RC blisters

Owned by print faults and poor maximum black. The general case, on film and on fibre paper, is blisters, and the mechanism lives there.

What you see

While the print is wet: small raised bubbles, in or under the emulsion, which move if pressed.

When it is dry: small crater-like depressions, best seen by tilting the print against a strong light so that the light grazes the surface. Kodak’s 1928 primer gives that description for dry film and it transfers: “tiny crater-like depressions when examined by reflected light”.

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

Likely causes

1. Carbon dioxide evolved in the layer, where a carbonate-bearing print developer met an acid bath.
2. A fixing bath with an excess of acid, together with a print that was not rinsed sufficiently before it.
3. A soft, over-swollen emulsion, from an over-alkaline developer or from warm working.
4. Drying or glazing outside the published limits — which is a real limit with a published number, and whose connection to blistering the course cannot establish. See the callout below.

What is happening: the chemistry and physics

Carbonate plus acid gives carbon dioxide, and gas evolved inside a swollen gelatin layer has nowhere to go quickly. If the gelatin is too weak to resist it, the layer lifts. Kodak’s 1928 primer states exactly that, and its two named conditions are a fixing bath carrying an excess of acid together with material that was not rinsed sufficiently, or a strongly acid rinse bath.

Kodak Limited confirmed the mechanism by selling a product against it. Their 1944 formulary introduces Kodalk, an alkali “intermediate in activity between sodium carbonate and borax”, on the grounds that a film 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”. Two publications sixteen years apart, one stating the mechanism and one removing the reactant.

What is specific to resin-coated paper is the geometry, and this is the course’s own reasoning. An RC print is a paper core sealed between two polyethylene skins, with the emulsion on top of the upper skin. Gas evolved in or under that emulsion cannot escape downwards through an impermeable layer, and the paper core — which on fibre paper absorbs a great deal — is sealed away from it. That is an inference from the published structure and is marked as one; no source in this course’s corpus states it.

!What the datasheets do and do not say about drying

The manifest for this entry asked for RC blister causes and drying temperature limits from the paper datasheets. The limits exist; the attribution does not.

What ILFORD publish: a hot-air drying limit of 85 °C (185 °F) for machine drying; that at room temperature prints dry in 10 to 20 minutes with no equipment; and that RC papers “should not be glazed/ferrotyped or dried on a drum or flatbed glazer – as this can cause the polyethylene in the paper to stick to the glazing surface”.

What they do not publish: any statement that heat, glazing or fast drying causes a blister. The glazing warning names sticking, which is a different failure.

So the course’s position is split. The gas mechanism above is well supported and is stated as established. The popular attribution of RC blisters to drying heat is not supported by anything in this course’s corpus, and this entry will not assert it. Respect the published drying limit because it is a published limit; do not treat it as the diagnosis.

If a Tier 1 or Tier 2 source establishing a thermal mechanism is found, this entry is where the correction belongs.

Diagnostic questions

• Look at the dry print 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.
• Were the bubbles there while the print was wet? A bubble present in the fixer or the wash points at the gas mechanism; one that appeared only during drying is the case the course cannot attribute.
• Are they distributed without regard to the picture? They should be.
• What alkali is in the print developer? A carbonate developer supplies the reactant.
• Was there a stop bath or a rinse between developer and fixer? No rinse at all is the condition Kodak names.
• How acid is the fixer? An over-acid bath is the other named condition.
• Was the session warm, and were the baths within a few degrees of one another?
• How was it dried, and at what temperature? Against ILFORD’s 85 °C machine limit and their warning against a drum or flatbed glazer.

Corrective action

None. The gelatin has been displaced and has dried in its new shape.

The rest of the session is worth saving: correct the conditions below and print again.

Prevention

Use a stop bath or at least a rinse between developer and fixer. ILFORD publish ILFOSTOP at 1+19 for 10 seconds and describe its function as stopping development immediately, reducing staining and extending the fixer’s life — and it is the same step that removes the carry-over this fault needs.

Keep the fixer at the acidity it is meant to have, and within its capacity.

Keep the session and the baths from running warm, and within a few degrees of one another. Both Kodak sources note that the trouble is more likely in hot weather.

Dry within the published limits. Room temperature and 10 to 20 minutes needs no equipment at all; a machine dryer for RC papers is the maker’s preferred route; 85 °C is the published ceiling for hot-air drying, and a drum or flatbed glazer is ruled out for a different reason — the polyethylene sticks.

And where it persists, consider the alkali. Kodak’s own answer was a developer whose accelerator does not evolve carbon dioxide on acidifying, which removes the reactant rather than managing the consequence.