Blistering from an over-hot hypo alum bath

Owned by Sulfide and Sepia Toning, which explains why the hypo-alum bath has to be hot — an alum solution is weakly acid, and acid added to plain thiosulfate precipitates the sulfur the toner works with, faster when it is warmed — and why it must not be hotter, setting the 1928 primer’s 49 to 52 °C working range and its warning of blistering above 54 °C beside the modern sheet’s hard ceiling. It carries Wall’s 1924 rule that this entry also rests on: let a print from a hot bath cool before washing. This entry was built earlier from Kodak T-1a and Kodak’s G-23.

What you see

Small raised bubbles in the emulsion, sometimes a single row where the print lay against the tray, sometimes scattered over the surface. Under a lamp they catch the light; wet, they can often be seen to move under gentle pressure. Some burst on drying and leave a small crater with a torn edge.

Kodak name the fault and its companion in the same clause: prints will blister or stain at higher temperatures. So a print that shows blisters and staining together has one cause, not two.

Likely causes

1. A bath above 49 °C. Kodak’s instruction for T-1a is to heat the tempering water to 49 °C (120 °F) and not to use a higher temperature.
2. Too long at that temperature. Their limit is 20 minutes, against a normal toning time of 12 to 15 minutes depending on the paper.
3. Washing the print while it is still hot. Wall’s 1924 manual is explicit: prints from a hot toning bath should be allowed to cool before washing or blisters are very likely to form.
4. An emulsion already weakened — by prolonged fixing, which Kodak warn expands the paper and lets solution into the base, or by a hot wash.

What is happening: the chemistry and physics

Gelatin softens with heat and with water, and both are at their extreme in this bath. Hypo alum sepia toning is the one procedure in ordinary black-and-white practice that runs at nearly 50 °C, and it runs there because the reaction needs it. That is a deliberate trade: the toner works, and the binder is close to the temperature at which it stops behaving as a solid.

A blister is that trade going wrong. Gas or liquid collects between the softened emulsion and the layer beneath it, and the softened emulsion has lost the strength to hold it flat. On a fibre print the layer beneath is the baryta and the paper; on a resin-coated print it is the polyethylene, which does not absorb, so anything that collects there has nowhere to go.

The Library of Congress make the general statement about the binder in the language of conservation: high humidity causes a gelatin binder to become soft and sticky, making it vulnerable to mechanical damage. A 50 °C bath is the acute version of that.

Kodak’s own 1928 primer gives a second number and a wider consequence: heat the bath above 130 °F (54 °C) and “blistering, staining and non-uniform toning will result” — three faults from one condition, which is why a print that blisters is usually a print that has stained and toned unevenly as well.

Wall’s 1924 manual adds the step that the modern sheet leaves out. Prints from a hot toning bath should be allowed to cool before washing, or blisters are very likely to form. That places at least part of the fault after the toner: a print carried straight from 49 °C into a cold wash meets a temperature step of twenty-five degrees or more while its binder is at its softest.

The course does not have a measured account of what the gas is or where it comes from, and it will not invent one. What is sourced is the condition — above 49 °C, or beyond 20 minutes at it — and the manufacturer’s own instruction not to exceed either.

!The confirming test: a thermometer in the toner, not in the water bath

Measure the temperature of the toning solution itself, in the tray, at the moment the print goes in.

Kodak’s method is a tray standing in a tempered water bath with the water heated to 49 °C. A tray heated directly, or a bath topped up with boiling water, can be a good deal hotter than the water around it and hotter still where the heat enters.

• Above 49 °C: the diagnosis is made, and the fix is a thermometer.
• At or below 49 °C, with blisters anyway: look at duration first, then at the temperature step from the wash, and then at whether the print was over-fixed.

The discriminator against frilling is where the damage is: frilling lifts the emulsion at the edges of the sheet, and blistering raises it in the middle of a face. The two share a mechanism — a softened binder losing its grip — and they do not share a location.

Diagnostic questions

• What did the thermometer read, and where was it? In the toner, not the water bath.
• How long was the print in? Kodak’s normal window is 12 to 15 minutes and their hard limit is 20.
• Was the print thoroughly washed first? Kodak’s own step 2 says so, and a print carrying fixer into a hot bath is carrying a silver solvent into it too.
• Was there sediment? Kodak’s step 3 is a warm rinse and a wipe with a soft sponge to remove sediment, which tells you sediment is expected — and a print wiped hot is a print handled at its most fragile.
• Was the print allowed to cool before washing? Wall’s instruction, and the one most likely to be skipped.
• Fibre or RC? The layer beneath the emulsion is different, and so is where the trapped material can go.

Corrective action

A blistered print cannot be repaired, and a burst blister is a physical loss.

Handle it as little as possible while wet and warm. The emulsion is at its weakest exactly when the fault is discovered.

Dry it flat and slowly, on the same reasoning as cracked emulsion from flattening: a damaged layer is less able to take a second stress.

Then re-tone a fresh print at the correct temperature, and note that Kodak’s own warning about this toner is that it causes a loss of print density and contrast, compensated by increasing exposure and development when you make the print — so a re-print is not simply another copy of the first.

Prevention

A tempered water bath and a thermometer, which is Kodak’s own arrangement and is the entire prevention.

Bring the print up to temperature before the bath, and let it cool before the wash. Wall’s 1924 instruction is the second half of that sentence and it is the half most often omitted.

Time the bath, and stop at 15 minutes rather than at 20.

Do not over-fix beforehand. Kodak note that prolonged fixing expands the paper and lets solution into the base, which is a weaker starting condition for anything hot.