Widely used from the 1870s into the early decades of the twentieth century, the photogravure process is a blend of photography and traditional intaglio printmaking. It was used to make high-quality reproductions of photographs in ink and also to make works some photographers considered to be original prints. The process begins with a photographic negative and involves several steps, including creating a positive, three-dimensional image made of gelatin on a resin-coated metal (usually copper) plate. When the plate is treated with acid, the gelatin layer (thinnest in the dark tones of the original and thickest in the highlights) resists being etched to varying degrees. When the finished plate is inked and printed onto paper, the tonal range of the image is reproduced. The process was especially popular for print editions and book illustrations.