By making images directly onto a photo-sensitised zinc or copper plate, Fox Talbot demonstrates the potential of photo-engraving. The principle was simple. A gelatine and bichromate mixture was coated onto a metal plate, then it is exposed to light through a photo-positive or actual object. Where the light reaches the plate, the gelatine-dichromate mix is hardened, while the image areas can be washed away, thus forming a mask for the etching fluid (acid). When the acid has ‘bitten’ to a suitable depth, the plate is washed, cleaned, then rolled-up with ink. The smooth non-images areas of the plate are wiped free of ink, which remains in the etched image areas, and is then printed in an etching press. The thick blankets of the etching press push the paper into the ink-filled image areas and the print is made. Fox Talbot added a kind of aquatint dusting of gum copal over the image, to create texture for the acid to bite into, and he marvelled at the results – the finest tracery of lines and subtle gradation of tones. Fox Talbot credited the origin of photo glyphic engraving to Wedgwood, who had experimented with these techniques in 1790s. Daguerre’s collaborator, Nicephore Niepce had also experimented (the Physautotype) in the 1820s. Late Karel Klic was to fully develop this technique as the Photogravure process (1878).