I printed a grayscale version of the final image on a transparency sheet and put it on a transparency projector and blew it up on the gymnasium wall. In an earlier experiment I found that using pencils to draw on this type of masonry is not efficient. I needed to sharpen the pencil every 10 seconds. So I thought charcoal sticks or oil pastels might be too messy. It was then I happened to have discovered Korn's Lithographic Crayons hidden in one of my cabinets.

They look like they could be 100 years old. Who knows how long they were in there.

I had the students trace the projected image with the crayons and it went pretty well.