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  1. #11

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    yes, of course, they all state that is not advisable remelting and they all suggest to break the emulsion in smaller quantities. But I never did that. And I know people who don't do that either.

    Anyway, I just got an idea.
    Why not making separate stock solutions of potassium bromide and silver nitrate? The Silver nitrate keeps very well inside a dark glass bottle. But how about potassium bromide in a gelatin solution, perhaps with some antiseptic additive?

    Everytime one needs some emulsion, the potassium bromide solution could be melted at a given temperature, and then some warm solution of silver nitrate could be mixed in for immediate use. If this is possible, it would shorten the emulsion making time.

    What do you think?

  2. #12
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    It is possible, but the antiseptic additive might affect the outcome. It would have to be tested. At Kodak we kept many stock solutions and premade gelatin for things like this.

    I keep premade gelatin in my darkroom all the time.

    PE

  3. #13

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    mmm by "affect the outcome" do you mean the emulsion charateristics are going to change time by time?

    I'm not thinking of storing pre made gelatin only, but gelatin mixed with potassium bromide in the right amount (in a dark bottle). If this is possible and the antiseptic additive affects only speed or contrast I could live with that... A few test strips at the beginning of a printing session would allow me to calibrate.The only things I'd like to avoid are possibility of pepper grains and fogging of course.

    What kind of antiseptic would you suggest? Would be a small quantity of denaturated alcohol a possible alternative to thymol?

  4. #14
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    Do not use conventional denatured alcohol if it becomes cloudy when you add it to water. This denaturing ingredient will interfere with the emulsion making process. Try to use pure ethyl alcohol if possible with no denaturant.

    Thymol is probably the best because it is not very toxic, easy to work with and readily available. It will change the emulsion slightly though as any organic chemical will so you may have to tinker with the forumla a tiny bit. It may not have any effect if you stay below a given level.

    PE

  5. #15

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    You can keep unwashed emulsions for a long time if refrigerated with no detectable photographic effect. I have some two years old samples in my fridge and when I used them last time they were just like the day I made them. But this requires (1) proper emulsion formula for unwashed use, (2) proper emulsion stabilizer and (3) biocide. The problem with unwashed emulsions is that ripening continues during storage. Overripening leads to fog and loss of speed and contrast. You must stop this ongoing process by using proper stabilizer. Use of formulae that are suitable for unwashed use is also a big help. Usually, chloride emulsions (including chlorobromide and chloroiodobromide emulsions) are quite usable unwashed, if the formula is adjusted properly. Bromide emulsions are in general much slower and doesn't keep as well, if used unwashed. (There are ways to do this but you'll need a ;lot more elaborate setup than washed process.) I'd use benzotriazole as the stabilizer if you don't have access to anything better. Generally, for bromide emulsions, there are much better choices but they are not very easily available unless you have an account with lab chemical supply houses.

    In reality, if you want good, fast emulsion of good contrast, you're better off mastering the washing process. There is no way around it if you want faster emulsions.

    I bet commercial liquid emulsions are all washed, and probably all bromide based. They also most likely include proper emulsion stabilizers and biocide. When made properly like this, the emulsion is quite robust unless you contaminate or expose to light.

    Biocide is necessary because bacteria and fungi can decompose gelatin and physically and chemically change the gelatin. Bacterial growth slows down at refrigeration temp but does not stop it. Fungi are also more resistant to lower temp. I use o-phenylphenolate or 2-phenylphenol sodium salt with good results. I have more info on the wiki section of my website.

    Did you use pepper fog with deionized water? When you mix silver nitrate with tap water containing chlorinating agent, some insoluble silver compounds form (looks cloudy) and they may provide an effective nuclei to grow on, providing a big fog center. I wouldn't use that kind of water. If water is perfectly fine but still get pepper fog, that's probably because of poor quality gelatin. The best is to ditch that gelatin and get a different batch of inert photographic gelatin. Also, too little gelatin (less than 0.5% in the kettle) or ineffective mixing can cause pepper fog, but these are rare based on your formula.

  6. #16

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    Oh by the way, ethanol is ineffective as the biocide in emulsion. The bactericide action of ethanol requires that the ethanol concentration to be 40% or higher. You cant use that kind of amount in emulsions.

  7. #17

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    Questions about stabilizers

    "Overripening leads to fog and loss of speed and contrast. You must stop this ongoing process by using proper stabilizer.

    I bet commercial liquid emulsions are all washed, and probably all bromide based."


    I was looking at the MSDS of Liquid Light and noticed that besides the bromide and chloride, there was trace amounts of Cadmium salts.

    Are the Cadmium salts used for contrast increase, reduction of fog, and would the Cadmum salts act as a stabilizer?

    Just one loud remark: I AM NOT ADVOCATING USING CADMIUM SALTS IN SMALL LAB EMULSION PRODUCTION. They are very toxic and bad for the envirnment.


    rongui

  8. #18
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    Cadmium is used to control curve shape (contrast) in many chloro bromide and bromide emulsions. It is used in trace amounts, and can be safely used this way, but in large operations such as Kodak or Fuji, are an environmental problem.

    Cadmium was removed from all Kodak products in the 60s.

    A bromide formula such as the one above should keep several months in the refrigerator and not go into fog. I have kept one that is very similar for months with no problem and no antifoggant or stabilizer. I did have thymol in it to prevent mold and fungus growth.

    Unless it is finished with sulfur, it will not perform in any suitable way as an enlarging paper but with sulfur, there will be a very large increase in speed and contrast. Once finished with sulfur, the emulsion should be used immediately unless a stabilzer is added.

    Washing is not necessary provided the right formula is used and provided the right final ingredients are added.

    PE

  9. #19
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    Enlarging Speed AgBr Emulsion

    This is the first post of a picture made using my bromide enlarging speed emulsion.

    The emulsion keeps for about 6 months in the refrigerator (with no addenda but Thymol) before sulfur sensitization and the paper itself keeps for about 9 months with little change.

    It is very similar to the one posted above in the OP. It was washed. I have gotten similar results from the same emulsion unwashed, but it requires different addenda and a longer sulfur finish.

    This is a portion of a portrait taken by a friend and given to me. The exposure was 12" at f11/f16 and the developer was Dektol 1:1 for 1 minute. The print was stopped, fixed and washed.

    The support is Strathmore Smooth.

    PE
    Attached Thumbnails Attached Thumbnails print portion of portrait.jpg  

  10. #20

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    thanks everyone for the informations...

    I made a very small batch this evening (50 ml) using separate solutions and adding denaturate alcohol to the gelatin/potassium bromide solution. Unfortunately that wasn't a good idea: while drying on the paper, the emulsion darkened and printed deep brown. Even a second unexposed sheet turned brown when placed in the developer. My fault - I add plenty of denaturated alcohol at home and wanted to try that. I use the same alcohol for a POP emulsion based on chloride salts and got no problem there.

    Tomorrow I'll buy some pure ethyl alcohol. I never heard of thymol, perhaps is popular in the US. I made some research and in Italy is used mainly to fight a kind of parasite in bee colonies. Dunno where to find it.

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