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  1. #1

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    Determining emulsion ISO

    This may have already been discussed, but, when making an emulsion is there a good way to acurately determine the ISO (or ASA) of the emulsion?
    Is it possible to 'set' the speed to a predetermined factor -- meaning can one make an ISO 100 B&W emulsion with any fair degree of acuracy?
    If not, why?

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    Quote Originally Posted by robopro View Post
    This may have already been discussed, but, when making an emulsion is there a good way to acurately determine the ISO (or ASA) of the emulsion?
    The emulsion has to be coated and made into the form of use first. Then follow the ISO method for the particular kind of the emulsion. Different kinds of emulsion use different measurement to determine the speed.

    Is it possible to 'set' the speed to a predetermined factor -- meaning can one make an ISO 100 B&W emulsion with any fair degree of acuracy?
    If not, why?
    For most of the time, no. There are many factors that affect speed and you don't know the speed until you make a test batch with exactly the same material, method and technique.

    Experienced emulsion makers have accumulation of data from an emulsion made with one or two variables shifted within a reasonable range. In this case, it is possible to aim a specific speed a priori. Even so, you typically make a test batch to confirm that the emulsion is exactly what you need.

    In reality, pictorial photographic emulsions are a blend or multilayer coating of 3-5 individually prepared emulsions. (In case of color, for each color layer, there are at least 3 emulsions involved.) The speed of the material is again influenced by the blending of the individual emulsions as well, although it is largely determined by the fastest one.

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    An easy way to determine the useful ISO of an emulsion is to take a picture of the MacBeth color checker or any other similar step scale and the negative with the scale 'centered' in the range of zones is the proper ISO.

    In other words, if you get a full range of steps, you have exposed it correctly, but if you get two steps of the same or approximately the same density, you are too close to either end of the scale. Each step of the neutral scale should show a pronounced difference in density.

    As for making emulsions with a given speed, it is quite normal to divide emulsion grain sizes into speed ranges, and emulsion content into speed ranges. Therefore, you can normally construct a nomograph of 'average' speeds for any given set of emulsion size / grain content (Br, Cl, I) and come up with a general range for a given emulsion. Superimposed on this is the crystal habit of the grain, with different shape grains giving different speeds.

    It is possible to change the speed range drastically by adding ingredients to either enhance or repress the speed for various reason.

    When you blend three emulsions, it is generally the middle component which is of most concern to the user (and the film designer), as that is where the image is normally placed in a correct exposure when using negative color and B&W films. Placing the image on the fast or slow component implies either under or over exposure (toe or shoulder). While this is sometimes to be desired, it is not the optimum.

    When you blend emulsions, care must be taken to prevent unwanted changes or avoid unwanted mismatches in the emulsions during keeping and coating which can cause changes in their response to light. This can introduce bumps in the characteristic curve.

    PE

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    Interesting. I know that film speed is determined by the size, shape, and density of the silver halide chrystals in the emulsion, but As I'm using an albumen emulsion right now that requires sensitization by soaking in a silver nitrate solution, it isn't really possible to 'force' crystals to grow -- you just get what you get. I was hoping there may be some sort of quick 'litmus test' I don't know about I could do to get an idea of the general speed range.
    I'm determining exposure times by trial and error and if I could get a better idea of the true speed (probably around a 6 but I'm just guessing) I could control my exposures better.
    I'll give you suggestion a try.

    I suppose if I'm going to start taking this hobby seriously I should take some time and actually learn what I'm doing.

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    You cannot 'force' growth by the soaking method, nor can you chemically sensitize the emulsion with sulfur or gold. You may be able to spectrally sensitize it, but that would give you just speed in the region of spectral sensitization. This would only be important in B&W printing of negatives, not really as much in the camera except to improve the tonal reproduction.

    PE

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    Quote Originally Posted by robopro View Post
    Interesting. I know that film speed is determined by the size, shape, and density of the silver halide chrystals in the emulsion
    That's a VERY crude relation. I can make 0.5 micron film/plate emulsion that varies in terms of photographic speed from single digit to three digits ASA speed. Even if the percentage of iodide is held fixed, the sensitivity varies a LOT depending on where in the grain I place the iodide. (Think of a multi-layer tennis ball and which layer contains iodide.)

