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One of the advantages of powdered chemicals is a long shelf-life. But once you mix it the clock starts ticking. I was wondering if it's possible to buy a larger batch of powdered chemicals and just mix as needed, being very careful to weigh the amount of powder and use the proper amount of water to keep the ratios correct.

Some problems that I might foresee with this are:

  • A single-powder mix might actually consist of different constituents which aren't uniformly distributed in the bag. Could be the case for B+W developer.
  • The correct ratio of water to powder isn't a linear relationship.
  • A multi-powder mix might need different ratios of the powder if it isn't used in full (non-linear relationship between the parts). For example C-41 kits which usually have 3 parts for the developer.
  • My assumption about shelf life is wrong and the powder oxidizes over a limited time frame (say several years).

Is any of this actually the case? Is any specific chemical in the process more prone to it (developer, fixer, bleach)? Or is it a non-issue?

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You are basically on the right track.

If you have a powder mix, as is common for black and white developers, the different substances are not uniformly distributed, but depending on grain size, density and perhaps even grain shape. It may work, but in general, you can't divide such a powder mix and expect the ratio of the different substances to remain the same.

Even dry chemicals may degrade over time. The chemical may react with oxygen or humidity in the air or simply decompose, e.g. due to exposure to light or heat. Dry chemicals usually keep longer if you store them in a dry, cold and dark environment.

The other points you are mentioning are rather non-issues. The ratio between water and dry chemicals is a linear relationship.

  • Really good to know! Perhaps in that case thoroughly mixing up the dry powder before adding a portion of it to water would suffice for getting proper content ratios. I'm glad to have this warning before trying to be economical when I start developing and not knowing why things work out badly. – G_H Feb 7 at 15:08
  • @G_H No, as I said, you won't in general be able to mix the powder enough to split it evenly. For an extreme, but perhaps comprehensible example, consider what happens if you put 50 marbles and two table spoons of sand in a box. Will you be able to shake or mix that, so that the sand distributes evenly enough between the marbles, allowing you to split the content in two and have 25 marbles and a table spoon of sand in each half? – jarnbjo Feb 7 at 17:07
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    "You can't divide such a powder mix and expect the ratio of the different substances to remain the same" - A mistake I made recently. 4 times it worked out just fine, and on the fifth attempt I got some lovely clear negatives (including edge markings) out of it. I've learnt my lesson and I went out and bought a compressible storage container. – John Feb 8 at 0:33
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    @xiota When baking bread, the ingredients stay distributed because of the dough's high viscosity. If you look at just the dry ingredients you will observe the same problem. Whole grains will e.g. not distribute evenly in a bowl of fine flour. For an example with perhaps more relevant particle sizes you can try to mix sugar and cinnamon and see how difficult that is. – jarnbjo Feb 8 at 8:41
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    @xiota I've both baked a lot of bread and mixed a lot of chemicals. Do you think that manufacturers warn against splitting the chemical bags just for fun? – jarnbjo Feb 8 at 8:46
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What amounts are you thinking about mixing? There's a minimum amount you have to mix at once because the development canisters need to be filled to work properly.

As for "uneven" mixing, how do you think the powdered chemicals got into the bags in the first place? Manufacturers do not mix chemicals for each bag separately. They make giant batches that get split up. If "uneven" mixtures were such a big problem, they wouldn't be able to sell you little bags of chemicals in the first place.

Particles with different densities can settle in layers. Just stir them around a bit before use. Otherwise, as long as you have accurate measuring devices, you can mix chemicals within an acceptable margin of error.

  • Are you talking about making the chemical mixtures yourself from scratch, or just diluting the desired ratio of commercial chemicals? – John Feb 8 at 0:43
  • Either way. Diffusion pretty much ensures commercial mixtures will be "even" enough, as long as the granules are evenly sized. (Not like marbles + sand, but if that were the case, I would boycott that manufacturer.) Just stir it around a bit before use. – xiota Feb 8 at 0:56

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