OK, maybe a bit long, but I'll try to keep it brief.
A typical offset lithography press has a number of rollers and a couple reservoirs. One reservoir for ink and another for water. There's a series of rollers that spread out the ink so it can coat the plate evenly.
The plate itself in modern systems is an aluminum plate with a coating that the ink will stich to. But when it's developed like a photo sections of it will absorb water. The plate is wet with water, and then is rolled over by the ink rollers. Actually this is almost constant, there's only about a 2 inch gap once the plate is wrapped around the roller that holds it. The ink is oil based, and won't stick to the damp areas.
Next, the plate rolls against a rubber mat called the blanket, transfering the ink to the blanket which is on its own roller.
Then the paper feeds between the blanket roller and another roller so the ink transfers to the paper.
If the plate isn't wet enough there will be extra ink, but usually a weak overall layer of whichever color it is.
If the plate is too wet the ink can float on the puddles. Uncommon, but spectacular in appearance.
The ink reservoir needs refilling occasionally, and the ink is very thick almost like a thin grease, so it needs to be spread out along the length of the reservoir. We used what was basically a putty knife. Sometimes there will be a bit of ink that drips off the knife and if it lands in just the right spot it ends up getting spread out like the area on this card.
There's a bunch more stuff that can go wrong, but those are the basics.
I've started putting together a guide to printing errors on cards with pics of various errors from my collection and explanations of most of them. Some of the very modern processes aren't as familiar to me, but the basics of how they're done are. (Stuff like foiling and die cutting)
Steve B
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