Do you print them or do you write them by hand? What kind of ink do you use?
They are inkjet-printed, and I processed the private keys entirely myself. Inkjet was deliberately chosen over several potential other possibilities including laser printing, thermal printing, dye sublimation, and laser engraving (all of which are available to me), with security and longevity being primary concerns.
The inkjet used to print the private keys is different from the inkjet on the outside of the series 1 coins.
If somehow the inkjet began to fade over time, we'd start to discover this as people opened their coins for redemption, long before the blacks would fade to white (assuming such was possible).
As for whether I'd write them by hand, no way - that would be egregious both in terms of difficulty and possibility for transcription mistakes. However, I did
inspect every single sheet of keys (330/sheet) by hand to ensure complete and legible printing, proper alignment, proper front-to-back registration and key correlation (public/private), as well as completeness of the print job without any duplications. They are then laser-cut into circles. So I am totally confident that every single key circle is good, and have no shivers about sending (for example) 1000 BTC to one of them when one goes in a gold coin.