pyrogallic acid

Collodion process

The collodion process is an early photographic process, which was quickly replaced at the end of the 19th century with today's gelatin emulsion process.

The process is said to have been invented, almost simultaneously, by Frederick Scott Archer and Gustave Le Gray in about 1850. During the following 30 to 40 years it was popular, and many photographers and experimenters refined the process.

The original idea

In 1851, Frederick Scott Archer introduced the wet plate process, sometimes referred to as the collodion process after the carrier material used. The process is very simple in concept. Bromide, iodide or chloride salts were dissolved in collodion, which is a solution of pyroxylin in alcohol and ether. This mixture was poured onto a cleaned glass plate, and allowed to sit for a few seconds. The plate was then placed into a solution of silver nitrate and water, which would convert the iodide, bromide or chloride salts to silver iodide, bromide or chloride, respectively. Once this reaction was complete, the plate was removed from the silver nitrate solution, and exposed in a camera while still wet. It was developed with a solution of iron sulfate, acetic acid and alcohol in water.

Advantages

It was the first widely used photographic process which resulted in a negative image on a transparent photographic medium. The other methods of its time, such as the Daguerreotype, produced a one-of-a-kind positive image, which could not be replicated easily. With the collodion process, however, the photographer could make an unlimited number of prints from a single negative; this was typically done on albumen paper.

In addition to the convenience of creating negatives, the collodion process had numerous other advantages. It was an inexpensive process, especially in comparison with the daguerreotype. The polishing equipment and fuming equipment needed for the daguerreotype could be dispensed with entirely. The support for the images was glass, which was far less expensive than silver-plated copper, and was more consistent than paper for paper negatives. It was also fast for the time, requiring only seconds for exposure.

Disadvantages of the process

The wet collodion process had a major disadvantage; the entire process, from coating to developing, had to be done before the plate dried. This gave the photographer about 10 minutes, sometimes less, to complete everything. The plate would be dripping silver nitrate solution, causing stains and troublesome build-ups in the camera and plate holders. This made it an inconvenient process for field use, as it required a portable darkroom.

The silver nitrate bath was also a source of significant trouble. Through repeated use, it would become saturated with alcohol, ether, iodide and bromide salts, dust and various organic matter. It would lose effectiveness, and would often result in mysterious failures of plates to produce an image.

The collodion process becomes widely adopted

Despite its disadvantage, wet plate collodion became enormously popular. It was used for portraiture, landscape work, architectural photography and art photography.

It is, in fact, still used by a number of artists and experimenters, owing to its unique and sometimes unpredictable qualities.

The Dry Process

The extreme inconvenience of shooting wet collodion in the field led to many attempts to develop a dry collodion process, which could be shot and developed some time after coating. A large number of methods were tried, though none were ever found to be truly practical and consistent in operation. Even well-known scientists such as Joseph Sidebotham, Richard Kennett, Major Russell and Frederick Charles Luther Wratten attempted, but never met with good results.

Typically, these methods involved coating or mixing the collodion with a substance that would prevent it from drying quickly. For as long as the collodion remained at least partially wet, it would retain some of its sensitivity. Common processes involved chemicals such as glycerin, magnesium nitrate, tannic acid and albumen. Others involved more unlikely substances, such as tea, coffee, honey, beer and seemingly unending combinations thereof.

Many of these worked to an extent; they allowed the plate to be exposed hours, or even days, after coating. They all possessed one chief disadvantage, however: regardless of the process used, they rendered the plate extremely slow. An image could require anywhere from three to ten times more exposure on a dry plate than on a wet plate.

Collodion Emulsion

In 1864 W. B. Bolton and B. J. Sayce published an idea for a process which would come to revolutionize photography. They suggested that sensitive silver salts be formed in a liquid collodion, rather than being precipitated, in-situ, on the surface of a plate. A light-sensitive plate could then be prepared by simply flowing this emulsion across the surface of a glass plate; no silver nitrate bath would be required.

This idea was soon brought to fruition. First, a printing emulsion was developed using silver chloride. These emulsions were slow, and could not be developed, so they were mostly used for positive printing. Shortly later, silver iodide and silver bromide emulsions were produced. These proved to be significantly faster, and the image could be brought out by development.

The emulsions also had the advantage that they could be washed. In the wet collodion process, silver nitrate would react with a haloid salt; potassium iodide, for example. This would result in a double replacement reaction. The silver and iodine ions in solution would react, forming silver iodide on the collodion film. However, at the same time, potassium nitrate would also be formed by the potassium ions from the iodide and the nitrate ions from the silver. This salt could not be removed in the wet process. However, with the emulsion process, it could be washed out after creation of the emulsion.

The speed of the emulsion process was unremarkable. It was not as fast as the ordinary wet process, but was not nearly as slow as the dry plate processes. Its chief advantage was that each plate would behave the same way; the inconsistencies observed in the ordinary process were rare.

Collodion Emulsion preparation Example

Below is an example of the preparation of a collodion emulsion, from the late 19th century. The language has been adapted to be more modern, and the units of measure have been converted to metric.

4.9 grams of pyroxylin are dissolved in 81.3 ml of alcohol, 148 ml of ether.

13 grams of zinc bromide are dissolved in 29.6 ml of alcohol. Four or five drops of nitric acid are added. This is added to half the collodion made above.

21.4 grams of silver nitrate are dissolved in 7.4 ml of water. 29.6 ml of alcohol are added. This is then poured into the other half of the collodion; the brominized collodion dropped in, slowly, while stirring.

The result is an emulsion of silver bromide. It is left to ripen for 10 to 20 hours, until it attains a creamy consistency. It may then be used or washed, as outlined below.

To wash, the emulsion is poured into a dish and the solvents are evaporated, until the collodion becomes gelatinous. It is then washed with water, followed by a washing in alcohol. After washing, it is re-dissolved in a mixture of ether and alcohol, and is ready for use.

Emulsions created in this manner could be used wet, but they were often coated on the plate and preserved in similar ways to the dry process.

Collodion emulsion plates were developed in an alkaline developer, not unlike those in common use today. An example formula follows.

Part A:

Pyrogallic acid 96 g Alcohol 1 oz.

Part B: Potassium bromide 12 g Distilled Water 30 ml

Part C: Ammonium carbonate 80 g Water 30 ml

When needed for use, mix 0.37 ml of A, 2.72 ml of B and 10.9 ml of C. Flow this over the plate until developed. If a dry plate is used, first wash the preservative off in running water.

Chemical Reactions Involved in the Process

The reactions involved in the collodion processes are the same as those which apply generically to photographic processes as a whole.

The iodide, chloride or bromide salts in the collodion react with silver nitrate to produce silver iodide, silver chloride or silver bromide. These are unstable to light, and are chemically changed upon exposure. They are reduced to metallic silver by the developer.

Wet Plate Today

The wet plate collodion process has undergone a revival over the past few decades. There are several practicing ambrotypists and tintypists who regularly set up and do images at Civil War reenactments. Many fine art photographers also use the process and its handcrafted individuality for gallery showings and personal work. There are several makers of reproduction equipment for the contemporary practitioner. The process is taught in workshops around the world and several workbooks and manuals are currently in print.

References

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