Woodburytypes are distinguished from other photomechanical processes by the fact that they are continuous-tone images. The process involves exposing unpigmented bichromated gelatin in contact with a negative. The gelatin hardens in proportion to the amount of light received. When the gelatin is washed, the unexposed portion dissolves, leaving behind a relief of the image. Under extremely high pressure, this relief is pressed into a sheet of soft lead, producing a mold of the image. This mold is then filled with pigmented gelatin and transferred to paper during printing. The process was invented in 1864 by Walter Woodbury and achieved acclaim for its exquisite rendering of pictorial detail and its permanency.
Video from the George Eastman Museum