From Greek words for glass and print, a hyalograph is a drawing on glass that has been roughened with a very fine and even grain. The tools used for the drawing are a lead-pencil, a stump, and/or a brush with Indian ink. The drawn on glass is then transferred by light to a sensitized copper plate. The plate is traditionally etched, inked and printed. The process was invented by M. Dujardin, the well-known heliogravure atelier, and employed mostly for scientific purposes. The process was also excellent for original work because the reproduction, being so very direct, loses less than other process —in fact, the loss is almost imperceptible, which cannot be said for many other photomechanical process.
 Hamerton, Philip Gilbert, Man in Art, studies in religious and historical art, portrait, and genre, Macmillan and co., London, 1892.
Hanson, David A. Checklist of Photomechanical Processes and Printing, 1825-1910. , 2017. p. 46.