Mordants are substances that dyers use to adhere dyes to fibers. Typically dyers use alum, copper, tin, tannic acid, or iron, among others. Each of the mordants will have a different effect on the color. A mordant can brighten, dull, or change a color all together. You can add the mordant to the fiber before, during, or after the dyeing process. My mixed mordant experiment goes like this: I would pre-mordant roving, then spin it into singles, then ply together singles that had different mordants. My prediction: if I dip a multi-mordant strand into the dye it should come out with two colors, giving it a barber-pole effect.
Next I did the same procedure with an alum mordant. After all the roving was dry I spun singles then plied the different mordant strands together.
The results were mixed. Some of the color variations were too subtle to notices. Others had the barber-pole effect that I was looking for. The yarn that I'm holding above was made with a tin/alum combination yarn dyed with red onion skins.