In my reading of older manuals and books, it appears that
a red lead/oil mixture was used for marking/scraping
up until (roughly) the mid 1930's, when the current
prussian blue/grease mixture was adopted.
According to Wikipedia, Prussian Blue (the pigment)
*massively* predates the 1930's.
So - does anyone know anything about
the use of red lead up until so late
a date, and why it finally (and belatedly)
gave way to prussian blue?
12 years ago