Inscriptions on the back of the canvas

[Michele Rushworth]

I would like to write the name of the portrait subject, title (if any), my full name and the year on the back of my paintings.

What should I use to write this on with, since it's raw linen back there? Graphite? Charcoal which is then sprayed with a fixative like Krylon? I don't want to cause long term damage to the linen fibers.

Thanks!