 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. I hope at this point now we have a greater appreciation for all that's going on and what seems to be a very simple looking silver halide grain. How the silver halide grain is formed, the impact of impurities on the performance of the silver halide, the role that defects and dislocations may have in the sensitivity and the chemical reactivity of silver halides. So how does the emulsion chemist think about all of this background information, if you will, in their emulsification making process? Let's go all the way back to the beginning when we talked about two ways of making an emulsion, a single jet and a double jet. And you might recall at the time I said that single jet was what was done in the early days where there wasn't much control and we progressed with our knowledge of solid state chemistry into a more double jet arrangement for emulsion making. And now you can understand why I said what I said. In a single jet experiment where we have the halide present in a fixed concentration and we're adding silver, there's really no way to control the ratio of concentrations between silver and halide and it's constantly changing as the emulsification process is occurring. So whenever a single jet experiments used to form emulsions, we get everything that we just discussed, a whole range and many dislocations and defects, random sizes and shapes and habits and a wide range or a large distribution of grain sizes. All of this will mean a fairly broad and unpredictable exposure sensitivity and development characteristic for those early materials to get better control to get better consistency on exposure and sensitivity. This is why all of the chemical manufacturers for silver halide emulsions went to a double jet, much more controlled. You could meter in both the silver and the halide to form just the grain you want at just the right size. So in double jet there were few dislocations, very uniform size. In the way I discussed where the two jets were opposed to one another, you would get the Tmax grain type technology that Kodak had. Very flat, that flat structure allowed for very high surface area. That means much, much higher exposure characteristics. This is where the films got faster because of this technology and with a much more limited distribution and sizes and defects and shapes. The predictability for exposure and development was much better with the latter materials than those materials used in the early part of the 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.