 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. In addition to the different spectral absorption properties of the silver halide grains, each silver halide system also has a very unique chemical adsorption property. Chemical adsorption is a very key component to the photographic chemical process. We need to be able to adsorb or bind onto the surface of the grain various chemistries to affect chemical development, fixing, and so forth to be able to generate images. It keely relies on the adsorption or the binding to various chemistries onto the surface of the grains. Now it's not intuitive, but as a grain gets smaller and smaller, its surface area actually gets larger and larger against the surface properties that matter the most. And so again, that's not intuitive, but think about it this way. As the grain gets smaller and smaller, more of its area, more of its surface actually becomes more accessible to chemistry. And so adsorption properties are key in chemical development processes, and they differ with each of the silver halide grains in terms of their absorption of silver ion, for example, of halide ion, of gelatin. The binding of gelatin onto the surface of the grain is a key component, a very important component, because you don't want your record of exposure to be floating around the colloidal emulsion during development. You want that grain that recorded that photon in that very particular position to be bound there. And it's gelatin's very unique property to bind onto the surface of the grain and hold it there is, again, a really important factor in being able to obtain high resolution photographic images. We also talked about the fact that if I wanted to extend my orthochromicity of the spectral sensitivity of my grain to a more panchromatic range, I'm going to need sensitizing dyes to do that. The silver halide grain alone is not capable of doing that. So in order for the spectral sensitizers to work, they have to bind directly onto the surface of the grain, so surface binding properties become important, as well as developer chemistry. For example, an illustration shown on the right, if you would suspend a silver bromide crystal into pure water, what you would find is that bromide preferentially binds, dissolves into the water and binds onto the surface of the silver bromide grain, then does silver at a much greater rate. What this does is creates a net negative charge, and as we get more into what a crystal is and the crystal habits, you'll be able to see how this works. But there tends to be more of a net negative charge on the grain, and this net negative charge is used extensively, both in photographic development and the binding of gelatin onto the grain, of the binding of spectral sensitizers. Keep this negative charge on the grain surface in the back of your mind. You'll see how we use that chemical adsorption property of the silver halide to our advantage to engineer products for doing light photography. 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 and photograph conservation committee for their work to make this program possible.