Nothing can be easier to prepare than the bath of nitrate of silver, and yet there is no preparation in the art of photography which produces so many difficulties and troubles to surmount as the sensitizing bath for the iodized or bromo-iodized collodion plates.
John Towler, The Silver Sunbeam
A nitrate bath is a solution comprised of silver nitrate (AgNO3), water, and tiny amounts of silver oxide and silver iodide. In modern times, you can of course purchase the bath of nitrate pre-prepared for use.
Essentially, what a nitrate bath does is makes certain gelatin substances light-sensitive (primarily, collodion). To prepare a "wet plate" for photography, you first coat one side in collodion and then place the plate in the nitrate bath. This must be done in a dark area to avoid exposing the nitrate bath and the plate to light, rendering it useless. After ten minutes or so, the plate is ready for a picture. The plate is placed inside a frame that fits in whatever camera is being used. The frame is placed in the camera, and the lens cap is removed, light floods the chamber with the frame, and the subject is captured onto the plate. After a few seconds, the cap is placed back on, and the picture is run through a cyanide bath and water bath to wash off any excess nitrate and collodion. This entire process is done at almost a dead run, to maximize exposure and minimize major errors.
Nitrate baths are very simple to make, even at home, assuming you have access to silver nitrate and silver iodide (you can find purchasable quantities in certain photography catalogs.) Simply add three ounces of silver nitrate to 36 ounces distilled water. Then add 6 grains of silver iodide to the solution, as well as 3-5 drops of nitric acid (also rather easily available). Place this entire solution in a wide and shallow tub for placing the plates in.
These days, of course, this method of photography has been outclassed by new technologies (not the least of which is the rise of digital cameras) and the nitrate bath is a bygone product of a pre-mass production photograph economy. Still, a few enthusiasts go through the rigorous process to capture the unique feel of a wet plate photograph, the predominant form of the late 19th century. And the actual chemical concept behind the plates is still used today - silver nitrate is still the ingredient used to capture light onto film in today's modern cameras.