it glow red (afterglow)?
(g) What is the odour produced when the material is removed from the flame? (If the material is still burning when removed from the flame, blow out the flame before you make your observation).
(h) What does the residue look like? Is it soft, in the shape of a fibre (ash) or bead- like or sticky? What colour is it?
(i) After it cools, touch the residue or ash. Is it hard, brittle (easy to crush) or soft and feathery?
Results
Compare results with typical fibre reactions given in table 2.2. When interpreting results remember that the presence of blends of fibres or dyes and finishes may affect results. Also, dyes and pigments may affect the colour of the residue.
Microscopy
Knowledge of fibre structure seen through the microscope and observing some of the differences among fibres in each group is helpful in understanding fibres and fabric behaviour.
Fibre manufacturers can use the microscopy test positively to identify many natural fibres.
Manufactured fibres are more difficult to identify as many of them look similar and variations in the manufacturing process may change their appearance .
Cross-sectional appearance may be helpful in identification (See diagram in section 3.1.1).
Solubility and melting-point
The generic class uses solubility tests to identify manufactured fibres and confirm identification of natural fibres. The unknown fibres are placed in different solvents to observe the solubility of the fibre. The melting point of, particularly, manufactured fibres may be used to identify fibres in support of other identification methods. Natural fibres, rayon, aramid and modacrylic fibres do not melt.
Wool Approaching flame
Curls away from flame
Wool In flame
Burns slowly
Wool On removal from flame
Self-extinguishes
Wool Odour
Burning hair
Wool Residue
Small, brittle, black bead
Silk Approaching flame
Curls away from flame
Silk In flame
Burns slowly, splutters
Silk On removal from flame
Usually self- extinguishes
Silk Odour
Burning hair
Silk Residue
Brittle, black bead
Cotton/ flax Approaching flame
No shrinkage
Cotton/ flax In flame
Burns quickly
Cotton/ flax On removal from flame
Continues to burn, afterglow
Cotton/ flax Odour
Burning paper
Cotton/ flax Residue
Grey, feathery ash
