The carbon process is a permanent, non-silver process. The most popular version was J.W Swan's, introduced in 1864. A tissue, coated with pigmented gelatin, is exposed under a negative. The exposed gelatin hardens and becomes insoluble in water. The tissue is then backed with a transfer sheet on the gelatin side and washed, which removes the original tissue and the unhardened gelatin. A positive relief image is produced, which is then transferred to a paper support. Carbon images were also transferred onto a variety of supports, including ceramic, glass, and metal. Popular 1870-1910. They are often indistinguishable from the photomechanical Woodburytype, which employs the carbon process in its manufacture.