Tag Archives: Silver bromide

Reliable, Total and Economical Coating and Sensitisation of Wet Collodion Glass Plates

Variously described as “historical”, “ethereal”, “otherworldly”, “alchemical”, the particular aesthetic of the wet plate continues to find use by contemporary photographers such as Sally Mann and Ben Cauchi.

The method of coating an even film of collodion on glass plates is quite an acquired skill, and by all accounts an art-form in itself: far from an exact science. Due to the necessity of keeping the plate wet to maintain sensitivity, the process was never industrialised, and each photographer produced and processed his own plates. Recently I have been meditating on the possibility of a more efficient, reliable, reproducible method of coating, sensitizing, and processing wet plates, requiring no particular skill and minimal wastage.

The method I eventually hit upon is derived from, of all things, the electronics industry. (consequently, I doubt that I would be able to patent, or otherwise claim the process) Mass-produced integrated circuits are manufactured upon silicon wafers, using a photography-related processes, called photolithography (which actually more closely resembles etching). This process necessitates the even and repeatable coating of the wafers with a photo-resist. The industrial method for accomplishing this is one called spin-coating, where the circular wafer is held level, and rotated (about the vertical axis) at a constant speed, while the liquefied emulsion is applied at the centre. So long as the rotation speed and the viscosity of the emulsion are controlled, the results are reliably consistent. The exact thickness of the emulsion is controllable, and a precise amount of emulsion maybe used.

It should be obvious how this spin-coating method might be applied to the coating, sensitisation, and processing of wet plates, to increase precision, reliability and efficiency. The apparatus need not be particularly sophisticated: reliable, repeatable results could be obtained with the re-purposed turntable, and a burette or pipettes.

But pause a moment, and consider: The traditional (hand-coated) wet-plate process itself has a particularly physical and performative quality, which lends the images an imperfect “contingency”(as Benjamin might say), and a hand-crafted kind of cachet. This leads me to wonder – having devised a method that removes much of the chance and skill from the wet-plate process – if it’s utilisation might destroy the very qualities of the process that make it interesting and desirable. Would a cleaner, more consistent, more efficient method render results that are too, in a word, industrial?

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Bonus Emulsion Formulae from “The Book of Photography”

“The Book of Photography” (Hasluck, 1907) chapter on “Plates and Films” includes two formulations for a silver (bromide) – gelatine emulsion. Starting with the simplest [formatting and numbering is mine]: ”

  1. gelatine, 30 grains, [dissolved in] water, 1 [fluid] ounce ;
  2. silver nitrate, 175 grs., water, 1/2 oz. ;
  3. potassium bromide, 140 grs., water, 1 oz. ;
  4. gelatine, 240 grs., water 2 oz.

… each of the ingredients [1, 2, 3, 4] dissolved separately[, and then combined ((1+2)+( 3+4))].” There’s also more sensitive (“more rapid”) emulsion given: ”

  1. Nelson’s gelatine No. 1 soluble, 30 grs., water, 1 oz. ;
  2. silver nitrate, 175 grs., water, 1/2 oz. ;
  3. potassium bromide, 130 grs., water, 1 oz. ;
  4. potassium iodide, 5 grs., water 1 oz. ;
  5. hard gelatine, 240 grs., water 2 oz.”

– which, I’d have thought, ought to give a slower emulsion, as AgI is nominally less photosensitive than AgBr. I presume the additional sensitivity has something to do with differing crystal habit.

Update: I’ve cracked it. Mixed AgI + AgBr grains are more sensitive because the iodide:

  1. introduces additional irregularities into the cubic AgBr crystal lattice,
  2. interacts with longer wavelengths of light than AgBr, extending its spectral range,
  3. accelerates crystal growth, and thus accelerates “ripening” of the emulsion.

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