 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. There are other defects that matter in the chemical and light sensitivity to silver halide grains and emulsions. These tend to be more macroscopic in nature than the more point defects that we've discussed in the previous slides. But they still matter in changing the reactivity of these silver halide crystals to chemical development, sensitization, and exposure. So the classes of the much more macroscopic defect tend to fall into a class called dislocations. These are shifts, sort of like twining that we discussed earlier, where we form clefts and ridges and edges. Anytime these defects form, these are going to be sites of different surface chemistry. They're going to have different combinations of ions present. Then you would find in the bulk of the crystal. In chemistry, we call these areas which have an increased chemical reactivity. They're much more chemically active. And so these clefts and dislocations form more trap sites. And here's another time that I've used this word trap site. In being more chemically reactive in terms of photographic chemistry and silver halide chemistry in particular, we're talking about the trapping of the mobility of ions, particularly of silver. So when we talk about latent image theory, I'm going to be referring to these trap sites. They can be correlated directly to all the concepts we've discussed up to this point. These are dislocations and defects within the crystal. So when I say trap site, I'm talking about a defect site. Either a point defect or a dislocation of some kind. They can be electron traps where they'll trap silver. This could be an example of an area which might have a more negative charge, which is attracting the silver. Or they can be an area where silver sort of congregated. And this would represent what's called a whole trap. And they form a trap which holds the reactive halide ion. Now when I use this term in latent image theory, this is where gelatin gets involved in the process. But we have to wait until we can discuss that in a lot more detail. Each crystal, as I mentioned before, has a plurality of defects and dislocations. And these are going to be sites where a lot of the photochemical reactivity is going to occur. And there's about a dozen or so of these trap sites in every crystal. 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.