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Lithography ("drawing on stone"): an old printing process based on chemical reactions between oil and water.
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It was discovered in 1798 that if limestone was ground perfectly flat and drawn upon with a grease-based crayon, and the stone was then treated with an acidic gum arabic solution, the open parts of the stone would attract and hold water, while the greasy areas of the stone repel the water. If an oil-based ink were rolled over the damp stone the ink would transfer to the greasy areas only leaving the damp areas free of ink. When run through a press the ink transfers to the paper. This creates very accurate prints. It was, and still is one of the best means of creating accurate editions of art prints. However there is one small problem -- the stones can weigh up to a ton or more. In addition, they don't really print too well on a perfectly flat non-absorbant surface like tin.
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In the late 19th century it was discovered that very thin metal plates could
be substituted for the stones, and since they only weighed a few pounds or even
less, they were easy to handle and no storage problem. Although commercial printers
no longer used stones, the term "lithograph" stuck. With the lightweight
metal plates, high-speed presses could be used, and they did beautiful work
on paper. But printing on very hard flat surfaces like tin was still an unsatisfactory
process.
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Offset Lithography
In the early 20th century several American printers figured out how to do it.
If the printer rolls a very large rubber roller over the printing plate the
image transfers beautifully to the rubber surface. If the roller is then rolled
over paper, cardboard, tin, or even glass the image "offsets" (transfers)
from the roller to the paper or tin beautifully. The transfer of ink from the
plate to the rubber roller and then to the paper or tin is called "offset
lithography". This is what is used on tin litho toys.
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Quality Control
Printing hundreds and thousands of items in more than one color demands absolute uniformity of size, shape, thickness, and smoothness of the surface to be printed. In the case of tin lithography the tin is treated exactly like paper. It must be cut to exact dimensions. Each color demands that the paper or tin go through the press again (for that particular color). Some tin litho toys went through the press many times. If there is the slightest lack of uniformity of material or miscalculation on the part of the printer or designer it will mean that registration is bad. Registration (each color being where it is supposed to be) is the key to color printing.
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