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Calotype
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Calotype or talbotype is an early photographic process introduced in 1841 by Henry Fox Talbot, using paper coated with silver iodide. The term calotype comes from the Greek for 'good', and for 'impression'.
sensitive element of a calotype is silver iodide. With exposure to light, silver iodide decomposes to silver leaving iodine as free element.
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Calotype or talbotype is an early photographic process introduced in 1841 by Henry Fox Talbot, using paper coated with silver iodide. The term calotype comes from the Greek for 'good', and for 'impression'.
How calotypes work
The sensitive element of a calotype is silver iodide. With exposure to light, silver iodide decomposes to silver leaving iodine as free element. Excess silver iodide is washed away after oxidizing the pure silver with a second application of gallo-nitrate. As silver oxide is black, the resulting image is visible. Potassium bromide then is used to stabilize the silver oxide.
The salted paper's sensitive element is silver chloride formed when the salt (sodium chloride) reacts with silver nitrate. Silver chloride decomposes when in contact with light forming silver and chlorine evaporates. Excess silver chloride is washed out of the paper and the silver oxidizes in contact with gallo-nitrate. The silver oxide is stabilized on the paper with hypo.
Silver chloride makes better prints because it is less sensitive to temperature. During long exposures in sunlight the temperature on the paper can be considerable. (Taken from Alf B. Meier's publication Basic Photography with permission of the author)
How to make a calotype
You need two soft brushes, several vials, white watercolor paper preferably without watermarks, silver nitrate, potassium iodide, acetic acid (33%) and gallic acid (100%) a warm plate and a copying frame that consists of a wood or metal board covered by a piece of glass.
Whenever you are involved in this procedure you should wear protective goggles, an apron and gloves. If you are allergic to iodine, silver or potassium you should not try this.
Do all photosensitive work in a dark room, dim light (about a candle's worth) can be used during processing. This process is quite insensible to red and brown safe lights.
Whilst in plain daylight you can mix 6.5 grams of silver nitrate with 170 ml of water. Put the silver nitrate into the water, close the bottle and shake well until mixed.. It helps to heat the water a little. To make the results a little more consistent, distilled water should be used. Now put 57 grams of potassium iodide in 1000 ml of water and mix until totally dissolved.
Take a soft brush and "paint" the paper with the silver nitrate solution, made from a one to one mixture of the above compounds, until it is completely covered by it. Blot and dry the paper until it is only a little humid. Now put the paper into a tray with the potassium iodide solution for 2 to 3 minutes. Drain it, rinse it, blot it and dry it.
Talbot called this product iodized paper. Iodized paper can be stored indefinitely if it is in a dark and cool place. Be careful with the rinsing water, especially when rinsing silver nitrate. The first rinse should be done in a tray and the contents of this tray disposed in the same way as standard photo fixing agents. If in doubt call the environment protection office in your town.
You cannot use iodized paper to make pictures. It is not sensitive enough. To sensitize it you need a solution of 6.5 grams of silver nitrate, 57 ml of water and 10 ml of acetic acid and you also need another solution consisting of 100 ml of water and 1g of gallic acid. These solutions last for a unlimited time as long as they are not mixed.
Mix the above solutions in a relation 1:1 to have what Talbot called gallo-nitrate of silver. This solution can be kept for about a month in absolute darkness. Shortly before using the paper "paint" the coated side with gallo-nitrate, let it settle in for a few minutes and then rinse the paper in water. Blot and dry it. As soon as the paper passes from wet to humid your image media is ready to be used. It can also be dried and stored for a few days in absolute darkness.
If you kept to the formula the sensitivity of this medium should be 6/3 ISO but has the strange property of getting more sensitive the longer it is stored.
Once you have exposed your media in the camera you have to develop it immediately. To do that you brush some gallo-nitrate over it and warm it on the warm plate or with a blow drier. Do that until you have a detailed negative of your image. Once you are satisfied with the result rinse the paper in water, blot it and put it into the potassium bromide solution for a few minutes, after that rinse and dry. Now you should have your first calotype. Sometimes these were waxed to make them more transparent. Using carbon drawing fixer (you can get them at an artist's supply store) helps the durability (If none is available cheap hair spray does the trick).
Now you have to get that nice negative into positive. To do that you can use calotype paper. Put it into the copying frame and your calotype face down on it. Now put your copying frame out in the sun for about 10 minutes (on a cloudy day make it 20). Develop your calotype as before and you should have a nice picture.
Copies on so-called salted papers give much better results. To do that first dip the paper in a solution of 100 grams of salt (no matter if sea salt or rock salt) in 1 liter of water. (If you live near an ocean you can also use seawater). Results are better if you add a pinch of sodium bicarbonate. Dry the paper and prepare it as before. Expose in the frame for about 10 minutes.
When opening the frame in dim light a photograph should be visible. Rinse the print in water for about 20 minutes. A second gallo-nitrate treatment is needed before exposing it to the light and you should fix it in sodium hyposulfite (about 10 grams per 100 milliliters of water) for about ten minutes and water again.
In Talbot's original instructions, there is no mention made of hyposulphite of soda as a fixing agent; that was first used by John Herschel in February 1840.
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