When I was looking for information about printmaking, I found lots of instructions for making and printing a plate, but very little about the stage that comes before that: choosing a subject, and deciding how to represent it in a print. Which is a shame, because I think it's often this early stage that determines success or failure.
Of course, the rules of composition also apply in printmaking. But in addition to that, a printmaker needs to develop an imagination of the possible - learning to work within the constraints of the medium.
In collagraph printmaking, I find these limitations the most challenging:
The maximum number of tonal values I can create in a collagraph print is four. So if I am considering a subject with a larger tonal range, I will need to decide if my subject will still work if I group some of these values together. I usually do a quick ink sketch to figure out what to do with the various midtones - do they sit in the middle, or at the light or dark end of the spectrum?
The opposite also holds true - a subject with a small tonal range that consists largely of midtones will rarely make an interesting print. That's why I prefer to go subject hunting on sunny days - I'm looking for the full tonal range, from the darkest dark to the lightest light. I also want these tonal values to form large, interesting shapes; if they are dotted all over the place, the print will look boring.
Another limitation is the fact that you can easily create dark lines on a light background by scratching or cutting into the board - whereas light lines on a dark background are much more difficult to achieve. You can paint or drizzle on PVA glue or gloss medium (like I did to create the white branches in the print below), but it's difficult to achieve anything delicate. You could try masking the lines by placing string onto the inked-up plate before running it through the press, but that's messy and imprecise. Of course, you could opt for relief inking with a roller, rather than intaglio inking.
Edge control is also tricky. In a collagraph, getting hard edges is easy - pretty much everything you paint or glue onto the plate will collect ink around its border and print with a hard edge. That can make atmospheric effects - like aerial perspective - hard to achieve. I have found that carborundum paste is great for creating dark shapes with soft edges, but creating a soft transition between a light area and a midtone is more difficult. In fact, I think this is best done when wiping (rather than building) the plate. For example, to create a sky with subtle white clouds, I would cover the entire sky area with PVA glue when making the plate. Once the plate has been inked up, I would wipe the sky very gently, leaving quite a lot of ink on it, then use cotton buds to remove more ink to create the clouds. An alternative would be to use the silk aquatint technique, which does allow you to create gentle tonal gradations. Wax crayons can also be used to subtly lighten areas.