When your skin is so dry and ashy, you can write/draw on it with your fingernail or any other object.
Have you ever taken your fingernail and scratched your skin and noticed a white, flaky line was left behind? This is usually caused by dry skin. Well, an ash tatt would be just that, but adding a bit of flare to the scratch mark. A simple design on the skin made by your fingernail or any other object and left there as a result of ashy dry skin. And the best thing about it is...it's NOT PERMANENT. :)