Uneven coating thickness

Lifted from Break/Fix — emulsion coating defects, entry 5 of twelve.

What you see

One region of the sheet consistently heavier than another, with no edge between them. A heavier bottom, a heavier corner, or a wedge running across the sheet.

The absence of an edge is what puts this in the gradient family rather than in the line-and-ridge family. A stop-and-restart line has an edge you can put a fingernail on; a gradient does not.

On the print, a heavier region gives more density for the same exposure and, on a hand-coated paper, a different maximum black.

Likely causes

Three, and they are separable.

1. The bed is not level. The wet layer flows downhill before it sets.
2. The emulsion is too warm. It stays fluid long enough to flow even on a level bed.
3. The viscosity is wrong — too little gelatin for the finished volume, so the layer is thin and will not hold the thickness it was laid at.

What is happening: the chemistry and physics

A freshly laid layer is a fluid film with a free surface, and it goes where gravity and its own surface tension take it until gelation stops it. Setting is the formation of hydrogen bonds between gelatin chains as the temperature falls, so the time available for flow is set by the temperature and the time available is what decides the gradient.

Coat at the top of the temperature window and the layer is fluid for longer; coat cool and it is fixed in place sooner but drags into ridges behind the rod. The window between the two is the working range, and it is narrow.

Viscosity is the third lever, and it is the one that is usually misdiagnosed as the first two. Noodle washing dilutes an emulsion — the gelatin concentration can fall to nearly half — so a melt made to a recipe’s figure may not be at the recipe’s figure by the time it reaches the jug.

Diagnostic questions

• Watch the plate clear in the fixer, and time it. A plate clears last where it is thickest, so the pattern of clearing is a thickness map you get for free. A plate that cleared evenly was coated evenly, whatever else is wrong with it.
• Is the direction the same on every sheet, or different on each? The same direction every time means the bed. Different directions mean temperature or the stroke.
• What was the temperature in the emulsion, not in the water bath?
• Measure the finished volume and compute the gelatin concentration. Do not inherit the recipe’s figure; the wash changed it. A coating solution needs its gelatin not to drop appreciably below 5 per cent.

Corrective action

Re-run the puddle test on the bed — a water puddle finds a slope that a spirit level on a warped board will not.

Coat at the top of your temperature window rather than above it.

For a thin, streaky coat specifically, the published remedy is to thicken the melt by adding 2 to 3 g of gelatin at a time, remelting and testing a small coat between each addition. Add it in steps, because you cannot take it out again.

Prevention

Level with a water puddle, not a spirit level. Keep the thermometer in the emulsion, not in the bath. Record the coating temperature in the coating record every time, so that a gradient can be correlated with a number rather than with a memory.

Measure the finished volume of every melt rather than inheriting it from the recipe, and compute the gelatin concentration from what you measured.