This paper examines mechanical drawings made between 1811 and 1835 by Cesar Nicolas Leblanc, draftsman and engraver for the Conservatoire des Arts et Metiers, for publication by the French patent ministry. It compares Leblanc's engravings with the original drawings of the same machines, drawings which mechanics and inventors had submitted with their patent applications and upon which Leblanc based his published copies. The paper further compares Leblanc's patent engravings with his engravings of other machines, ones of proven utility. Leblanc developed a visual vocabulary to accompany Conservatoire Director Gerard-Joseph Christian's philosophy of industrial mechanics, and used it to expose a crucial ambiguity in French patenting practice. Patented machines were unproven machines, Leblanc's patent engravings argued, while his other engravings endorsed machines of known value.