I personally prefer the bleach test. It tells me whether or not a fibre is primarily protein, but it can also tell me if I've got a blend. It's also fairly diagnostic for determining silk apart from other protein fibres. I also like the fact that I can re-use the bleach. I don't keep bleach on hand for many purposes, but the bleach I use for yarn tests can be poured back into a glass jar and re-used. Not everybody knows about the bleach test, so I thought I'd do a post about it.
You need to use real chlorine bleach - not the "colour-safe" stuff. You don't need much of it though. I use a small glass dish and cut pieces of the yarns that I'm not sure about. Usually they're yarns from thrift stores, grab bags, or trades, and the content label is long gone. Today I tested a group of yarns I thought were mostly acrylics. I was surprised!
Here are my yarns at the beginning of the test. I've just poured a wee bit of bleach in the dish - enough to cover the yarn. I use an old fork to push the yarn into the bleach so it is saturated. Do take a picture at the beginning of the test because you might forget which yarns were which....and some of them might be about to disappear...
The yarn on the far left - the dark red one - is my 100% wool control sample, so I can compare other yarns to it.
Within about 5 minutes, many of the yarns were telling me that they contained animal fibres. As you can see in the picture below, many of them are getting a halo of bubbles around them. That's a sign that they contain protein fibres that are in the process of dissolving.
I was surprised to see the purplish bulky yarn was among the bubblers. I had thought it was completely synthetic. You can see it is definitely releasing bubbles, whereas the blue and green yarns to the left of it are not, nor is the cranberry to the right. The pink yarn on the left is definitely a wool.
Here's a clear view of my control 100% wool sample, bubbling away. The green and red yarns below it are doing the same, but the teal is not.
You can also see that the thick purplish yarn has stopped bubbling. This tells me that it's a blend. It had a small percentage of protein fibre, likely wool, which dissolved initially. Now it's just the synthetic fibre that's left over.
And here was the scene after an hour. The wool (or other protein) fibres are completely dissolved. The five strands you see that never bubbled are completely synthetic, and the thick purple was a blend that bubbled for just a short time.
I didn't include any cotton yarns in this batch, but cotton, flax, and other plant fibres are unaffected by the bleach bath. I can usually tell the difference between cottons and synthetics by feel, so I don't use the bleach test to differentiate between those fibre types. If you're not comfortable discerning between plant and synthetic by feel, the burn test will certainly help you figure out which is which! I also notice (but I'm not going to say that it's a guarantee) that the acrylics and other synthetics tend not to lose their colour in the bleach, even though it's 100% bleach. You can see those yarns in the picture above are the same colours they started out as! On the other hand, cotton and flax tend to actually bleach out their colour.
Another result that you'll see with this test is something that bubbles a lot and then leaves a fine thread behind. This is the case with wool yarns that include nylon. The nylon stays and the wool dissolves. If you use your fork or other tool to swirl around any residues, you'll see if they're connected (i.e. synthetic leftovers) or just blobs of disintegrating protein fibre.
Here's an example of that in a test I did previously and photographed. The dark yarn on the far left is a wool with nylon, and the black boucle in the middle is a likely mohair boucle with a binder thread. The yellow, second from the left, is silk. The far right orange one is a cotton. The others are wool or other animal fibre.
Here you can see the dark wool on the right has partially dissolved, and the boucle has dissolved but left the binder thread, which is now on the right of the picture after I swirled things around a bit. The cotton is in the middle and it has lost its colour. The yellowish blob in the top is silk, but all the other wools have dissolved. The yellowish-brown thin piece along the bottom was an added afterthought, not in the first picture, so ignore it.
At the end of that test, you can see what was left - the nylon on the left from the sock blend (which is folded in half), the centre is the boucle binder, also probably nylon, and the right is the now-bleached cotton.