In the 1830ies, a new era began as many chemists found different ways to capture the environment with silver and light. A camera obscura was used to collect the light of an object through a lens and concentrate the rays of light on photosensitive material.
Here is a very low-level approach to explain how a photograph forms from silver and light – on the example of a collodion wet plate.
The photosensitive plate
Collodion consists of two main ingredients: nitrocellulose and salts. The nitrocellulose itself is dissolved in alcohol and ether, so the photographic collodion is a viscose liquid that can be poured on a piece of glass or black aluminium.
Let’s have a quick look on the salts that are used for collodion. In the early days of collodion wetplate photography, mostly iodides were used, for example cadmium iodine or ammonium iodine. Later, bromides were added in order to improve the midtones of the scenery. Frequently, cadmium bromide or potassium bromide were standard ingredients.
All salts are dissolved in a small quantity of water and added to the raw collodion solution.
A remark here: there are different mixtures if you want to create positives (like tintypes or ambrotypes) or negatives on clear glass. While the focus for positives lies in getting a very bright image (light is reflected by the silver), negatives need to have a high density to print on silver chloride paper.
As a next step, the plate is inserted into the bath of silver nitrate. Typically, a 9% solution is used, so 90g of silver nitrate AgNO3 dissolved in 1l of distilled water.
Now the salts dissolved in the collodion are forming silver salts in a few minutes. This is called “double decomposition“ or salt metathesis. For example, the cadium bromide (CdBr) is formed to silver bromide (AgBr) and cadmium iodine (CdI) is formed to silver iodine (CdI) with this formula:
AgNO3 + CdBr —> AgBr + CdNO3
AgNO3 + CdI —> AgI + CdNO3
Those silver iodides (AgI) and silver bromides (AgBr) are distributed on the collodion plate, inside the collodion layer, they are also called silver halides.
After some minutes in the silver nitrate bath, the plate is almost white.
Exposing the plate: the latent image
As soon as light with a certain wavelength (UV light and blue light for collodion wetplate) hits the silver halides, an excitation of electrons is taking place: the photodissociation (or photolysis).
A photon hits out one electron of the bromide, the electron reduces the silverion (Ag+) to elemental silver (Ag). If you imagine photographing a scenery with a table and a white table cloth, much light is reflected by the cloth thus forming a lot of elemental silver on the plate. This elemental silver is very weak and not visible and therefore called “latent image“.
Development of the plate: reducing silver ions
After the (correct) exposure, we have:
- silver halides in the dark areas of the photographed scenery
- elemental silver where light hit the silver halides
- silver nitrate that is still on the plate, where no double decomposition took place
The normal iron developer reduces the silver ions in the area, where elemental silver is formed during the exposure. More and more silver is formed, until the development is stopped. If one would continue the development, silver would be formed in areas that did not receive light, making the plate very foggy.
In the end, you can see the image on the plate, where silverions were reduced. Dark areas look very pale and blueish.
Removing unexposed silver halides: the image appears from silver and light
After the development, silver halides are still present in the dark, unexposed areas of the image. Herschel found out, that sodium thiosulfate can be used to remove unexposed silver halides and thus form an image, that can last.
Chemically, silver halides are converted into a different form that can be washed out afterwards:
2 Na2S2O3 + AgBr —> Na3[Ag(S2O3)2] + NaBr
After this fixation step, the plate must be washed to remove all sodium thiosulfate.
Conclusion
It‘s remarkable, that people in the 19th century found out those mechanisms sometimes by trial and error, sometimes because they had a profound knowledge in chemistry. The basic principles how a photograph forms out of silver and light are truly captivating.
If you ever want to try out this process, you can book a workshop or a portrait session with me.