Thorium used in dye transfer paper was there as a white colored mordant that tied the dyes in place. The Thorium cation (positive ion) formed a very insoluable salt with the anionic (negative) sulfonic acid dyes used in dye transfer. In the last instant product that Kodak was going to introduce, they used Nickel as the salt for assisting in both complexing and mordanting the dye in place. That improved dye stability as well.

So, metals, and radioactive materials have a long history in photo products.

PE