Knead the dough. Cream the shortening and eggs. Sear the steak. Even if you never set foot in a kitchen, you probably understand what those instructions mean. That is because our culture is intertwined with food. We've grown up with the vocabulary. Familiarity informs us that cooking is a thing we can do if we choose to.
Although it sometimes feels as though our culture is just as intertwined with photography, it's been a long time since most photographers made their own materials. As the processes disappeared behind factory walls, so did the vocabulary and the DIY knowledge and comfort. That's not actually a bad thing for us. We get to dive into a "new" photographic medium, with all the excitement and satifaction that comes with learning something both useful and beautiful.
Making emulsions is no different from making food. A recipe is a recipe. With a few basic ingredients, a handful of vocabulary words and techniques, and a simple space to work, you can make your own black & white printing paper, dry plates, and film.
How a silver gelatin emulsion is different from other processes
Although a silver gelatin emulsion contains one or more salts, it is not a "salt print" (also called a "salted paper print"). A salt print is made by first soaking paper in a salt solution (common table salt, sodium chloride) and then drying the paper. This is followed by brushing the (literally) "salted paper" with a solution of silver nitrate soon before exposure. The salt and silver combine in the paper. The resulting image takes on the texture of the base watercolor paper and appears to be a part of the paper. The prints can be very lovely, but the process has inherent technical limitations. It requires strong UV light to make exposures. This makes it unsuitable for enlarging in the traditional sense. Of course, in our times of enlarged digital negatives for contact printing, this is much less an issue than it was in the past. Because the image is sunk into the paper, image fine detail can be lost. It can be difficult to achieve high-contrast prints. Because it is such slow (low ISO) process, it is not suitable for making negatives.
A silver gelatin emulsion more precisely would be called a "silver halide in gelatin" emulsion. A halide is the active part of a salt. The type of salt determines the specific halide—for photography these are chloride, bromide, and iodide. It is the process of combining a salt with silver nitrate, while both are suspended in gelatin, that makes the magic. If we were to take a solution of salt and water and add it to a solution of silver nitrate and water, the resulting combination would drop straight out of solution (precipitate) as worthless clumps. But, add even a little bit of dissolved gelatin to either solution, or to both, and the minute crystals that form become photosensitive and stay suspended in the resulting viscous emulsion. There is a fair amount of chemistry underlying the phenomenon, but it's not something we have to understand to make emulsions, just as we don't have to understand the biochemistry of yeast to bake a loaf of bread.
With paper emulsions, because the emulsion is made before it is coated on paper, the image sits on the paper surface. It is less influenced by paper texture. Fine detail is preserved. The chemistry of the process lends itself to a variety of contrast ranges. The emulsions are capable of much higher ISO values than other traditional photographic processes. This means you can make enlarging paper, and paper negatives, and film, and glass dry plate negatives.
As with all specialty endeavors, emulsion making has its own jargon. To the beginner, the vocabulary may seem esoteric and daunting, but that's primarily because until recently DIY silver gelatin has been a remote concept. Fortunately, that's changing. Another source of confusion arises from the early history of the technology.
Silver gelatin materials production started out in the kitchen—many kitchens, actually. George Eastman started Kodak in his mother's kitchen. Sometimes it was the wife of a photography entrepreneur who made the materials. Even then, from the very beginning, the individual dry plate and paper manufacturers kept their recipes and techniques secret. This included vocabulary. Because so little information was exchanged, an emulsion maker often had his or her own in-house vocabulary.