Although items dyed in indigo do not need a mordant, thiourea dioxide is a key component to the dye bath. Thio removes oxygen from the bath and allows indigo to "stick" to the fibers. Amount needed per dye bath will vary. Though it is added in a small quantity for the stock solution, more may be needed during the course of a session to keep your dye bath in optimum condition for dyeing. You are purchasing 6 ounces of thiourea dioxide.