 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 can be very unique effects now when we take advantage of this known fact that one face grows faster than another in a cubic relationship. We can get the various types of silver halides that we have sometimes seen in the more modern 20th century silver halide emulsions, the plates, and plates that seem to be growing on top of plates. When there are a series of plates growing one on top of the other, this is known as twining. Plates and twines occur when one face grows preferentially faster than all the others. And this, again, we notice that the one-one-one face is the most sensitive to differences in silver or halide concentration because these are faces which are pure in one of the ions. If I have an acid-based emulsion, I'm going to get accelerated growth of the one-one-one face. If I have a highly basic ammonia or anamanical emulsion, then I'm going to get very accelerated rate of growth along the one-one-one. So there's a real balance there, I can kind of dial or tweak in between the two to get the different crystal shapes that I want. Plates are very important because they provide a lot of surface area. They provide surface area for photons, which translates into exposure, and they provide a surface by which we can deposit chemistry, sensitizers, developers, and some of the chemistry that we'll talk about a little bit later. This formation of twin plates is the introduction of what is most critical about silver halide grains, the reason why they work. And in solid-state chemistry, the reasons why silver halide works is it's because they're defective. They actually have defects, and a twine creates a defect, it creates sites or edges which enhanced chemical activity, where enhanced chemical activity will occur. At the bottom of this slide, if you'll see I'm moving ions in the direction of the one-one-one face, and they're all the same ions, either you can imagine these being all halides or all silver, a twine is where they get out of step. They start growing in one direction, and then they sort of slip, and the plate starts growing into the other direction, and you end up with this twine plate, a set of plates with an edge as shown on the right-hand side of the slide. It's a mirror image. It starts out one plate shape, and then it starts taking off into another direction, and it's almost like you've taken two plates, and you've glued them together. This is known as a twine, and it creates what is known as an edge defect, and on these edges you've got high surface area, you've got imbalance of surface ions, and this is going to be the site where a lot of very interesting photochemistry happens. 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.