Printing with ink and watercolour

I was inspired by the amazing woodcuts and watercolour washes in the illustrations  by Holly Meade  for 'In the Sea' by David Elliott. I cut stamps (out of rubber, not wood!) and used oil based ink to print them onto paper and some lines with a lithographic crayon. Then I did a couple of watercolour washes over the top.

All was going well until I went to wash my brushes and found that they have a sticky black coating. Dunked them in turps, wiped with rags, then rinsed with soap, warm water, more soap. Hope they will be okay. Was it the printers ink or the crayon, I wonder? I guess you have to wait more than an hour for the ink to dry. Still, I like the effect.