Cracked emulsion from flattening a bone-dry fibre print

Owned by Lab: washing, drying, flattening and mounting the print, which reaches this entry’s conclusion from the mechanics of the layer rather than from the conservation literature: dry gelatin behaves as a glass rather than a rubber, so bending strain at the emulsion surface is relieved by fracture instead of deformation, and the rule that follows is to add moisture before you add force. A print that resists is reporting that it is too dry. The environmental half — humidity variation, rolling, an embrittled support — is Part XII’s, and this entry was assembled from it before Part XIX was written.

What you see

A network of fine cracks in the emulsion, usually appearing at the moment a curled or rolled print is pressed or unrolled, and following the direction of the bend. Held at an angle to a lamp they show as a fine craquelure; flat on, they may be almost invisible until the print is tilted.

They are in the binder, not in the paper, which is why they show as broken sheen rather than as a tear.

Likely causes

1. Flattening a print that dried at low humidity. The Library of Congress state the mechanism directly: low RH causes the binder to shrink and crack and the secondary support to curl. The two halves of that sentence are the whole fault — the curl is what makes you want to flatten it, and the shrinkage is what makes flattening dangerous.
2. Drying too fast or too hot. Kodak’s instruction for toned fibre prints is to dry them slowly, between blotters or on a rack, with any dryer set to its lowest speed and temperature.
3. Rolling for storage. The Library of Congress advise against rolling photographs because a print may crack when it is unrolled for use.
4. A print already brittle. An acidic mount or an adverse environment makes paper yellow and brittle, and a brittle support takes the binder with it when it is bent.

What is happening: the chemistry and physics

A print is two materials with different responses to water, bonded together. The gelatin binder swells and shrinks a great deal with humidity; the paper support swells and shrinks by a different amount and mostly across the grain. The American Institute for Conservation’s photographic materials group make the general finding on albumen prints and it applies to any binder on paper: overall cracking of the binder is likely caused by variations in humidity that produced differential dimensional changes between the image layer and the support.

At low humidity the binder is at its most shrunken and its least extensible. Bending it then asks the outer surface of the curve to stretch, and it cannot — so it fails in tension, in a network of cracks at right angles to the strain. The same bend at 50 per cent relative humidity is taken up by a layer that still has some water in it and some give.

This is why the remedy is humidity rather than force, and why it is not a matter of pressing harder.

!The confirming test: humidify a corner before you flatten anything

Do the test on a scrap or on a margin, not on the print.

Put the print in a closed box with a tray of water — not touching it — and leave it overnight, so that it takes up moisture from the air rather than from liquid. Then try the bend.

• It flattens without cracking after humidification and cracked before: the diagnosis is confirmed, and the procedure for the rest of the print is the one you have just run.
• It cracks even humidified: the binder is already damaged or the support is embrittled, and the print should be supported rather than flattened — the Library of Congress recommend stiff card supports beneath brittle or broken photographs.

Never humidify by wetting. High RH takes gelatin the other way: the Library of Congress note that it becomes soft and sticky, and a sticky emulsion pressed under a weight is a print bonded to whatever it was pressed against.

Diagnostic questions

• Was the print rolled, and for how long? A roll sets a curl, and the curl is what invites the fault.
• How was it dried? A hot glazer or a fast dryer leaves a print drier than the room and more curled than it needs to be.
• What is the humidity where the print has been kept? Both extremes cause damage and they cause different damage.
• Is the paper yellowed or brittle? Then the support is part of the problem, and an acidic mount is the likely history.
• Do the cracks follow the direction of the bend? They should. Cracks that do not are probably drying marks or handling damage rather than this.

Corrective action

Nothing repairs a cracked binder in this course. The cracks are physical losses in a layer a few micrometres thick, and there is no assessed treatment here for consolidating them.

Stop flattening. Support the print instead — a stiff card beneath it, in a folder, flat.

Humidify slowly and then dry flat under light weight, for any print that must be flattened. Overnight in a humid chamber, then between clean blotters under a board, then several days.

Then look at the whole environment, because a print that dried this hard has probably been living in a room that is drying everything in it.

Prevention

Dry slowly and flat. Kodak’s own instruction for toned fibre prints — blotters or a rack, and the lowest dryer settings — is the prevention as well as the finish.

Do not roll fibre prints for storage, on the Library of Congress’s own advice, and store flat in appropriate enclosures.

Keep the storage humidity moderate and stable. The instruction that matters is stable: the AIC’s finding attributes the cracking to variations in humidity, not to one value of it.

Avoid acidic mounts, which are a slow route to the same brittleness — and see edge fading from mount and adhesive for what else they do.