" Any suitable material", So reads the patent. I think that the success or failure of this kinde of gizzmo would depend on the surface characteristics of the roller. Having worked with coatings rolled on via multi roller machines, the interaction between the coating and the roller is crucial to how the coating is deposited on the substrate. Varrious synthetic "rubber" materials are used. I think that George probably knew, from experience, what kind of rubber worked best for the emulsions of the day, but considered that proprietary info.