Engineer's Blue: Prussian Blue Ground into a Marking Paste

Engineer's blue is a heavily pigmented paste that shows where two surfaces touch: smear it thinly on a true flat, rub the work on it, and the blue comes off only on the high spots, which are then scraped down. Joseph Whitworth made accurate flat surfaces this way in the 1830s. It is Prussian blue ground into a grease. The picture: how the blue marks the high spots (our drawing).