A brief guide to photographs on glass

Albumen negatives

The first successful method of photography on glass was the albumen process, developed in 1848 by Abel Niépce de Saint-Victor. A glass plate was coated with albumen extracted from egg white and treated with light-sensitive chemicals. Very fine detail was captured in the negative, but exposures of 5–15 minutes were required depending on the light. This made it unsuitable for portraiture, although it could be used for landscapes and architectural studies.

The albumen on glass process remained in use until the 1860s, although it never achieved widespread popularity. Hence, albumen negatives are rarely found. They can only be distinguished from wet collodion negatives by complex chemical tests.

Collodion negatives

1851 marked the beginning of a new era in photography with the introduction of the first fully practical process for negatives on glass. The invention, which quickly replaced all earlier photographic processes, was F. Scott Archer’s wet collodion process. A sheet of glass was hand-coated with collodion (an explosive solution of guncotton dissolved in ether) containing salt. The plate was then treated with silver nitrate, which reacted with the salt to form light-sensitive silver chloride. The plate had to be exposed in the camera and developed before it dried and became impervious to the processing solutions, hence the description ‘wet’.

The wet collodion process required considerable manual dexterity as all the preparations and processing had to be carried out on the spot, which meant carrying a complete portable darkroom around for landscape photography. The disadvantages were, however, compensated for by the detail that could be captured on the negative and the greater sensitivity of the process. Exposure times varied from a few seconds to a few minutes, making it the most sensitive photographic process up to that time. Such short exposures meant that the process was well suited to portrait photography, as well as landscape and architectural studies.

Collodion negatives have a characteristic milky or creamy look when seen against a black background, reflecting a positive image. The coating is usually uneven at the edges and corners; at least one corner may be free of coating where it was held by the photographer during the preparation of the plate.

The wet collodion process remained in general use until the mid-1880s and is sometimes still used today in the printing industry.

Gelatin dry plate

The inconvenience of the wet collodion process led to the demand for plates prepared in advance and stored for several days or weeks before exposure and/or processing. In 1871, Dr. R.L. Maddox introduced the gelatin dry plate process. After various improvements, the process went into general manufacture in 1878, rapidly replacing the wet collodion process. The plates were bought ready-prepared and could be stored for several weeks or months before exposure and development. The development of the plates was still carried out by the photographer.

When first introduced, the gelatin dry plates were 180 times slower than the wet collodion ones. Rapid improvements in their sensitivity meant that by the 1870s, truly instantaneous photographs—with exposures of a fraction of a second—were possible.

Gelatin dry plates were coated by machine and thus show even coatings right up to the edge of the plate (deterioration may have caused some flaking in places). The image is generally dark when seen in reflected light (i.e. dark when viewed in front of a black background) although tarnishing may make it appear silvery and reflective. The negatives are frequently found stored in the manufacturers’ boxes.

Gelatin dry plates continued to be used until the 1970s.