I ended up putting mine in at 450 for about an hour. It might have been a bit excessive, but the Sharpie has survived several trips through the dishwasher already without any signs of wear. Note: check other bake at 450 & "special" pen link in comments on this site.
I made this ornament for my friend, Jennifer :) --She's a Wizard of Oz fan too!! :) :) I used clear ornament, glitter, vinyl and my silhouette cameo machine :)
Wizard of Oz Hand Painted Glass Ornament by Bogies on Etsy, $22.00
Just print off any picture you want, then spray paint a piece of wood black, cut the picture to match the size of the wood. Using Mod podge coat the wood then lay the picture on top. Once it has dried thoroughly, use sandpaper to rough up the edges, then put a layer of mod podge over the picture. paint some eye hooks and use pretty ribbon as a hanger and voila