 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. The ability for a silver halide grain to absorb light and for us to be able to develop an image recorded in a silver halide crystal comes in the fact that the crystal is not perfect. It contains defects. The most important class of defect, which is required for photography, even the work, is known as the point defect or a freckle defect. Now, a freckle defect is actually a very simple defect. It's simply a silver ion that falls out of its position into what's called an interstitial or in-between site. Silver is smaller than the halide, so the motion of the silver by this defect is by far greater than that of the halide. When we're talking about even something as large as an iodide ion in relation to silver, silver is moving around the grain. That's the important bit, the silver moves. This is highly critical to the photographic process. If this did not occur, if this defect did not occur, we would not have silver halide as a photographic medium. And so silver is moving almost 1,000 to 10,000 times more often than a halide does in the grain by this freckle defect. This motion, this falling out of its position into the interstitial areas is what gives silver halide that conductivity, that silver or ion motion. That silver motion is going to be critical for silver development. And what also results when that silver drops out of its position is a defect site which creates a net charge. If, as shown in this illustration, my silver positive ion falls into an interstitial site, at the surface now I have a net negative charge. I mentioned before that the net charge of a grain is in fact negative. This comes from the fact that silver, two results, one is halides tend to deposit more onto the surface where the silver is. So we get an abundance of more halide ion on the surface then of silver. But it also occurs in the fact that the silver is diving into the crystal by these freckle defects, leaving more halide on the surface, giving it a net negative charge. And these are critically important. You'll see again how they all play together. So this freckle defect is a big deal and it's required for photography to happen. Remember it when we talk about the latent imagery on how an exposure happens and how we get to have what is known as a latent image site. 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.