Early Photographic Processes
Wet Collodion Glass Negatives
Chemists and photographers experimented during the 1840s with a range of light-sensitive emulsions and various substrates, not just glass and paper, looking for a method to produce negatives of high resolution. In 1847, glass plates, coated with an albumen layer containing light sensitive salts, were introduced, and adopted with moderate success. In this case, although rendition of detail was satisfactory, the low light sensitivity of the plate was still a problem.
Finally, in April 1851, the English sculptor and photographer Frederick Scott Archer introduced a new method. He coated the glass plates with a substance called collodion, a solution of cellulose nitrate (gun cotton) dissolved in a mixture of alcohol and ether. This substance was mixed with potassium iodide and bromide, and carefully poured over a glass plate. The procedure had to be carried out as fast as possible because the high evaporation rates of ether and alcohol caused the emulsion to lose its flowing properties quickly; it became viscous, and therefore difficult to handle. The next step was to sensitise the plate, by way of floating it onto a silver nitrate solution. Exposure was carried out while the glass plate was still damp, or ‘tacky’. The image was developed with ferro-sulphate or pirogallic acid, fixed with potassium cyanide or sodium thiosulphate, and washed. The resulting image was sometimes intensified, and finally the negative was varnished in order to protect its surface from abrasion.
In spite of being a complex and manually difficult procedure, wet collodion negatives were universally adopted, because they gave an answer to the main problems presented by previous methods. The increased light sensitivity meant that exposure times could be greatly reduced and the resulting image had a fine resolution and tonal range.
To read more about TRW’s use of this process, click here.
Albumen Prints
This photographic printing process, introduced by the French photographer Blanquart Evrard in May 1850, was widely used throughout the 19th century. The prints were formed on thin paper coated on one side with albumen (egg white), which contained the silver photographic image.
The basic method employed to produce an albumen print was as follows:
Coating: The paper used to manufacture albumen prints was a lightweight high quality woven writing paper with an average composition of 85% linen and 15% cotton. The paper was coated by floating it onto a mixture of egg white and sodium or ammonium chloride. The albumen acted as a carrier and binder of the image, and prevented the silver salts from sinking into the paper fibres, thus giving a sharper definition and smoother surface to the final photograph.
Sensitising: Once the albumen coating was dry, the paper was sensitised by floating the albumen side on to an aqueous solution of silver nitrate. This enabled light-sensitive silver chloride crystals to form in the albumen layer. The coating was allowed to dry.
Printing: The photographic image was then obtained by the printing-out method: that is, by placing the sensitised paper in a contact printing frame with the negative, usually a wet or (later) dry collodion glass negative. This was exposed directly to a light source for a long enough time to produce the required print density. Sunny weather was desirable!
Processing: The print was then rinsed with water, toned (not always), fixed, and washed again, and then carefully dried flat.
Tinting: Colours were applied to the surface of the prints at this point if required. The paints used were transparent watercolours, applied with a brush to each print. The technique required considerable skill, and in the case of a stereo pair, both prints had to be coloured exactly the same (except for an occasional special effect) – an even more demanding job.
Finish: Albumen photographs were sometimes calendered or burnished to give additional shine to the surface. In order to give support to the thin paper and counteract the curling tendency, albumen prints (whether stereo or not) were often mounted onto cards or boards.
When in good condition, albumen prints present a crisp photographic image with very good detail definition and rendering of tonal progression. The surface of these photographs, if unvarnished, has an eggshell finish.
The stereo cards which have survived best to the present day are often the non-tinted examples. Often, on the tinted views, the colouring looks crude, especially if the prints underneath have faded, exposing the imperfections of the colouring. But the best of the hand-tinted cards, if well-preserved, are exquisite treasures – in beauty, the equal of any 21st century colour photograph.
To read more about the use of this process by TRW, click here .
The Daguerreotype Process
The discovery of this method of securing a permanent photographic image was announced in 1839 in Paris by L.J.M. Daguerre (hence the name) and Nicephore Niepce in Paris. Daguerreotypes were made in England, France, Germany and the USA, from the 1840s onwards, by practitioners who often added their own refinements to the process. It was successfully employed in portraiture, but daguerreotypes were also made of landscapes, interiors and even astronomical subjects. By the mid 1850s, its popularity was giving way to other types of photographs.
A daguerreotype is a photographic image formed in a silver layer on a copper plate. Daguerreotypes have a mirror-like quality, due to the reflection of the highly polished silver surface. The image is laterally reversed, and depending on the angle of illumination, can look like a negative.
The process evolved and improved during the 1840s and 1850s, but the basic method was as follows:
Preparing the plate: Daguerreotype plates were sold, ready-made in standard sizes. The silver-plated side had to be cleaned and buffed to a high polish. This was important in order to achieve an even and smooth image, as well as an adequate tonal range. The buffing process was carried out in different stages, using ever-finer abrasive powders and a fine cloth.
Sensitising: The polished silver side was sensitised with fumes of iodine to produce light sensitive silver iodide. This was carried out in a light-tight container. The duration of the exposure to iodine vapours had to be evaluated by the photographer. Factors such as relative humidity and temperature affected the rate of silver iodide formation. A second sensitising step, using other compounds such as bromide or chloride, was added from the early 1840s. This considerably shortened exposure times.
Exposure: The sensitised plate was exposed in a camera. As camera lenses became more efficient and the light sensitivity of plates increased, due to double (or triple) sensitising, exposure times were reduced to a few seconds, depending on the atmospheric conditions. The resulting image was ‘latent’ – ie not visible until ‘developed’.
Developing: The latent image was developed by exposing the plate to mercury vapour, the mercury forming an amalgam with the silver metal in the highlights. The darker areas are in fact pure silver metal. The plate was then rinsed in water, and the image was fixed with a sodium thiosulphate solution.
Toning: Gold toning with a gold chloride solution was introduced in 1841 by the French physicist, Hyppolite Fizeau. It gave the image a richer tonality and acted as a protective layer.
Tinting: The tinting of daguerreotypes was a very skilled job. It was vital to keep any physical contact with the image to an absolute minimum. Powder colours were dropped on to the plate, using brushes and a special piece of apparatus designed to keep the colouring artist’s hand suspended just above the surface. In some cases, gold highlights were also added to emphasise jewellery, buttons, or medals in the image
Mounting: The final part of the making of a daguerreotype was to seal it against outside elements. Finished daguerreotypes are not prone to fading due to incident light, but, left open to the air, they quickly tarnish – the surface is oxidised, developing a brownish tint, and often the other colours normally seen on tarnished silver plate, which can be quite attractive if slight, but will eventually completely obscure the image.
To find out more about how stereo daguerreotypes were mounted and TRW’s use of this method, click here .
