I made these Sharpie mugs for my cousin’s engagement.
I used the method of baking where you put the mugs in a cold oven, turn it on to 220°C for 45min, then turn the oven off and leave the mugs in the oven until cool. I believe this avoids any possible cracking of the mugs by rapid temperature changes (although they should be pretty sturdy since tea and coffee are hot!). It also ensures they get plenty of setting time.
I used a regular black Sharpie, which seemed to work really well. I also used some Sharpie Pro coloured pens that we have. The colours were rubbish! They went on the mugs fine, and you can see in the photo (the mugs in the timber table) there’s a nice little red flower bud on one of the mugs, but after baking the red went yellow and the blue went a lighter musty yellow. Thankfully the red-yellow was a pleasant enough colour, and the little bits of green stayed about the same. I might have gone over the yellowed blue with green, but baking seemed to change the surface of the mugs a bit. The baked pen & glaze couldn’t be coloured over again: the pen just didn’t stick.
I will try this again some time, either without colour, or with paint pens if I really want colour.