Hillotype process
The Hillotype is perhaps the oddest duck of all the color processes. Reverend Hill did something few people have ever done in history: he invented a process literally decades more advanced than all of civilization's knowledge. It would take a half-century before the rest of the world caught up to his achievement. There are very few similar examples in history: Charles Babbage designed a computer central processing unit, (which he called an "Analytical Engine"), in 1837; Jules Verne described a spaceship in 1865; the Amazon Indians could etch metal in the 1500's. Just as precocious as these feats, Rev. Levi Hill invented a permanent, natural color process which allowed him to take color photographs before the Civil War.
It seems that Hill was working largely by instinct when he produced his first colored images. Other photographers thought he was a fraud and violently tried to debunk his claim. Unfortunately, Hill had hand-colored parts of some of his photographs and his enemies seized upon those examples as evidence that he had not invented an automatic process, he was merely trying to pass off hand-colored images as a true color process. Hill had written a book describing his method, but angry competitors threatened to destroy his lab and got a court order to destroy all the copies of his book because Hill had libeled them. Hill died in 1865.
Fortunately, several dozen of his color photographs, "Hillotypes", survive and are kept at the Smithsonian Institution. In addition, some copies of Hill's book which described his procedure, "A Treatise on Heliochromy" (1851) still exist. In 1981 a professor of photography, Joseph Boudreau used the book to reproduce Hill's process. Boudreau wrote an essay about his effort and created several color photographs which were reprinted in an anthology about photography, (I forget the title).
From Boudreau's explanation and the color prints he made it seems to me that Hill had managed both to color-sensitize a daguerreotype plate and to produce a final color print by a kind of "dye-bleach" process. Actually, it was more of a "pigment-bleach" process. Apparently Hill took a normal daguerreotype unexposed plate--which is silver metal on copper plate--and subjected it to various acids and chemicals which would chemically tone the plate. Since the blue color ultimately produced looks like a cyanotype I think he partly toned the plate Prussian Blue. Other chemicals toned the plate a faint red. The mixture of all the tones produced a dark gray plate which was then made ready for exposure in the camera by turning the silver into a silver halide via an acid bath. Exposures could take thirty minutes on a sunny day. The plates were then developed according to the daguerreotype recipe which produced a direct positive.
Imagine that toning covers the plate completely with specks of colored grains of toner. A speck of Prussian Blue would press against the light sensitive surface of the silver halide. Since the speck is blue/green in color it must reflect blue/green light into our eyes; therefore, it must absorb red light. When red light falls upon the speck of Prussian Blue the speck absorbs energy from the red light and injects electrons into the light-sensitive silver halide, causing this pixel of the plate to be exposed. To develop as a daguerreotype the exposed plate is held over a bowl of heated mercury and the mercury atoms adhere to the exposed silver halide. Since the mercury atoms are whiter in color, any place where light has struck appears lighter than the background. The mercury atoms will blot out the Prussian Blue speck in a similar manner, so any place where red light has fallen the Prussian Blue specks will be turned white and the surrounding red-colored specks will stay red, leaving the pixel mainly red in color. A similar reaction occurs with other colors of light. The final plate is an amalgam of mercury, silver and the chemicals which make up the colored specks of the toning.
Hillotypes have survived over 150 years. Probably the methods used to preserve daguerreotypes are suitable for them.
