The process of preparing the base with a binder for photographic coating is called subbing. The process involved putting a mixture of water, gelatin, and solvents onto the nitrate or acetate film base. The solvents dissolve the top layer of the base, allowing the gelatin to stick to the base. Once the solvents and water have evaporated, the gelatin layer is ready for coating. Polyester is handled slightly differently, because of its increased durability and the increased difficulty of dissolving the top layer of the base. In the MELT CASTING PROCESS, a special chemical layer is applied, before the polyester is stretched into its final form. This chemical layer makes it easier to then attach a gelatin layer in the manner described above.