I think learning to do hatching, cross-hatching, or stippling are very important skills when working in pen and ink. My students, for the most part, understand how to get a variety of values with pencil, colored pencil, or charcoal. However, with something like ink, it is impossible to get shades of gray. It's either black where you use it, or white where you don't. I found a wonderful idea for hatching or cross-hatching on Stuck in the Mud Pottery. I believe this lesson is for high school students, but I was able to adapt it to the 5th grade level.