 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. When we view high magnification micrographs of silver halide grains, we notice that they have very regular shapes. They will have a shape ranging anywhere from a cube to either a hexagonal shape or even an octahedral shape, but that's about all you normally see. It's very regular. The crystals that are formed in the silver halide families all follow a cubic pattern. They make cubic shapes where their edges are of various sizes. And this is what changes their dimension from being a perfect cube to maybe more of a hexagonal or octahedral type shape. And how can we control these shapes and being able to make and predict the exposure characteristics for these silver halide grains? Well, to be able to study crystal shapes or habits, we need to know an area of chemistry known as solid-state chemistry. What I'll do is introduce some of the terms and have you go back and fill some of the details in at your own reading. But essentially all we're going to be using is designations that allows us to move into a three-dimensional cubic space, a room, if you will. I need to have a map where if you and I are going to discuss where we are and where we're going or where a certain location is in the room, we need to be able to know how to translate from where we are to where we're going. And in these crystal habits and the grains, remember there are a regular array of positive silver ions and negative halide ions, and they alternate positive, negative, positive, negative, and so on. But within each plane, they're going to be similar. There'll be planes of positive silvers and planes of negative halide. Remember earlier I said this is an important concept and we'll come to know more about why this is as we move forward. Looking at the slide shown, you'll see two designations. Suppose we move in just two different directions from the center point of a crystal. We're standing in the middle of a cube and we want to move. We have two distinct ways we can move under what is known as the Miller indices or the index by which we map our motion. I can move in the B direction and the B direction under the Miller system is two, zero, zero. Or I can move in the A direction. In the A direction I'm moving along the one, one, one direction. Or these two faces once formed are normally referred to in solid state chemistry as the two, zero, zero face or the one, one, one face. Now you can imagine just in the way I drew it that I can generate two types of crystals. If I have my B direction or my two, zero, zero faces which are all at right angles to one another both up and down and left to right. If these grow out the fastest in relation to A what I'm going to end up with is a cube. If on the other hand I have my one, one, one face growing at about the same rate that my two, zero, zero face grows this is where I end up with a one, two, three, four, five, six, seven, eight sided at most an eight sided or octahedral shaped crystal. If there is not such an equal growth amongst the two faces I may end up with only a hexagonally shaped crystal or maybe it has growth that is faster on three sides but not one so I get preferential growth for example on the two, zero, zero faces but my one, one, one is it's just hanging in there giving me these clipped corners if you will and this is how I get that very characteristic hexagonal shape where there's three equal long sides and three equal short sides that make up the grain which you see for example in the logo for I, S, and T what are these conditions that changes the rate of growth along either the A direction or the B direction? Well what we know then is that just mathematically not even thinking about the chemistry of the emulsion making process but just mathematically if I can get my A to grow faster than my B but not so fast I mean it can't grow so that I end up with a cube again if it grows about three times faster then with my A growing faster than my B I'm going to end up with the octahedra as shown I'm going to end up with one, one, one faces if however my B grows at just a little bit faster rate than the A this has to be only the square root of three faster about 1.5 times or so but just a little bit faster then I'm going to end up with a very cubic characteristic shape that we associate with other cubic crystals such as table salt so now let's picturing this graphic in our minds let's now turn these faces in the silver halide chemistry to illustrate just how we get the various shapes that we see in silver halide emulsions 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 and photograph conservation committee for their work to make this program possible