You can label them as unknown:
Products containing unknown fibers30
If a textile product is made — in whole or in part — from scraps, clippings, rags, secondhand fibers or fabrics, or other textile waste materials of unknown and, for practical purposes, undeterminable fiber content, the disclosure may indicate that this is the case. For example:
Made of clippings of unknown fiber content
100% unknown fibers — rags
All undetermined fibers — textile by-products
100% miscellaneous pieces of undetermined fiber content
Secondhand materials — fiber content unknown
45% Rayon
30% Acetate
25% Unknown fiber content
75% Recycled Wool
25% Unknown Reclaimed Fibers
60% Cotton
40% Unknown fibers — scraps
If on the other hand, the fiber content is known or can be readily determined, the full content disclosure must be given.
Sale of remnants and products made of remnants31
Remnants for sale in a retail store do not have to be labeled individually if a display sign indicates that they are “remnants of unknown fiber content and origin.” Similarly, remnants of known fiber do not have to be labeled individually if a sign indicates the content. For example: “remnants, 100% cotton,” “remnants, 50% rayon, 50% acetate.”
If a finished product is made of remnants of undetermined content, the label would read: “Made of remnants of undetermined fiber content and origin,” or some equivalent statement.