I want to tell you the history of photography: the new exploration of photosensitive materials

In the last article, we witnessed the birth of three practical photography methods. However, due to the poor performance of these masters used in early photography, there are always various defects in use: unable to copy, poor imaging quality or too long exposure time. Then, can you make a master, which can greatly shorten the exposure time, improve the image quality and facilitate the printing of a large number of photos?

/kloc-scientists in the 0/9th century first thought of using a glass plate instead of the paper base used in Carlo's photography as the bottom plate, and coating the glass plate with photosensitive materials. Theoretically, the image quality of the substrate made by this method is far superior to that of the paper negative. Moreover, because of the light transmittance of glass, images can be easily printed on photographic paper. The problem now is that the glass surface is very smooth, how to make the photosensitive material adhere to its surface?

1839, the British astronomer Sir John Herschel tried to solve this problem. He put the glass plate in a pot filled with suspension of silver chloride, and let silver chloride particles slowly deposit on the surface of the plate, and took a photo with this plate as the bottom plate, which is the earliest existing glass bottom plate.

However, such a simple and direct method is not effective. First of all, the deposition process is too slow and lacks maneuverability; Secondly, because silver chloride is directly deposited on the surface of glass plate, it lacks adhesion and is easy to be washed off during development and fixing.

1847 In June, Nipps's nephew Abel Nipps (1805- 1870) used egg white as an adhesive, coated it on the surface of the glass plate, dried it, and coated it with photosensitizer for shooting. This method works well. Compared with the paper negatives used in Carlo's photography, the obtained images are clearer and the particles are more delicate, which quickly gained wide popularity. As a result, abel Nieps obtained the patent right of egg white glass photography.

Shortly after the advent of egg white glass negative film, it was found that the negative film shot by egg white glass method was very suitable for slide show, so egg white glass slide show was invented. Others developed egg white photographic paper based on the negative of "egg white glass". The manufacturing method is to coat a mixture of egg white and ammonium chloride on the surface of white paper, dry it and store it, and coat silver nitrate solution to make it sensitive before printing photos. This kind of photographic paper has good imaging effect and smooth surface, which is very popular with everyone. 1866, Britain consumed as many as 500,000 eggs a month just to make egg white photographic paper.

Although the effect of egg white glass photography is ideal, there are still some shortcomings: the solubility of photosensitizer in egg white is limited, which leads to its low photosensitivity and requires nearly ten minutes of exposure time; Moreover, over time, the egg white will turn yellow and brittle, and even fall off the glass plate. These shortcomings directly restrict the development of egg white glass photography.

185 1 year, a substance called "collodion" replaced egg white. "Collodion", namely "nitrocellulose" and "nitrocellulose", is a polymer that can be esterified by absorbent cotton and nitric acid, and is insoluble in water. Its physical properties are basically the same as cotton, and it is highly flammable and explosive, also known as "smokeless powder". Dissolving collodion in the mixed solution of ethanol and ether can obtain a yellowish viscous liquid, namely collodion. When applied to the damaged parts of clothes, it will form a transparent and hard protective film after drying, so it was often used to repair clothes at that time.

Fredrick Scott Archer, an Englishman, found that the adhesion of collodion was stronger than that of egg white. He added potassium iodide to collodion, then coated a thin layer of mixed solution on the glass plate, and then dipped the glass plate into silver nitrate solution to generate silver iodide, which made it photosensitive. After shooting for more than ten seconds, you can get a clear negative by developing and fixing it.

The collodion will crystallize after drying, which greatly reduces the sensitivity of the master. Shooting, developing and fixing all need to be done when the master is wet, so this method is called "wet plate photography". Because the photographic master needs to be used now, it can't be done in advance. At that time, photographers who used this method needed to carry a tent as a darkroom when they went out to shoot, which was used to coat and wash the master. Also bring a bunch of chemical reagents in glass bottles, some of which are highly toxic and some are flammable and explosive. Moreover, the size of the photo format is far smaller than that of the ordinary painter's drawing paper. However, due to its good quality, short shooting time and large number of photos, wet lithography has been widely used for nearly 30 years.

In view of the shortcomings of collodion wet plate photography, such as the negative can not be stored for a long time, some people try to improve it.

1855, a physicist named J·M· Taupenot was inspired by the egg white glass method, and a layer of egg white was coated on the surface of collodion coating to keep the moisture of collodion, so that the bottom plate could be dried and preserved. However, this kind of backplane also has the problems of low sensitivity and long exposure time. Moreover, its technical difficulty is very high, and it is necessary to strictly follow Tao Pengnuo's operation steps to get the ideal effect. Later generations have made various improvements on it, and replacing egg white with dehydrated honey and other substances can achieve better results.

Although the master of dry printing collodion method has low sensitivity and short storage time, dry printing photography greatly lightens the burden of photographers and frees them from the tedious work of coating sensitizers on site, so it became popular in the late19th century. It has greatly promoted the popularization of photography in society. Some people commented that it (dry collodion photography) made photography really invented and made photography more practical and popular.

Although the collodion dry plate is very convenient to use, its exposure time of tens of seconds or even minutes is far from meeting the needs of portrait photography. It is even more impossible to photograph moving objects.

1868, based on the characteristics of gelatin, a man named W. H. Harrison first put forward the idea of replacing collodion with gelatin. Gelatin is a colorless to yellowish transparent or translucent flaky or powdery substance formed by partial degradation of collagen in connective tissues such as skin, bones and muscle membranes of animals. It absorbs water and expands in cold water, but can dissolve in hot water to form a transparent colloidal liquid. Gelatin has the following characteristics: it is insoluble in organic solvents and cold water, and can withstand the washing process without dissolution or shedding; Swelling by water, the solution can penetrate into gelatin; The melting point is between 24 and 28 degrees Celsius, which is convenient for heating and coating.

187 1 In September, British doctor and photographer Richard dukes (18 16- 1902) published an article in a famous British photography magazine, which realized this idea for the first time: he mixed silver bromide into heated and melted gelatin to make it.

This simple dry plate has low sensitivity. Fortunately, Marduk did not apply for a patent for his own method, which is convenient for others to improve. After improvement by several scientists, the new gelatin dry plate can shorten the exposure time to tens of seconds, and the sensitivity is about equivalent to today's ISO50.

It was not until the appearance of gelatin xerography that photographers had an ideal photosensitive material in all aspects: convenient manufacture, convenient storage, good photosensitive performance and good imaging quality. The greatly shortened exposure time also makes the flash come in handy, and the photographer has a broader creative space.

Some businessmen smell business opportunities and start to set up factories to mass-produce photosensitive plates. Photographers don't need to prepare in advance, and they can buy stable quality plates in the store at any time. In addition to the original works, cameras, photographic paper and chemicals needed for photography have all started mass production, and a huge consumer market has begun to take shape.

Since then, photography is no longer the patent of a few people who are proficient in chemistry. Ordinary people can also buy cameras and masters, as long as they learn to control the exposure time, they can take clear photos. This has greatly promoted the development of photography.

Note: Cover image source (all rights reserved): Alex Timmermans.

That's all for this issue. Please see the next article for more details.

refer to

[1] is also acceptable. History of photography [M]. Chongqing: Chongqing Publishing House, 2008+00.

[2] Tang Tongyi. The history of world photography development: the journey of photography exploration [M]. Nanjing: Nanjing Normal University Press, 2006.38+0.

[3] Li. History of world photography [M]. Heilongjiang: Heilongjiang People's Publishing House, 2004.6438+0

[4] Sun Nuo. The past of wet printing and dry printing [DB/OL]. /7/view-4838809.htm，20 18. 10. 19。