Albumen layer dissolving during sensitising

Owned by Egg Albumen: A Protein Binder and What It Does to Silver, which names this entry when it treats the float time and the bath temperature as limits rather than guidelines, because on a sheet that has not been separately hardened the silver bath is in a race between coagulating the layer and dissolving it. The sensitising lab sends the same failure here from its troubleshooting table and states the part that catches people out: the fresh bath is the dangerous one. This entry was written earlier from the albumen encyclopaedia entry and Part V’s work on what makes a protein layer stay where it is put.

What you see

Coating that leaves the sheet in the bath. On a single-coated paper the layer clouds the silver solution and thins visibly. On a double-coated paper the failure is at the second coating step rather than at sensitisation: the sheet comes out of the second albumen bath no thicker than it went in.

There is a milder relative worth naming in the same breath, because it is diagnosed at the same moment: white circles on the finished print where air bubbles were trapped under the sheet. Reilly names both faults in the same passage, and both are floating faults.

Likely causes

1. No hardening step between coatings. Reilly’s finding is unambiguous: without one, there is no gain in thickness, because the second coating step dissolved off the albumen remaining from the first.
2. A hardening step that did not work. For the alcohol route, pure alcohol is too strong and unevenly coagulates the albumen layer, while too dilute a solution is not strong enough to coagulate it before it partially dissolves into the water. Seventy per cent is the figure experience settled on.
3. A sheet too dry. Reilly: if the sheets are excessively dry at sensitisation they will not properly absorb the silver nitrate solution.
4. Handling in the bath. Air trapped under the sheet, silver reaching the back, or lifting too fast so that runoff marks the coating.

What is happening: the chemistry and physics

Albumen is a water-soluble protein, and photography is wet. The obvious question — why does an albumen print not simply dissolve in the wash — has a specific answer, and it is the key to the whole entry.

The silver bath is also the hardening bath. Reilly: the reason albumen does not dissolve off the sheet during processing is that contact with silver nitrate in the sensitizing bath coagulates it and forms a new insoluble silver-albumen complex called silver albumenate. And that compound is not merely structural — it is itself light-sensitive and makes an important contribution to image formation, which is why residual silver albumenate is also the cause of highlight yellowing.

So a single coating is protected the moment it is sensitised, and a second coating is not. The second float happens in water, before any silver has been near the sheet, and water is exactly what the first coating is soluble in. Hence the three historical hardening routes, all of which denature the protein by a different means: six months in a warm loft (a slow cure, the commonest nineteenth-century method), a current of steam (cooking it), or 70 per cent isopropyl alcohol (the practical modern one).

The alcohol has a competing requirement, which is why the concentration is narrow. It must coagulate the protein faster than the water in it dissolves the protein. Too strong and the coagulation is uneven; too weak and the dissolution wins.

And there is one detail that looks like fussiness and is chemistry. Reilly instructs that whatever chloride the albumen contains should also be added to the alcohol solution — if the albumen carries 2 per cent ammonium chloride, so should the alcohol — to prevent the chlorides leaching out. The chloride is what becomes silver chloride at sensitisation; lose it and you have hardened the layer and removed the image.

!The confirming test: float a corner and weigh the difference

Cut two strips from a coated sheet. Float one on plain water for the length of the second coating operation, then dry both and compare them by weight and by gloss.

• The floated strip is lighter and duller: the coating is still soluble, and any second coating will dissolve it. Harden first.
• They match: the layer is hardened, and the fault is elsewhere.

Then check the sheet’s condition before the silver bath. Reilly’s remedy for over-dry paper is an overnight rest in a damp place, which also makes the sheets more supple and easier to handle — and his own floating figures are 10 to 12 per cent silver nitrate for two and a half to three minutes, with no additives required in the ordinary course of printing.

And watch the lift. Reilly’s two instructions here are precise and they diagnose two different marks: lifting must be very slow, so that no silver solution reaches the back of the sheet and so that the runoff does not mark the coating; and no air may be trapped under the sheet, because bubbles prevent sensitization where they occur and cause white circles on the face of the finished print.

Diagnostic questions

• Is this a first or a second coating? The whole fault lives at the second.
• Was the first coating hardened, and how? Loft, steam or alcohol, and at what strength.
• What was the alcohol concentration? Seventy per cent, on Reilly’s account of what experience settled.
• Did the alcohol carry the same chloride as the albumen? If not, the chloride has been leaching out.
• How dry was the sheet? Too dry and it will not take the silver up.
• How was it lifted, and was there air underneath? The two floating faults, and both leave signatures.

Corrective action

Harden between coatings, then re-coat. There is no recovering a coating that has gone into the bath.

Use 70 per cent isopropyl alcohol, matched for chloride, and pull the sheets slowly through the tray, hang them to dry, then flatten them under weights so that they are manageable for the second float.

Mark the edge that hung lowest and hang it as the top next time, which is Reilly’s own instruction and which evens out the run-off between the two coatings.

And weigh the benefit honestly. Reilly notes that two coatings give a paper that is quite glossy and may even be so heavily coated that it is brittle and hard to tone, that thicker coatings become increasingly less permeable, making toning and fixing more difficult, and that double-coated papers curl more and are harder to manipulate. Double coating is a choice with costs, not an improvement.

Prevention

Hardening is a step in the workflow, not an optional refinement, and its absence is the whole of this fault.

Keep coated sheets cool and dry — they keep very well — and if they must be rolled, roll them albumen side out, which minimises cracking of the layer.

Rest the sheets before sensitising, so that they are neither wet nor bone dry.

Float carefully. Slowly on, no air underneath, slowly off, nothing on the back.