Thought I would share what I have found to work for marking test tiles easily and cheaply. The Dixon (Hi-Heat) China Marker pencil in brown lasts through ^7 oxidation firings on my test tiles. It stays clear and legible and is way cheaper than the ceramic underglaze pencils. ($7- for a dozen china pencils on Amazon versus around $12 for one ug pencil) It's kind of waxy so doesn't smear or rub off if you wipe glaze drips off bottom of test tiles. It doesn't leave any ghost marks on the kiln shelves.
It does burn out with glaze overtop but for test tiles it's brilliant. Haven't fired it hotter than ^7 so don't know if it will do ^10 but I think it would.