 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. So before moving on to an understanding of how these silver halides capture light and then turn that light into a silver image, we need to look a little bit more in detail at the atomic level, what's going on during crystal growth. This has been studied to great detail by all the giants of photographic chemistry back in the day. They needed to know exactly what was going on to be able to control the emulsion-making process, and they learned a great deal using a lot of very innovative techniques. And what they learned was when a halide salt was mixed with a silver salt, they would first mix. What was very important is that you needed a nucleating event. Anybody that's ever made crystals in secondary school from various salts know that the crystal growth happens by the start of a seeding event, and then boom, it seems like the whole crystal just falls out of solution. That's exactly what goes on in the emulsion-making process. It first starts out with that very characteristic gelatin color. There's nothing present, and then the two salts are added, and it's almost like they explode in the formation of silver halide grains. So what ends up happening through the photographic chemistry studies is that there is what's called a nucleating event in crystal growth that's called a seeding event. All that needs to happen is about six ions or so, six ion pairs need to congregate and associate to start the crystal-forming process. Then they coalesce. So once that nucleation happens, that's where you get the really rapid crystal growth. And all of a sudden, all over, there are crystals of various shapes and sizes being formed because of difference in time and temperature and stir rate. There's a whole bunch of different sizes and shapes of crystals floating around in the emulsion. Now, what an emulsion chemist will do next is ripen that batch or let it cook for a while, and there's a reason for this. The silver halide grains need to go through a process which is known as Ostwald ripening. And what Ostwald ripening does is it sacrifices, during the cooking process, there's a sacrifice of the smaller silver halide grains that will re-dissolve and re-deposit onto the larger grains that have much more surface area and so larger grains tend to dominate relative to the smaller grains. So Ostwald ripening at a certain temperature and stir rate is a process where we get dissolution of the smaller grains to diffusion of those ions towards the larger grains and then the re-depositing of those ions onto the larger grains. This has a way of shifting the distribution of small crystals into larger ones. And so the emulsion chemist would wait and sample the emulsion until the right distribution was achieved through this ripening or cooking process. What also would happen with time is after the grains had started ripening, there would also be re-crystallization that happens. Now, remember, a silver halide emulsion, particularly the more advanced ones later in the 20th century, were just not one type. They were actually a mix of all three halides, both chlorine, bromine, and iodine. Towards the later days, they're almost predominantly bromide with a little bit of iodine. And so what would happen was going back to our knowledge of the chemical properties of silver halides, we know that silver iodide is the least soluble. So during the crystal growth processes, what would happen would be you'd get your silver iodide in a greater proportion form first and through the Oswald ripening processes, the smaller grains would re-dissolve. And with bromide present, bromide would act as a solvent. And this is not the last time we're going to hear about bromide ion acting as a solvent, as a silver grain solvent. So when bromide is added and present, what will end up happening is it will help dissolve the silver iodide that's present, the iodide dissolves. The bromide is a much more reactive halide ion and the bromide would take its place, but because the iodide is much less soluble, it would also fill back in at the crystal. And you'd start getting crystals with a mix of silver, bromide, iodide, or silver chloride, bromide, and iodide. In the literature, when you see the designation silver chloride, bromide, iodide, it's always written in order from the highest concentration to the lowest concentration found within a grain. So when I'm writing in the, this slide, silver bromo iodide, and this would be called silver bromo iodide, then there would be a much more bromide present than iodide. Iodide would be the smallest component. If I had a silver chlorobromo iodide grain when it contained all three, then the predominant form present would be of the chloride, with then the bromide being next, followed by the iodide. So anytime these crystals would start to intermix and form a mixture of halides within the silver halide grain, you would also introduce defects within the grains. This is another way the chemist would engineer defects into the silver halide grain, so that we could have a grain with various sensitivity and chemical reactivity properties. So now we know how silver halide grains are formed. We know about their mixed composition. We know the importance of defects, and that defects are actually very good for photographic processes. And so it's going to take a study and a consideration of what is known as latent image theory to find out how all these chemical and physical properties fit together to allow silver halide to be the light sensitive recording material that we've used for the better part of the 19th and 20th century. 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.