Blisters

Owned by the emulsion under stress.

What you see

While wet: small raised bubbles in the emulsion layer, between the emulsion and its support, that move if you press them.

When dry, Kodak’s 1928 primer gives the description to match against, and it is the one worth memorising because the dry appearance is what most people meet: “tiny crater-like depressions when examined by reflected light”. The gas has gone, the lifted gelatin has collapsed, and what is left is a dimpled surface that catches the light at a grazing angle.

They are usually many rather than one, distributed over the sheet without regard to the picture, and they are in the emulsion rather than on it.

Likely causes

1. Carbon dioxide from the developer’s carbonate meeting an acid bath, which is the principal mechanism.
2. A fixing bath with an excess of acid, together with films that were not rinsed sufficiently.
3. A strongly acid rinse bath between developer and fixer.
4. A layer that was too soft to resist the gas — from an over-alkaline developer, from warm working, or from a bath that was not hardening as it should.

What is happening: the chemistry and physics

Carbonate plus acid gives carbon dioxide. That is ordinary chemistry, and it happens wherever the two meet — including inside a swollen gelatin layer that has just come out of a carbonate developer and is carrying that developer in its pores.

Gas evolved inside a soft gel has nowhere to go quickly. It accumulates, and if the gelatin is too weak to resist it, it lifts the layer. The 1928 primer states exactly that: “carbon dioxide gas is evolved which produces blisters if the gelatin is too soft to withstand the disruptive action of the gas.”

Two conditions therefore have to be met together — gas produced, and a layer soft enough to lift — which is why the primer’s own list of contributing factors includes hot weather and a bath that is not hardening adequately, both of which act on the second condition rather than the first.

⚗A manufacturer staked a product on this mechanism

Kodak Limited’s 1944 formulary introduces Kodalk, an alkali “intermediate in activity between sodium carbonate and borax”, and sells it on precisely this fault: films developed in a Kodalk developer “will not blister when placed in an acid fixing bath, even at high temperatures”, because Kodalk “does not evolve carbon dioxide on acidifying” — with the further benefit of “less tendency to precipitate aluminium sulphite sludge from fixing baths containing alum”.

Two Kodak publications, sixteen years apart, one describing the mechanism and one marketing a substitution that removes the reactant. That is about as strong a confirmation as the historical literature offers.

!Where the sources put the gas, and where they do not

The mechanism above is well supported. One detail of the way it is usually retold is not.

The account you commonly meet places the evolution at a strongly acid stop bath. Both Kodak sources place it at the acid fixing bath; the 1928 primer names “a strongly acid rinse bath” as an additional condition rather than the principal one.

The chemistry does not care which acid bath it is. But the conditions the sources give — an over-acid fixer, and insufficient rinsing before it — point at the fixer, and so does the prevention.

Diagnostic questions

• Look at the dry material at a grazing angle under a strong light. Craters that catch the light are blisters. A flat mark in the plane of the layer is something else.
• Are they distributed without regard to the picture? They should be. Anything that follows the image is a development or flow fault.
• What alkali is in your developer? A carbonate developer supplies the reactant; a borax or metaborate developer supplies far less of it.
• What is between developer and fixer? No rinse at all is the condition the primer names.
• How acid is the fixer? An acid fixer past its intended pH, or a hardening fixer over-acidified, is the other named condition.
• Was the session warm, and were the baths at different temperatures? Both soften the layer. ILFORD ask for all process solutions within 5 °C of the developer.
• Is it a hand-coated layer? Then the substratum and the hardening are the variables, and Abney’s 1885 account of the substratum’s role in preventing frilling and blistering is the place to start.

Corrective action

None. The gelatin has been physically displaced and has dried in its new shape. A blistered negative is a blistered negative.

The batch is worth saving: stop, correct the conditions below, and process the rest.

Prevention

Rinse or use a stop bath between developer and fixer. ILFORD publish ILFOSTOP at 1+19 for 10 seconds, and describe its function as stopping development immediately and reducing carry-over of excess developer into the fixer — which is exactly the carry-over this fault needs. A plain water rinse is better than nothing.

Keep the fixer at the acidity it is meant to have. An over-acid bath is the primer’s first named condition.

Keep the baths close in temperature and the session cool. Both Kodak sources note that the trouble is more likely in hot weather.

Where it persists, change the alkali. That is what Kodak’s own answer was: a developer whose accelerator does not evolve carbon dioxide on acidifying cannot produce this fault at all.