An ad for "DuoShade" paper by Grafix from the Spring 1988 issue of WITTYWORLD Magazine.
Duotone paper was the choice a many cartoonists, particularly editorial cartoonists. It looked like a sheet of bristol and you could ink on it. And then, once you were done with the inking, you could open up a small bottle of clear liquid and brush on a clear chemical that would react with the paper, revealing shading lines or dots.
I've been told that the chemicals caused cancer. I've never seen proof, but several pros have told me that it was dangerous.
Regardless, the need for this product has virtually gone away with the advent of visual manipulation programs like PhotoShop. Maybe if you're a tough Reaganesque cowboy, you're way too tough to get cancer.