Ever since high school I've been drawing tattoos on people with pens or markers. Then one time in botany class a senior approached me about drawing him a tattoo. He was heading to the Navy after graduation and wanted to get it before he got in (as they frown upon them once you're in).
So I drew him up a wicked Grim Reaper shoulder piece. The Reaper was holding his scythe above his head and flipping a boney middle finger. The guy loved it and went off and got it tattooed. I was amazed when he came back to see my drawing on his arm.
Drawing is a lot like handwriting. You can recognize your own handwriting almost instantly. Even though you dont pay exact attention to the curve on your "r's" or how you cross your "t's", you still know it from staring at it all the time. So when I saw the tattoo on his arm I could see it as my drawing even if no one else ever knew me or knew I did it - I would recognize it on him till the day I die.
That's a real amazing thing about having your art on someone's skin. It's an incredible feeling of memorializing your own work on a canvas that will last for 1000's of years. More important though it's humbling to know that someone enjoyed what you created so much and it spoke to them in such a strong way that they want to display it for the rest of their life.
Since that first tattoo back in high school I now (as of today) have 3 more pieces of work on people. One guy from work and the shop girl at the tattoo shop I go to recently got a voodoo doll/angel on her leg.
What's even cooler about this one is that he kept my lettering on the banner on the right. I just put it on there to measure the spacing, but he liked it enough to keep it. So that's my handwriting too.
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Here's a view of the original voodoo doll drawing that she used for her tattoo.