Myths in B&W Photography #3

[rowland_mowrey]

<p>#3: You should use an acid / alkaline / neutral fix for B&W. It should be Sodium Hypo based / Ammonium Hypo based / Something else.....</p>

<p>Well, all of the above are possible.</p>

<p>Any Sodium or Ammonium Hypo based fixer is usable with B&W films and papers as long as the pH remains between 4.5 and 9.0. However, to have a hardening fix, the pH must be in the lower range, about 4.5 or the hardener will precipitate. At this pH, the fix will have a strong odor of SO2 and Acetic Acid, and will be less stable to decomposition. Wash times will be long due to the fact that a low pH causes gelatin to swell far less than when the gelatin is neutral or acid.</p>

<p>At a pH of 8 - 9, gelatin swell is at its maximum and therefore fix times are shorter and wash times are shorter. Stability is greatest and decomposition of the fix is slow. What you sacrifice is the capacity of the fix unless you use a stop or running water rinse after the developer. And yes, you can use a stop bath before an alkaline fix. This is provided the fix is buffered properly. Usually, you can tell this is so if the fix concentrate has some sediment in it before diluting with water. This is precipitated buffer. This type of fix will gradually release ammonia gas during use and may be a bother to some people.</p>

<p>At a pH of 5 - 7, you make a compromise of all features. Swell is medium, fix rate is medium unless some chemical adjustment is made to the fix and wash times are medium unless something is done to accelerate the wash rate. This is a compromise fix.</p>

<p>Sodium based fixes are slow, but Ammonium based fixes are fast and are the basic foundation of all rapid fixes on the market. Sodium and Ammonium ions are the only two positive ions that should be added to a fix bath. For example, both Mees and Haist cite the fact that Potassium ion will just about poison a fix and Calcium and Magnesium ions will slow a fix down drastically.</p>

<p>For any fix, a manufacturer cannot give you a true wash time or capacity. You should test your fix and wash for suitability using the standard retained hypo test, retained silver test and the hypo exhaustion test. The latter is the most subjective and hard to interpret, but basically once you have this established you should be in a good position.</p>

<p>Ron Mowrey</p>

[bill_clark___minnetonka_mi]

<p>Thanks for the info. It's nice to see this for refresh of the knowlege in my cranium.<br /> I use a two bath fix for film. Two bath works well especially with certain T-grained films.<br>

Don't use two bath fix for prints. Maybe I should?</p>

[Alan Johnson]

"Potassium ion will just about poison a fix"

Bit exaggerated?

I don't see reports of problems with potassium carbonate in FX-2 or Pyrocat HD.

[photojim]

<p>Potassium-based stop baths are deadly problematic; Anchell and Troop write about that. My guess is that a citric or acetic acid stop bath or running water fix get rid of the potassium well enough to avoid the problem.</p>

[rowland_mowrey]

<p>The Potassium used in developers such as KI and other potassium salts, are indeed pretty much removed by the rinse or stop following the developer step. This has been tested.</p>

<p>But, if you should use Potassium Sulfite or Potassium Thiosulfate or Potassium Thiocyanate in the fix itself, you will see a big dropoff in activity as reported by Haist and Mees & James. A&T echo that warning.</p>

<p>Ron Mowrey</p>