If you use artist’s charcoal for instance, you can get something called a ‘fixer’, (which is basically just vary thin hairspray) which puts a clear protective coating over whatever it is you’re trying to preserve. That might work.
But I’d just use normal stamping ink, like us humans use when we give our fingerprints. Just take care to clean it real quick before he goes on to walk all over your furniture 🙂
