A comparison was made between keratometry readings and the corneal curvature as read from a shadowgraph representation of a dental stone impression of the same eye in 13 instances. The act of taking an impression does not significantly alter the shape of the cornea.
Miller D, Holmberg A, Carroll J, Exford J, Boyd M. Effect of Impression Taking on the Shape of the Cornea in Scleral Lens Fitting. Arch Ophthalmol. 1967;78(3):331-333. doi:10.1001/archopht.1967.00980030333012