 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. Let's review our first defect. The first defect takes advantage of the differences in the rate of growth of a cubic grain in these two face directions that we've talked about, the 2-0-0 and the 1-1-1, where the 1-1-1 will be pure in one of either of the two ions as you march in that direction. And so any large imbalance of either one of the ions, either silver or the halide, will generate a more rapid growth along the 1-1-1, and this is where we see more multi-sided regular grains. That actually, because of that direction, tend to be more plate-shaped than cube-shaped. And we can see a twine or a twin of these two plates when there is an excessive excess, if that makes any sense of one of these ions, either the halide or the silver. We also know that the crystal growth depends upon the size of the halide itself. Once we start getting a large concentration of either the larger radii halides, either bromide or iodide, we don't see cubes. You will not see cubes in those emulsions. They tend to be more hexagonally-shaped plates. So let's go on and discuss more of the defects, which are critical to photographic chemistry. And this next one is the most critical defect. You wouldn't often think that defect is good in anything, but in photographic chemistry, defect is everything. 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.