[Blas]

If you want to take it to the 'N"th degree....
When you purchase one, they are usually edged in a cotton material piping. Remove a length of the thread used to hold on the edging and using a match or lighter to see if it burns or melts. Cotton will burn and polyester will melt. You don't want polyester thread on the rag because it scratches. If you drop it on the ground while working with it, start a fresh rag until the dropped rag is cleaned. Amazing what it picks up off the ground when dropped. After washing the rag, inspect each rag for foreign material before reusing...
Almost as bad as dropping a paint clay bar on the ground and then continuing to use it....