The use of salt for dying backpacks. A quote from the web:
Table salt is fine for dyeing, especially if it’s non-iodized, but sea salt is not a good choice because of the minerals in it, as well as the unnecessary expense. Pickling salt is better because it does not have additives.
What salt does, in high water ratio immersion dyeing, is reduce the tendency of the fabric to repel the dye. Since both the fabric and the dye are negatively charged they tend to repel each other. The addition of large quantities of salt reduces the electronegativity of both fiber and dye, making it easier for the dye to associate with the fiber so that it is located handy for the formation of a chemical bond.
In low water immersion dyeing, the purpose of salt is different. It reduces the solubility of the dye, which is probably why it results in somewhat different effects in the uneven coloration that results from the constriction of the fabric by the small size of the container it’s in. Adding salt to low water immersion dyeing can increase the crystal-like patterns produced by this method of dyeing.