25 years ago, when I moved from New York City to Pennsylvania I gathered bags of black walnuts and wearing cloth garden gloves cleaned them all. When I had finished and took off the gloves... my hands were black! A friend called her farmer grandpa to ask how to remove the stain... and he just laughed. Took a couple of weeks to wear off. To top it off... I lined up the walnuts to dry out on my porch... and when I came out a few hours later there were only three left... the squirrels were thrilled.