Silver is not a light-sensitive material. Only when mixed with nitric acid or further mixed with halogens does the solution darken into metallic silver.
Silver nitrate worked in forming the first photographic images. Yet needed an extra kick in reducing exposure times and increasing detail.
cover photo by Ricardo Gomez Angel
Silver Nitrate and Light
For over 300 years, chemists and inventors have been using silver to create photographs. Alchemists knew about the metals' properties since the 13th century. They had created silver nitrate – a solution of silver broken down in nitric acid. This realisation was revisited in 1717. German Inventor Johann H. Schulze witnessed letters forming in a silver and chalk solution when in direct sunlight. It was the first foray into chemical photography.
However, it was a while before it had a use in photography – There wasn’t a need for it. In 1802, Thomas Wedgwood was the first photographer to use silver nitrate to create crude photographic negatives. The paper was coated in silver nitrate and placed in a camera obscura to record the world outside. These negatives could make a positive image through contact printing.
Improvements - Silver Halides
Before (and during) the use of silver, other solutions were used to record a scene. Niepce had used asphalt, (Bitumen of Judea), and Sir John Herschel experimented with flower petals and vegetable juices. Iron was also used in the Kallitype and other processes, yet some also needed silver to process. Daguerre used silver on copper for his process, where iodine vapors made it light-sensitive.
Silver nitrate was light sensitive, yet needed improving. Not only were they dangerous to the skin, but they also needed a long time to expose. Silver halides were the next step, and made possible by the discovery and availability of halogens. Chlorine (1774), Iodine (1811), and Bromine (1825). By mixing silver nitrate and a halogen, the silver halides were created and used, one way or another, until the present day.
Why are Silver Halides Better?
The silver halide process is one of, if not the most important of all analog photographic systems in place. The main reason is sensitivity – not much radiant energy is needed to create an image. Silver nitrate didn’t have the same benefit, as its sensitivity is inferior to silver halides.
The second reason is its flexibility. Silver halides create ranges of sensitive film, from 50 to over 3200 ASA for the most common. Contrast and tonal ranges also differ depending on how the photographic film or paper manufacturer sees fit.
How are Silver Halides Used?
Three halogens can be used to create light-sensitive material film or paper for photography. Silver chloride and silver bromide can be used separately or combined in the development stage. Silver iodine is usually combined with silver chloride or silver bromide. This is common except for Daguerreotype and wet plate collodion processes.
In modern paper and film, the silver halides, amongst other chemicals, rest in a gelatine solution called photographic emulsion. Monochromatic papers and film have a single emulsion layer, whereas color paper and films require an emulsion layer for each of the following; cyan, yellow, and magenta