 Welcome to the series Photographic Chemistry presented by the Foundation of the American Institute for Conservation of Historic and Artistic Works. This program was made possible by grants from the National Endowment for the Humanities and the Andrew W. Mellon Foundation. Each program in this series is presented as a short video. Depending on your video viewer, you should be able to pause, return to a previous section, or skip ahead to a later section by using a scroll bar or on-screen icons. You will find an outline of the course and short quizzes to test your understanding on the course webpage. We begin our investigation of the light sensitivity of silver salts by first learning how silver halides are formed. As shown on this slide, we see what is referred to as the emulsification equation. Now a little bit later on, I will describe in more detail why this equation is called an emulsification equation. But for right now, let's just focus on the formation of the very light sensitive silver halide salts that we've used in traditional film and paper-based photography. This is a chemical equation which shows how we combine two soluble salts in water to form a very insoluble silver halide salt. A combination which is actually engineered for a particular type of light sensitivity that we're trying to develop for our applications, our photographic applications. So this equation starts off with a very common silver source, and the silver source is silver nitrate. It's almost always silver nitrate because this is a very soluble form of silver that we can deliver in a water-based solution. And we learn from general chemistry that all nitrate salts are soluble in water. So this is why you see the silver source in most all applications is silver nitrate. And now we have some choices. We are going to combine that or mix the silver nitrate solution with different types of halide salts. It could be potassium, sodium, and in the earlier days they use cadmium and lithium forms of the halides. And again, as we review a halide is in the group 7 or the fluorine group of the periodic table. And I'll get to why we have missing members in our emulsification equation in a moment. But these are your combinations and they're written separated by a space for a reason. In other words, if I want to make a silver chloride photographic paper, for example, I have some choices. I could use the potassium chloride salt, the sodium chloride salt. I can mix in potassium chloride with cadmium chloride, and there's advantages and disadvantages to using cadmium and lithium. Again, we'll go over that more in a later section on latent image formation. But for right now, the space indicates your freedom of being able to choose what halide salt you bring into the mix. And so when we mix a halide salt with the silver, we get what's called a double replacement reaction in chemistry. And so we get precipitating a silver halide salt. And depending upon which one we choose, we can obtain either the silver chloride, the silver bromide, or the silver iodine. And when we mix salts, we will learn a little bit later, we actually get grains which contain all three in various proportions. And each proportion has a very unique impact on its light sensitivity. Now what's also formed is soluble salt. Again, depending upon which halide source we used, for example, if we used a potassium chloride to make our sodium chloride grain, then we're going to have a salt suspended in solution, which is going to be potassium nitrate, and it's going to be dissolved. A little bit later, we're going to talk about a process called noodling. And this noodling process in the emulsion making process is a step where we have to remove those salts from the emulsion. We got to clean it up as it were. So the noodling step, they'll refer to, again, later in the role of gelatin section of this presentation. We will noodle because of the presence of these soluble salts that we need to remove. So this is our base formula. This is our recipe for making films and papers which are silver halide sensitive. Now, just as a brief aside, we're missing some members of the halide family. Why can't we use, for example, silver fluoride? Well, we could. And silver fluoride would make a fantastic light sensitive silver halide material. The unfortunate problem is, though, that each of these halide salts comes with different chemical properties. Silver fluoride, for example, is extremely soluble. It is some four or five times more soluble than table salt and water. So you could possibly make a silver fluoride sensitized photographic paper, for example, when the minute you put that paper into your developing solution, which contains water, you would see your image come up and then just dissolve away because silver fluoride is just that soluble. This is why you won't see fluorine in the chemistry of the light sensitivity, light sensitive silver halide grains. Now, there's also a member at the bottom. Silver estentine would also be a very light sensitive silver halide grain similar to silver chloride bromide and iodine. The unfortunate part is that estentine is radioactive, so it's not very practical to use. Estentine salts in the formation of silver materials. All you would have because the silver halide grains are very sensitive to radiation. And again, we'll go over those details in the formation of the latent image. If we introduced any radioactive material to these films, you would expose them the minute you made them. And so this equation, the emulsionification equation, describes the beginning of our studies of how silver halides are formed and how their properties are determined by how we mix these two components, the silver nitrate and the selected silver halide salt together to form the silver halide. You have completed this unit. Depending on your video viewer, you should be able to scroll back to any point in the video as desired. The short quiz found in the course materials on the website may help you confirm your understanding of the concepts introduced here. Many thanks to the instructor, production editor, coordinator and the collaborative workshops in Photograph Conservation Committee for their work to make this program possible.