Found a cute little Rembrandt study on my sketchbook tday. A teacher once told me that Master copies heightens our sense of beauty, thus I always try to squeeze in one or two every week. This one was done in ballpointpen and no I did not use it to make the dense blacks, I exercised good judgment and switched to a brushpen for the blacks.
I have gotten better at switching tools in the middle of drawing or painting. Awareness of tools is a discipline in itself. While working, I have to constantly ask myself (for the sake of efficiency) "Am I using the best tool for this task?"