Most printing technology was based on letterpress, the printing of images that projected above nonprinting areas. In 1976, Aloys Senefelder invented a planographic, or flat plane, printing process later called lithography. He found that an image, no matter how detailed, that was drawn with a greasy substance on the face of a water-absorbent stone and then linked could be printed onto paper with absolute fidelity. Lithography was ideally suited for illustration and enjoyed a phenomenal popularity during the 19th century, especially for color printing, which required a separate stone to print each color. Eventually, it was found that the image on the stone could be transferred, using a special starch-coated transfer paper, from the stone to a metal plate that was used for the actual printing.
Photoengraving
Early in the 19th century it was discover that certain materials hardened and became insoluble in direct proportion to the amount of light exposure they received. By 1880s it had become possible to expose a metal plate, coated with a light-sensitive substance, to a photographic negative, creating a positive image that could be engrave in relief on the plate. Early photoengraving was line cuts: they could reproduce only in black and white. The invention of the half screen made it possible to reproduce a continuous tone image, one in which a range of blacks and grey was present.
Offset Lithography
Lithographic metal plates had only rarely been used for commercial printing, in part because the image on the plate was often worn through by the printing paper. In 1904 an American printer, Ira W Rubel, accidentally discovered that the lithographic image could be transferred, or offset, to a rubber cylinder that could then print as perfectly as the plate and would become the most popular printing process because of its economy, long plate life, and ability to print on many different textures.
Gravure
Although intaglio techniques, such as etching and engraving, had long been used as printing methods by artists, the development of an intaglio printing plate, one whose image was incised below the plate surface, was developed only late in the 19th century for commercial printing. Some people use a gelatin-coated, light sensitive tissue to transfer a grid pattern unto a copper cylinder was etched and linked, it printed an image whose varying tones were achieved by varying depth of the ink-filled cells that the grid pattern had produced. This process gave rise the field of gravure printing, which includes Rotogravure, a high speed, high volume technique -essentially the same process used- that to day print the newspaper and magazines color separations, as well as a huge variety of packaging materials.
Other Printing Process
Flexography, a relief process utilizing flexible rubber or plastic plates, is particularly well suited for printing on many kinds of nonpaper packaging materials and recently, has been used successfully to print newspaper. Screen process printing makes use of a finely woven metal or synthetic mesh to which a nonporous stencil is mechanically or the image is to be printed. Applications of screen printing include the printing of poster, fabrics and electronic circuit boards.
Color Printing
Halftone color printing, the process still used today to reproduce full color, was introduced in the 1890s, but many years passed before its full potential was realized. Although color reproduction theory was fairly well understood, the lack of color film restricted color work to studios where the necessary separation negative had to be made directly from the subject, under the most exacting conditions. Reliable color film became available in the 1930s and color reproduction grew both more common and more accurate.