I widened my trials and melted many a pen in the oven but I can conclude the following pens are safe for 115oC (Fimo curing).
- Papermate Comfortmate - capped ball pen (this also comes in retractable but haven't tried those)
- Papermate 2000 Stick
- Papermate Flexigrip Elite (I had the retractable ones cause I couldn't find the capped ones and only tested the barrel)
- Bic round stic ( I found some in a pound shop - hurrah!)
- Bic soft feel retractable pen but you can take the retractable part out and it is only the very end cap
- Staples own brand metal barrelled roller ball pen with cap - cost about £1.99
The following pens melt/distort in a 115oC oven
- Staedtler Sticks
- Reynolds by papermate
- Staples own - Sonix ball point stick pens
- Staples own cushiongrip ball point pens (retractable)
- Papermate 2020 Stick
Hope that helps and I hope to be able to show you some covered pens soon