 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. Now that we know a little about the chemical and physical properties of the silver halide family, let's see how those chemical and physical properties are leveraged in producing various types of silver halide crystal grains. These conditions are often times referred to as the precipitation conditions. They are the conditions which are found in either the single or the double jet experiment that we discussed earlier. There are usually two conditions, and they involve whether there is an excess of one ion over the other. In the acidic emulsion, there is an excess of halide. Now again, there's a chemical reason for this. The salts of halide, for example, table salt, sodium chloride, is a salt of a strong base sodium hydroxide with a strong acid, which is hydrochloric acid. You put those together, you produce salt and water. Now on balance, which base and which acid is strongest determines what happens to the pH when you dissolve a salt. For example, we'll discuss a lot later, and you know from your experience that when you take baking soda and you dissolve a little bit of that in water, the water becomes kind of slimy. It's on the basic side. That's because baking sodium or sodium carbonate is the salt of a strong base sodium hydroxide and a weak acid, carbonic acid. So that salt, when dissolved back into water, remembers where it came from, and the overall net result is a stronger basic solution. This is why the pH of a bicarbonate solution is somewhere around 9 or 10. In precipitation conditions, the silver halides that are formed by the halide salts, sodium chloride, potassium iodide, and the list that we showed in the emulsification equation, when you place these in water, they tend to be on the acidic side. They tend to make the emulsion more on the acidic side of a neutral pH of 7. Depending upon the salt, this is why it's called a neutral or acid emulsion. It hevers around 5, 6, 7 in pH. And this happens when there's an excess of halide, and it depends on just how much excess there is of the halide that will determine where the pH lies. These are critical adjustments in the emulsification process. Smaller changes in pH, remember, is a large scale. So we get large changes in the availability of H plus ion or acid in the chemistry with changes in pH. Now the other condition is the opposite. And this is because we like to form an emulsion that is in excess of silver. Well to do this, we have to solubilize the silver before it has a chance to make a halide. So we need to stabilize it. And what we use as a stabilizer is ammonia. And so when ammonium hydroxide is added, it's known as an anemonical emulsion, meaning a emulsion that contains ammonia. And what happens is the ammonia stabilizes the silver in solution, the silver ion, so that we can get more of it in the solution to react with the halide. So both of these conditions control the rate of growth. Now let me say before we move on how important these two concepts are. And you'll see why they matter in just a few discussions. In one condition, we have an excess of halide, the acid emulsion. In the other condition, we have an excess of silver in the emulsion. And these will play to determine the crystal shape or what is known as the crystal habit in the emulsion making process. So remember this point as we move forward and you'll begin to see where it becomes important on which condition we choose and which process we use to produce a silver halide emulsion. 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.