How do I know if a fabric is cotton?

[Sarah]

I bought a set of sheets off of ebay that say 100% cotton, (not good quality at all!) but they are shiny, very thin and static-y.  Is there a way to tell if they are cotton?  I hate to waste the dye if they aren't. 

[mustdye]

I've had a problem with new sheets having a stain guard on them or something.
I bought new twice...
I now look for sheets to dye at resale shops and estate sales.

[ktaltre]

If the sheet fabric is static-y, there is probably polyester in it.
A quick way (like in a store) to tell if cloth is all cotton or close to it, is to take a corner and bunch it up tightly in your hand. If it wrinkles nicely, then it's cotton. Linen will do this also.
If the bunched up fabric springs back with little or no wrinkling, then there is polyester in it.
Then there is the burn test. Cotton smells like paper burning with a lite colored feathery ash.
Polyester shrinks from the flame, has a sweet smell and leaves a hard black or brown bead.
I just went and squished an all cotton t-shirt - the cotton knit wrinkles nicely too. I don't have any 50/50 ploycotton shirts to squish; next time I go shopping, I'll squish some 50/50 knits and report back, haha.
Sheets are notorious for having finishes on them for stains and wrinkles. I've read that one can remove the finish by scouring the sheet using very hot water, soda ash and synthrapol.
Dharma sells wide pfd muslin. Joann's Fabrics sells wide muslin. I have had good dye results with wide muslin; I do scour it, or at least wash with synthrapol in hot water first.
I'm an hour and a half from an Ikea store; I want to try their cheap and medium price sheets. I LWI dyed a set last fall for a client and the sheets took the dye well. I need to take a drive..............
k. taltre

[Sarah]

I went ahead and wasted the dye - the sheets surely are not cotton   Thanks for the input!

[ktaltre]

Sarah,
You could "dye" the strange content sheets with watered down fabric paints.
M. Fowler does this with sheets in his dvd. He ties his pattern and squirts on the watery paints like dye.
Then you untie and spread out the sheet outside to dry.
I did a sheet like that in the class I took from him and it turned out quite well.
I didn't heat set it, but didn't wash it for a couple of months and then it washed fine.
k. taltre