 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 now that we have the recipe for making a light-sensitized silver emulsion, now we go to the cooking step. And this is often times referred to as silver halide cooking or the cooking step. As much like baking a cake, not only do the ingredients have an impact on the final outcome of the product, but the process and the way they are mixed has a final impact on the quality of the product. Back in the early days, the way that silver halide materials were produced was by a process known as a single jet emulsion making process. In the single jet process, the order of addition is as follows. Gelatin is suspended in water, and dissolved within the gelatin solution is the silver halide salt. And again, this gelatin making process is not anything different than the way you would make the product jello at home. It's made in exactly the same way. In fact, if you didn't know this, it's the higher quality gelatin, and we'll talk more about the properties and where gelatin comes from, but it's the higher quality gelatin that went to the Kodaks and the Fuji's of the world. The lower quality fraction is actually what's in the jello box that you take home. A very high quality of gelatin is necessary for a high quality silver halide emulsion to be formed. And so this would be dissolved into a water-based solution, just like you would do at home with jello, and you would add the halide salt to the gelatin solution. It's called single jet because what would happen next is that the silver nitrate solution would be jetted into at a fixed rate, at a controlled rate, at a controlled temperature to form the silver emulsion, the precipitation of the silver halide grains within the gelatin. And the process, the rate, the temperature, a process known as ripening, how long you let it cook, all of this determined the final outcome of the product. Now in a single jet, you have a lot less control of the process. You're starting out with about one molar salt, halide salt in the gelatin solution, and molarity is a concentration measure, and I refer you back to your general chemistry studies on what molarity means, and we're going to be referring to molarity quite often in this initial stages of light sensitivity of silver salts. So you have about a one molar or so solution of salt in this gelatin, in your jello, if you will, and then you would add to it somewhere on the order of six molar silver salt in the jet, and the way that the silver salt would be added is by a constant rate with stirring and the stir rate, again, all these processes matter, but you can imagine that as the silver is being added, the concentration of the halide salts changing in the pot. And in the silver, in the single jet experiment, then you have a lot less control, the variety of different types of silver halide grains, which would be formed, the habits, and that is the shape of the grain, as we'll describe in a moment, would be varied, the size of the grains would be varied, and therefore its photographic properties would not be as consistent and would vary from batch to batch. So it was learned about the middle part of the 20th century that a finer control process was acquired to get more reproducibility in the photographic making process. And so this is where a double jet pot was developed. And in the double jet experiment, where we have the difference between a double jet and a single jet is in a double jet, we have the gelatin dissolved in a water-based solution, again just like you would do with gel at home, and then we are going to mix in two separate jets the silver salt with the halide salt, and again rate, stir rate, temperature, process, all of this mattered to produce a much higher quality, much more uniform grain. In fact, developed at Kodak, they learned that they could develop the tea grain. If you remember back in the middle 90s, early to middle 90s, the development of what's called the T-Max line of silver halide materials, these were photographic plates of silver halide grains, and these plates had a lot of surface area, they were very uniform, they had a very distinct shape, and therefore the sensitivities of the films were much more consistent batch to batch. And the reason why this was, Kodak learned, that they could do a double jet experiment, but what they found was that they took the halide salt and injected it from the bottom with the silver salt injected from the top and had the two chemistries meet in the middle, they could control the habit formation or the grain shape formation in a lot more controlled way, a lot more predictable way. Now the process by which this would occur chemically is what we are going to go over next, and what's going on here, remember I mentioned in the emulsification equation, we're adding a soluble silver salt to a soluble halide salt to produce an insoluble silver halide grain. We need that insolubility for our image to remain on the paper in the film as we develop it in a water based solution, so how does that all happen? So the minute we mix the silver with the halide to combine to form the silver halide grain, that silver halide is insoluble, it is going to crash out of solution and go through a process known as chemically as precipitation, and this process is how we're going to make the emulsion of insoluble silver halide grain supported by a gelatin matrix, and how that occurs will be found on the next slide. 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.