If the yarn is solely of one category (cellulosic, synthetic or protein), then a burn test, if correctly done, can easily reveal which category it is in. This means that silk, polyester and rayon, though all can be lustrous and can have long fibres, burn in a completely different manner.
- Protein (animal fibres such as wool, silk, alpaca, mohair):
When approaching the flame, they tend to shrink away and are hard to light.
When in the flame, they are hard to ignite, look like they’re melting or bubbling up, and tend to self-extinguish.
Upon removal from flame, the odour of the smoke is like burned hair and the ash is a crushable/crumbly dull black ash, often in a ball.
- Cellulosic (plant fibres such as cotton, linen, ramie and viscose rayon which is a man-made fibre made from regenerated cellulose such as cotton linters and wood)
When approaching the flame, they ignite easily.
When in the flame, they burn clearly and you can often continue to see the yarn structure, such as plies, etc, all the way through to the ash
Upon removal from flame – blow gently to extinguish and try not to blow away the ash
They will glow and then go out
The odour is of burning paper (also cellulose)
The ash will be soft and grey and easily powders away – you can often still see the yarn structure in the ash
- Synthetics (plastic-based such as polyester, nylon, olefin)
These all burn somewhat differently – some just fuse together, some burn wildly, melt quickly and drip hot molten plastic. The smell can also vary from celery-like to very acrid.
The common characteristic is that the ash, when cooled, is usually hard – sometimes it is black, sometimes light coloured.
**Watch out – if there is any chance you are burning a synthetic, be careful to hold it out over a non-flammable surface (aluminium foil works well) and be ready to drop it if it starts to burn rapidly or molten and very hot plastic begins to drip – you can burn your hand or ruin or ignite whatever it drips onto. Better yet, hold it with tweezers!
General notes and disclaimers to the above:
- Yarns will act differently according to the amount of twist, eg a tightly twisted yarn will not ignite as quickly as a loosely twisted one
- Yarns with fibres or mixed categories , eg a blend of cellulose and synthetic (such as a cotton/polyester) will give mixed results. You will then need to carefully observe all the different steps in a burn test – not just the ash – to play detective and try to guess what it is composed of
- If you get “mixed” results as above, try burning individual plies separately, or also any nubs or slubs separately.
A fail-proof test for protein fibres, learned from a wool council, is to put a small sample in straight chlorine bleach. If it’s protein – and silk is – it will completely “disappear” in a relatively short time. Some have taken only five or ten minutes, others have taken as long as an hour or two.
A caveat or two: be careful handling the bleach, of course, and realize that if you leave the sample in bleach for days and days, cellulose will also disintegrate.
And the difference between rayon and cotton is that rayon is weaker when it is wet and cotton is stronger when wet. So wet the middle of a thread with some spittle or water and pull the thread with both hands until it breaks. In case of rayon it will break at the wet spot, cotton will break on a dry place.