Lithography

"litho-" = stone; "-graph" = write;
"Lithograph" = Writing on Stone

The basic premise of lithography is that oil and water don't mix, they repel each other.

Stone lithography, the original art of printing multiple copies from an image drawn on stone, is performed by drawing with grease pencil, ink, or any oil-based medium on a thick, flat, nearly polished limestone slab. The non-image areas are etched with gum Arabic, then the slab is moistened with water so that printing ink does not stick to the non-image areas. When an ink roller is rolled over the stone, the ink adheres only to the image.

Modern lithographic printing really isn't much different in concept from stone lithography. Instead of a limestone slab, a flexible aluminum plate is used. Images are chemically bonded to a plate with a lacquer-based photosensitive coating instead of oil-based pigments and asphaltum. A water and acid mixture is used to prevent the ink from adhering to the non-image areas. After the plates are exposed and processed, gum Arabic is still used to etch the non-image areas.



General Operation

1. A right-reading (non-mirrored) image is etched onto the printing plate
2. Printing plates are mounted on plate cylinders of the press.
3. Paper is pulled off the stack by the feeder unit and fed into the press. Grippers hold the paper onto the impression cylinder.
4. Ink is distributed to the plate through several rubber or silicone-based ink rollers. Fountain solution, the water and acid etching solution that prevents ink from sticking to non-image areas, is distributed through a chrome fountain roller.
5. Ink is transferred from the plate to the blanket cylinder which is wrapped with a fabric-backed rubber blanket. The image on the blanket is now mirror-imaged.
6. The blanket cylinder and the impression cylinder meet and roll against each other under high pressure and the reversed image transfers to the paper as a right-reading image.
7. The paper is released and fed to each successive ink unit, laying all the colors on the paper, until it reaches the delivery unit and is stacked on a rolling pallet.

The most elusive task in printing color is controlling the many factors that affect how the ink actually gets to the paper. Cylinder pressure, blanket condition and packing, fountain acidity, powder usage, press speed and quality, ink density, and grade of stock used all affect the difficulty of printing the project. Of all these factors, ink density is the most difficult and time consuming factor for a press operator to control consistently. The Tobias IQ 200 (Scanning Densitometer) is an invaluable tool for controlling ink distribution and density on the press.