Pub. 3 2013 Issue 1

21 Mining Focus on a piece of film, and that is a more effective method for developing countries that don’t have the same digital technology as the U.S. Although x-rays can work directly on film for a traditional x-ray, it is more common to convert the energy from the x-ray into light by making use of intensifying screens. The light is what forms the image on the film. The film structure consists of several layers on both the top and the bottom sides of the film. The base of the film is between 170 and 200 microns. The base is what makes the film strong and flexible. It has to be transparent to light and stay dimensionally stable over time. In the early days of x-ray technology, people used glass and cellulose nitrate. When it is manufactured today, it usually consists of cellulose triacetate and polyester. An adhesive is applied over the base, followed by an emulsion that is 5 to 10 microns thick, and then finally the whole thing is covered by something called a supercoat. The supercoat prevents the emulsion from being damaged by mechanical equipment. Gelatin and silver halide are the two most important ingredients in the emulsion. People make gelatin from bone. It is an ideal suspension medium because it stops silver halide grains from clumping together. Also, processing chemicals can go through the gelatin quickly without changing its strength or its permanence. What is silver halide? It is a light-sensitive compound made from silver and a halogen. (Halogens are a group in the periodic table. The list has five elements on it: fluorine, chlorine, bromine, iodine, and astatine.) Silver halide consists of a small amount of silver iodide (one to ten percent) and 90 to 99 percent silver bromide. Each grain has a diameter of about 2.3 mm and as many as a billion silver ions. In turn, each milliliter of emulsion has billions of grains. By itself, the silver bromide has low photographic sensitivity, but the sensitivity can be increased by using a reducing agent that has sulfur in it while heating the silver bromide under controlled conditions. The process creates silver sulfide on the surface of the crystal. People call this part of the crystal a sensitivity speck. It traps electrons, and the electrons in turn start forming the image. Silver and Water Purification Probably the most important medical application of silver is the ability of silver cations to purify water. Impure water presents serious medical problems in developing countries. When silver is used with oxygen, their catalytic action is a powerful sanitizer for algae and bacteria. It works better than chlorine and is becoming the U.S. method of choice for filtration in hospitals, water systems within communities, spas, and pools. For one example of how silver can help prevent serious illness, consider Legionaire’s disease. This disease is caused when there is too much buildup of Legionella bacteria in connections, pipes, and water tanks; silver gets rid of it. Some hospitals have hot-water pipes made of copper and silver because of the double protection they give against both Legionella infections and MRSA. X Probably the most important medical application of silver is the ability of silver cations to purify water.