6th Grade Self-Portraits (a learning experience)

This is only my 2nd year of teaching Art in public/charter schools, so I'm still experimenting with a lot of techniques and media. I've been wanting to try oil pastel batik with a class, and since my 6th grade classes are the only ones for which I have sink access (I use a science classroom), I thought this would be a good place to experiment. So glad I've been telling my kids all year that art is an experiment and a terrific forum through which to practice our problem-solving skills!

Demo: Aside from a few gray lines where
the ink hadn't dried completely on the
paper, it worked just fine...

My demo turned out exactly as planned. I created the line drawing with white chalk pastel, then filled the image with oil pastel, laying it down heavily and leaving the white chalk lines visible. I then painted black India ink over the entire image, waited for it to dry (about 25 min), and rinsed it in the sink in my office. It came out GREAT, and I was so excited to do this with my students.

So I went through the process with my 6th graders...they spent a lot of time on their under-drawings, pressing firmly, leaving chalk lines empty. They really did a fantastic job. Only when we inked the drawings and began washing them, the ink didn't come off like it did with my demo. I'm perplexed. The first few students who began rinsing their drawings were washing away the oil pastel, as well, in trying to get the ink off, and one ended up with a small hole. So...I put a pause on the project until we get back from Spring Break. My only solution for salvaging these (the kids have put SO much effort into them) is to turn them into scratch board drawings. One student has already started on this process, and while the result is a bit "dirtier" than we were going for, it's not such a bad solution. I think we'll be touching them up with more oil pastels, as well, particularly the eyes and other focal points.

Student self-portrait w/
scratchboard technique begun.

I report when we're done. (View the updated post here!) In the mean time, if this has happened to anyone else, I'm open to additional ideas. I've read that some people add dish soap to the ink (which I didn't do), so perhaps that's part of the problem? Also, some of the kids inked theirs in one class, then washed at the start of the next, a space of two days wait time. I thought that might be the problem, so I had a couple of students ink theirs while we had time to wait for them to dry (again, about 20-25 min), but that persistent ink wouldn't come off of theirs either. So we just decided to let them dry again and wait to try the scratchboard technique. Aside from drying time with the first set of student work, there's absolutely no difference in materials or technique from the demo I made and the students' work. Ah well...mistakes are the foundation for discovery. We'll work it out. :)