Camera Obscura

Materials and preparation to reduce negatives

Wikipedia says that hydrochloric acid concentrated more than 37% is a smoking gas, so I don’t think we have to use a gas that surpasses this concentration. Unfortunately this answer arrived after some tests with a hydrochloric acid solution that was less concentrated (0.1N), solution that functioned in any case.

If my chemistry memories are corrects, a solution of 0.1N of hydrochloric acid equals a concentration of 0.1M that is 36,5g of acid per liter of solution. As HCl density is 1,64g/ml, it corresponds to a solution of 0,2% that is 60 times more diluted than the concentrated hydrochloric acid. The proportion tells that 500ml of solution 0.1N contain almost 1ml of pure acid, while 30ml of concentrated acid contain almost 10ml. If all those calculations are corrects, I used a tenth of the hydrochloric acid indicated in the original formula. In any case, the solution seems working.

I put in a blender 300ml of tap water where I added (remember to add acid in water and not vice versa) 500ml of hydrochloric acid 0.1N. I made approximate doses, but as the recipe doesn’t give any advice about the requested concentration, I couldn’t make any equivalence. In any case, as hydrochloric acid is dangerous, I rather handle less concentrated solutions.

I added dichromate potassium and alum potassium to this solution, following the book indications. I made all the operations in daylight and it doesn’t seem to cause any problem

Negatives are Adox films, developed in ID11 B dilution, 1 to 5 minute, with an excessive contrast.

I re-developed using Perceptol diluted 1+4. This development should be like D-23 that is no more on the market (even if, I might say, the formula is reported on The darkroom cookbook).

Observation on the behavior of Eder reducer

Eder harmonizer reducer bleach negatives. I noticed though that the power of the solution rapidly decreases. After a dozen of bleached negatives 24×30cm, it takes more than 10 minute, while the first one took only one minute. This could be due to the fact that I’m using a solution that is less concentrated in hydrochloric acid.

I noticed any yellow stain, as the book said. Bleached negatives have a faint grey-green color and it doesn’t change even after an hour of wash.

The re-development seemed to me complete after 2-5 minutes, depending on negatives. Working with daylight allows an easy estimation of the negative, but the method is very approximate. In any case when working on accurate sensitometry normally you never have to reduce negatives.

Re-developed negatives color is brown and reminds in a certain way films developed with pyrogallol. It made me think that those kinds of negatives would have filtered a lot of UV giving particularly long exposition time. In reality it didn’t happen, so it seems like the coloration occurs in the visible part or that it doesn’t interest the wavelength sensitive to cyanotype or Van Dyke solutions.

A silver-gray layer settles in the re-development basin, more quickly than the one usually settled in the development basins. In any case, it is easy to remove with a little scratch.

---

To read the rest of this article go to page: 1 2 3