Invented in 1842, the cyanotype process uses the action of sunlight on paper coated with light-sensitive iron compounds (rather than the more typical silver salts) against which is placed a photographic negative, a drawing, or an object. The resulting image appears white to shaded, against a background of rich color (the iron-based pigment, Prussian blue) and has a soft, matte surface. Subsequent washing intensified and fixed the image color. Early use of the cyanotype process included images of natural history specimens. From the late -nineteenth century, commercially-available papers were widely used for the common "blueprint" for architectural or other technical plans. The process continues in use today.