This is a fantastic lesson I've incorporated into my "Art of China" unit.
I pinned this lesson months and months ago and finally got around to it this year
with my Grade 9/10 class.
It's found HERE on one of my all time favorite art blogs: http://arteascuola.com/
She gives all the instructions on her blog.
The only thing we did differently was use white acrylic paint instead of gesso. We also used simple bamboo sticks/skewers for the scratching part. My kids loved this lesson- a couple even saying it was their favorite all year :)