1,727. Writing a Get on Shabbos or Yom Tov

Hilchos Geirushin 3:19

If a man writes a get on Shabbos or Yom Kippur violating the prohibition only unintentionally and he then gives it to his wife, divorce is effected. If the get was written and signed on Shabbos or Yom Kippur willfully violating the law and he then gives it to her, then divorce is not effected because the witnesses who signed are disqualified under Torah law. If the get was written on a yom tov in willful violation of the law and he then gave it to his wife in front of qualified witnesses on yom tov, the get is invalid at the rabbinic level.

Hilchos Geirushin 4:1

A get can only be written with a permanent substance like ink or various paints. If it was written with something impermanent, like spirits or fruit juice, the get is invalid. If a get was written with lead, a stylus or charcoal, it is valid, though these things should optimally not be used.