My searches have found that this refers to the raw latex from the Rubber Tree. This liquid can be obtained unsmoked (as a liquid) or smoked (as a rubbery solid). The rubbery solid is (was) used to make automobile tires and dissolved in the proper solvents made up the original latex cements. The pure liquid latex was sold on the market years ago as a sort of glue as well.
Today, the entire science of Latices is a part of polymer science that involves making tailored latex compositions. The Autochromes probably used the original sap. It is akin to a sticky oil dispersed in water which dries down rather hard when all of the solvents evaporate. Since the particles don't like to coalesce in solution, you can make 3 color tinted latices and these can be combined without forming one large glob of material, but rather will remain separate.