    As I'm using an albumen emulsion right now that requires sensitization by soaking in a silver nitrate solution, it isn't really possible to 'force' crystals to grow -- you just get what you get.
    That method is considerably more limiting compared to modern emulsion technology and can't really control anything important in what I mentioned above...

    I was hoping there may be some sort of quick 'litmus test' I don't know about I could do to get an idea of the general speed range.
    I'm determining exposure times by trial and error and if I could get a better idea of the true speed (probably around a 6 but I'm just guessing) I could control my exposures better.
    I'll give you suggestion a try.
    I'd contact print a step wedge for a predetermined exposure. But the measurement of speed for print material is done in a different way than negative films (for example, measures the exposure necessary to get midtone, not shadow) and I still recommend to review the ISO method and the idea behing it.

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    Tennis balls are hollow. A golf ball would be a better analogy.

    Placement of the various chemicals within a crystal is very important, but is often very difficult to achieve without extensive equipment. It is difficult to prove as well without very delicate analytical techniques that are able to probe the crystals on the micron or sub micron level.

    Besides Iodide, mentioned above, it is possible to include Rhodium, Iridium, Osmium, Lead, Cadmium, Mercury and Copper salts in various locations within a silver halide crystal to control speed and curve shape, as well as latent image keeping and reciprocity. And, just because I list them here does not mean that I advocate their use.

    Unless you have a firm grounding in emulsion making chemistry, or you have a fixed formula given to you, you will most likely be groping in the dark. Outside of the patent literature, which is purpously made obscure most of the time, very little of this is published. Mees and James is one of the best textbooks on this subject, but contains little specific information.

    PE

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    That's a VERY crude relation.

    Yes, it is, I know, but I'm afraid if I start trying to get too technical I'm going to show off my ignorance even more than I am now! :-). I'm a little too busy to try it right now but I've been thinking I may try making a gelatin emulsion. Of course it won't be on the level you guys are doing, but what the heck? I copied 4 recipes off the web and have read about a dozen patents and I think I'm starting to get at least a basic feel for it. Also think I might try using sodium thiosulphate to try and bump the speed up. One patent I read called for adding 6.7 grams of 1% solution to every 2000 grams gelatin and ripening at 50C for an hour. I managed to break that down to 1 small drop for every 40 grams.
    One of the recipes I found calls for 2 ripening periods, 1 for 2 hours and 1 for 1.5 hours, so I'm thinking I should add it during the second phase.
    What do you guys think?

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    I think that you should base the addition of hypo on silver rather than gelatin, and you should also consider the size of the grain. More hypo is needed as the grain size gets smaller.

    The first ripening is usually right after the emulsion is made and contains no hypo, and the second ripening which is called 'finishing' or 'chemical sensitization' is carried out in the presence of hypo.

    There are few real modern finish formulas published anywhere, but I will tell you that they are as complex as most modern emulsion formuas. The simple ones that use hypo and heat are really old and rather primitive, but they do work.

    Some emulsions do not require any finish depending on the speed you require. I have seen excellent camera speed (ISO 6) emulsions with no finish, so it really depends on what you want.

    PE

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    Well, you have to remember that in the patent they control the amount of silver halide and the grain size a lot more tightly than I could. I'll just have to guess, but 1 drop of a 1% solution per 40 grams of gelatin doesn't seem like much -- or am I missing something? Anyway, I can always bump the amount up or down and see what happens, right? And it'll be awhile before I can even get started so I have time for more research -- I may even find a better additive than hypo...

    I actually want to produce as fast an emulsion as possible in my kitchen without investing in a professional lab. I like the look of albumen and it is cool to see people's eyes pop open when I tell them how I made it, but the exposure times with pinhole are so long I'm really limited in what I can do. It would be cool if I could get the exposure times on an 11X14 format down to a couple of minutes -- or even a few seconds so I could do portraits.

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