I was reminded the other day of the challenges presented by working with paper. Frankly, it was a reminder I would have lived without, but I guess it was good for me. The fundamental problem is this: paper changes size.
Painters have the same problem to an extent. The cotton or linen substrate that they paint on absorbs moisture from the air (or gives it back to the air) and expands or contracts accordingly. Thus their canvas gets loose in the middle or stretches too tight and distorts the stretchers.
Paper is particularly fond of changing dimensions in the presence of changing levels of moisture. Try this experiment. Take 10 sheets of paper out of your computers printer. Put 5 of them, side by side across a table. Take the other 5, wet them (just dip them in a tray of water) and place them side by side immediately below the other 5. The wet papers should be 1/4″ to 3/8″ wider! Some papers expand more than others and some expand more in one direction than another.
As printmakers, we are constantly messing with wetting and drying our papers. Each time we do so, they change their sizes, not necessarily consistently. On top of that, we DO things to them. We smash them, form them, pull them: generally abuse them. Which of course makes them stretch, not always evenly.
All of this needs to be taken into account when printing a multi-color edition. The larger you work, the more you need to be aware of it as expansion is often a percentage of the dimensions and 10% of 50 inches is a lot more than 10% of 6 inches.
So, the problem I had was in printing a woodcut. I was working on the third color and I found that the paper was being pulled about 1/8″ off over the 35 inch length of the print. It was doing it consistently. After a lot of thought, I realized that the paper was being formed down onto the cut areas of the plate and, since this was happening mostly on one side of the plate, was pulling the paper toward it and stretching it. Of course this will also mess up subsequent layers as the stretching is more or less permanent so the image has moved relative to my registration points.
The solution to the problem will involve eliminating the forming by going to a harder tympan which will apply the printing pressure over a narrower range. It will mean that the printing pressure has to be more precise, but that is ok. The existing proofs are spoiled.
Like everything else in printmaking, you have to be aware of your materials and processes and how they work. Without that knowledge, the material is in control and you cannot get a consistent edition. Sometimes it takes a problem to remind me of that.