An excerpt from Anthotypes – Explore the darkroom in your garden and make photographs using plants. Sir John Herschels description of how the chemistry of anthotypes and how light changes pigment to make an anthotype.
Sir John Herschel describes what happens when an anthotype is made in his 1842 paper On the Action of the Rays of the Solar Spectrum on Vegetable Colours, and on some new Photographic Processes, in the Philosophical Transactions of the Royal Society, vol. 133 (1842), pp. 181-215.
“The action is positive, that is to say, light destroys color; either totally, or leaving a residual tint, on which it has no further, or a very much slower action. And thus is effected a sort of chromatic analysis, in which two distinct elements of color are separated, by destroying the one and leaving the other outstanding. The older the paper, or the tincture with which it is stained, the greater is the amount of this residual tint.”
Henry H. Snelling describes in History and Practice of the Art of Photography what takes place when the emulsion on the coated paper fades to a lighter color as the sunlight destroys the pigment of the exposed areas. He writes the following description of the chemistry of the anthotype process:
“From an examination of the researches of Sir John Herschel on the coloring matter of plants, it will be seen that the action of the sun’s rays is to destroy the color, effecting a sort of chromatic analysis, in which two distinct elements of color are separated, by destroying the one and leaving the other outstanding. The action is confined within the visible spectrum, and thus a broad distinction is exhibited between the action of the sun’s rays on vegetable juices and on argentine compounds, the latter being most sensibly affected by the invisible rays beyond the violet.”
He also writes in chapter X, page 65:
“Sir John Herschel attributes these changes to the escape of carbonic acid in some cases; to a chemical alteration, depending upon the absorption of oxygen, in others; and again in others, especially where the expressed juice coagulates on standing, to a loss of vitality, or disorganization of the molecules.”
The chemical process of anthotypes can also be described in a more philosophical and beautiful way, as done in 1843 in the book Photogenic manipulation by George Thomas Fisher, published by George Knight and Sons in London:
“…the flowers which, imbued with the principle of vitality, whatever that may be, resist the influence of all exterior agents, bud, bloom and flourish in beauty and fragrance, become subject, when the vital energy is exhausted, to these very influences, especially to that of light; the color vanishes or is changed; in fact, a photogenic process has taken place.”
The chemistry has, naturally, not changed since the invention of the anthotype.