 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. Translating now the rate of growth of the A direction relative to the B, in terms of silver halide, what do we find? Now, in the earliest silver halide days in the middle of the 19th century, what you'd find predominantly in the salted papers is a cubic grain. What this translates into is when chlorine is used as the halide, or chloride sources used as the halide, you almost always find a cubic grain as the end result. And what this translates into is when silver chloride is used in the emulsion-making process alone, the rate of the B direction grows faster than the rate of the A direction. In that ratio we just described on the previous slide, about a square root of three faster. And this is because chlorine's a little bit smaller ion. It tends to be about the same size as a silver ion. And so they tend to fit very regular and very neatly into an irregular cubic arrangement. And they pack very uniformly. And because of this, the rate of B and A are about the same with B being just a little bit faster as mentioned before, and we end up with a cubic grain. As we introduce bromide salts into the emulsion-making process, we see two results. If the silver is well in excess of bromide in concentration, then we still get that characteristic cubic grain. But if the bromide is in excess, this is when we start seeing predominantly hexagonally shaped grains or octahedrally shaped grains. And here is the first example of where I've used a difference in concentration and had it matter to the shape of the grain. So notice that when silver is in a higher concentration we get a cube. When bromide is in higher concentration we get a hexagonal or octahedral or multi-sided shaped grain. And this is because bromide is starting to become much, much larger. It's starting to become much more dominant in size. It's not so easy for the regular packed arrangement of positive, negative, positive, negative to happen in the grain. And so we get the A direction growing faster than the B. Now I'm going to mention this a little bit later, but I'll tell you why this matters now. That 1, 1, 1 face in a cubic grain for all the silver halide families is unique. And what's unique about it is that it only contains one of the ions as we march across the grain in the 1, 1, 1 direction. It alternates between being all silver or all halide and then all silver and then all halide. So this is why what we're really affecting is not so much the rate of growth of B because the rate of growth of B in that direction is, okay, pack a positive then a negative. Pack a positive then a negative and so on in a plane for what is practically infinity as far as the crystal is concerned in that direction. But in the 1, 1, 1 it is pack all silver then pack all halide then pack all silver. So the rate of growth of A is really going to be determined by the balance between the concentrations of the silver and the halide. And it shows in the result that I mentioned here in the middle of this slide. If you're silver heavy then we're going to get a rate of growth of the two faces to be about the same and we end up with a cube. But when we're bromide heavy when we have a high concentration of bromide this is when that 1, 1 face pack really much, much faster than the B direction of the 2, 0, 0 face and then we end up with more sides and it's very predictable this is why we get that very characteristically shaped hexagon and if we really control it well plates like Tmax that have that very hexagonal shaped shape to it or octahedral. To illustrate this even further when the halide gets to be even more of a larger presence in the crystal making process, iodide you never see a cube iodine is too large it it packs when it wants to pack it's too much a larger ion in comparison to silver and so what we always see happening is that it's able to pack really quickly along the 1, 1, 1 face then the coordinated effort of going okay silver you go and then I'll go and then you'll go and then I'll go that takes more time when the iodide is really bulky but when iodide and silver can all lay themselves down in one plane all together the process is much, much faster it would be as if you were stacking balls in a plane if it's just one color one shape you can stack stack stack stack stack really quickly rather than alternating between and so that that 1, 1, 1 face grows preferentially much faster and so you end up with the hexagon or octahedrally shaped very regular silver halide crystals 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