Marking fluid is a pigmented concoction used in the shop to make marks on metal parts. First the fluid is applied, dried, then scribed. Modern fluid is made from denatured alcohol, shellac (a resin), and gentian violet. My thoughts for sustainable marking fluid are to mix ethanol, flaxseed oil, and Prussian blue. Prussian blue is a pigment which may help the oil dry faster, and it is non-toxic (can be consumed to remove heavy metals). I imagine ethanol may also help the oil dry faster, but I am not a chemist.