Discussion in 'Silver Gelatin Based Emulsion Making & Coating' started by kevin klein, Sep 15, 2013.
Why is there a reserve gelatin added to some emulsions? What if it was all used at the start?
The amount of "precipitation gelatin" (the gelatin used at the beginning when you are putting the silver together with the salt to make a silver halide) is considered an emulsion variable. Some gelatin (or other colloid) has to be present to prevent big clumps of silver halide from falling straight to the bottom of the beaker as a useless mess, but generally the less that is present initially, the larger and faster the grains of sensitive silver formed. In other words, you'll get a much different emulsion if you start with all the gelatin. It's one of those things that is very educational to play around with. You might 'discover' an emulsion you really love.
So if all the gelatin is desolved first, the result will be smaller grains and slower speed?
Yes -- all other emulsion making manipulations being equal. The emulsion will also have a higher contrast (i.e. the distribution of grain sizes is less.) The sum of these characteristics is a bright, clean emulsion that handles detail well. Don't forget that to a large extent contrast can controlled with the appropriate exposure and developer/development strategy.
The contrast is generally controlled by addition time, the speed by gelatin quantity.
Fast pptn gives higher contrast and slow pptn gives lower contrast (in general).
Low gelatin, 1 - 5% is used for higher speed situations, and over 5% is used for slower speed emulsions, all else being equal.
Any Ammonia containing salts or silver halide solvents present during pptn will throw all of this out the window.
I know what you're saying, Ron, but I think there might be a small chance that it is confusing in the context of Kevin's questions.
% gelatin is not the first thing that comes to mind when thinking about grain sizes and size distribution curves. The rule of thumb about speed of precipitation (the first few minutes of making an emulsion when the salts and the silver are brought together, a.k.a. "addition") is "fast pptn gives higher contrast". True, as far as it goes, but it is based on the assumption that the emulsion maker is using the minimum possible amount of precipitation gelatin. It might help to look at the 'why' of that.
In the first moments of addition, the grains formed are smaller than microscopic, but they immediately start growing. Some dissolve and their materials go into larger crystals. Some just join together outright. The character of an emulsion is set by a number of factors, but primarily by how long the ripening goes on (the time given the emulsion after precipitation to grow crystals). During all this, gelatin is keeping things in suspension. Usually, it's just enough to keep things in suspension. Because, the higher the percent gelatin the chemicals are swimming in, (i.e. the thicker the soup) the harder it is for the crystals to mix it up and grow. The result is that the emulsion ends up with more small crystals and fewer crystals of a mixture of sizes. It is the mixture of sizes that is the primary mechanism of contrast. Slow precipitation is just another way to allow the small crystals to grow.
Between precipitation/addition time and % gelatin, along with the initial concentration of the halide and the silver, we have almost an infinite number of options for making an emulsion. I think this makes things easier, not harder, to get started cooking. If you start with a decent recipe and play around, while learning just a few facts, you simply can't go wrong.
Umm, Denise, it is time for me to expand on my comment above.
In a single run emulsion (Silver Nitrate into salt and gelatin), it is the size vs freqency distribution and grain type and size that matter. In this single run (SR), the concentration of salt decreases with time and thus a different size and different grain type form. The size and shape is all over the map with a slow addition, and this gives low contrast and (mainly) higher speed due to the larger crystals that can form during the long pptn.
With a fast addition, the grains are mostly the same size and shape because they all form pretty much at the same time and thus see the same salt (and gelatin) concentration. They are smaller and slower.
With any emulsion, all else being the same, lower gelatin gives larger grains and higher speed. More gelatin gives lower speed and finer grains. Keeping things contrast and doing a time of addition series, the speed goes up and contrast goes down. In the early days, this was the major means to control contrast. In fact, early high contrast emulsions were made by dumping the Silver Nitrate over the side in one big "splash". A side note: if gelatin at the start goes much below about 1 or 1.5%, you begin to form "crunchies" which are often the size, shape and feel of corn flakes. In fact, you can use a coffee filter to determine if your gelatin level is reasonably good. If you have a lot of residue in the filter, it is likely that gelatin is too low for the conditions. Adjusting conditions can be a long and difficult task for one not trained in the art.
See example below of pptn time vs contrast and speed!
Now, if a double run or an Ammonia salt is used, this is all quite different.
I hope this gives you a clear understanding. I would be happy to give a complete workshop on this and more, free to you Denise, should you ever make it to Rochester. But, I hope this further increases your understanding of emulsion making.
BTW, this figure is in my book along with a page or so of explanation.
Amazing, Ron. Simply amazing. I thought maybe, just maybe, we could share the sandbox. Even invite a few other kids to play. Nope? OK.
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