Plate coating and drying

Purpose

To take a bed of prepared glass from a melted emulsion to dry, cut, sleeved plates at one measured coating weight, every silver stream collected.

When to use it

Any coating of glass: camera plates, or a lantern slide.

Two things are already done: the emulsion is made, washed and finalled by the emulsion session, and the plates are cut, arrised, cleaned and dried with the subbing decision taken, on the plates lab.

Not paper: that bed belongs to the coating lesson.

Before you start

You should already know this emulsion’s coating window and how to compute a coating weight, from coating, drying and hardening; a lantern slide’s target is six to nine times lighter than a camera plate’s, and the plates lab does that arithmetic.

This is Level B work: cut glass, a bath near coating temperature, and wet silver-bearing gelatin under a safelight. Nitrile gloves, splash goggles, apron and closed shoes, over a tray. Handle plates by the edges alone, in cotton or clean nitrile gloves, with freshly washed hands — a fingerprint is a coating defect you will not see until the fixer. A chrome alum dip used in preparation sends its rinse by the chromium route.

On the bench: the bed glass, plastic wrap, four dam bars, chilled slab, water bath, thermometer, mesh filter, syringe and drying box from the coating station; a round-bowled tablespoon used for nothing else; six prepared plates; a sharp blade; paper sleeves, labels, a lightproof box; the silver-waste container; the batch record. Confirm the session sits inside the safe time from the safelight test.

Procedure

The bed

1. Level the bed glass, then confirm it with a puddle of water: pour one, watch which way it goes, adjust, and repeat until more water poured in makes the puddle grow round rather than run.
2. Moisten the glass, lay plastic wrap over it, tug the worst wrinkles out, and moisten the wrap.
3. Arrange the plates and the four dam bars on the wet wrap: a 1/16 to 1/8 inch (1.5 to 3 mm) gap between every plate and its neighbours, and the bars butted tight against each other around the outside. Six plates, or four. No more.
4. Set the drying box before anything is wet: fabric over the air vent, door shut, and a small radiator holding about 24 °C (75 °F) if the room is cool or damp.

Coating, under the safelight

5. Remelt the emulsion in the bath and bring it to the coating temperature the formula gives, with the thermometer in the emulsion and not in the bath.
6. Strain it through the mesh filter into the pouring vessel.
7. Draw one plate’s measured volume into the syringe — 15 mL for a 4 × 5 inch camera plate at the tested rate — and pour it onto the middle of the plate.
8. Spread it at once with the bottom of the round-bowled spoon held nearly flat, working the puddle out to all four edges and chasing bubbles to where they can break. One pass. Do not rework a plate. A bubble caught between two plates is left alone.
9. Repeat steps 7 and 8 across the bed at the same volume, writing it down as you go.

Set, cut, dry

10. Move the bed, or the plates one by one, onto the chilled slab and leave it undisturbed until the layer has gelled.
11. When the plates are dry or nearly dry, cut the emulsion with a sharp blade along every gap, plate to plate and plate to dam bar. Plates are never pulled apart.
12. Prop each plate up so the smear on its underside can dry, holding it by that underside if the edges are still tacky.
13. Dry in the box, cool and slow, and leave the door shut. Plates take longer than paper.
14. Peel the wrap off the bed with the waste emulsion on it into the silver-bearing solid waste, and wash the bars, spoon and syringe in warm water before the gelatin sets.

Storing

15. When the plates are completely dry, sleeve each in its own paper enclosure, emulsion side marked, labelled with the version code, coating weight and date by the labelling procedure.
16. Stand the sleeved plates vertically on their long edges in a lightproof box marked fragile/glass and heavy, filled out with board so nothing jostles, and store it cool, dry and dark — below 18 °C at 30 to 40 per cent relative humidity is the conservation figure.
17. Refrigerate leftover emulsion by its own procedure, and put every rinse, failed coat and offcut of dried layer into the silver container, under the disposal caveat.

How you know it worked

• The puddle grew round rather than running, before any emulsion was melted.
• Each plate carried the same measured volume, written down.
• Every plate lifted off the bed with an unbroken edge after the blade, and none was pulled.
• No plate is tacky, and the undersides are dry.
• Fix your first plate and watch it clear. Even clearing means a level bed and an even stroke; the last corner to clear is the thickest.
• Emulsion, plates and collected waste account for what was melted.

If a step fails

Step 1, the puddle keeps running. Shim and repeat. Do not coat plates on a bed you have not levelled: paper forgives a slope, glass does not, and the fault reads as an emulsion fault.

Step 3, emulsion escapes past the bars. The wrap was too dry, or the bars were not tight. Re-wet and re-lay before pouring; what escapes is silver-bearing waste.

Step 5, the emulsion is above or below its window. Bring it back before pouring; both faults are described on the coating lesson.

Step 6, the emulsion sets in the filter. It cooled too far. Warm it gently back towards the coating temperature and strain again; do not thin it with water, which changes the coating weight.

Step 8, a ridge runs across the plate. The stroke stopped or went back over a setting layer. The plate is not recoverable; coat two sacrificial plates first. See uneven coating thickness.

Step 8, the layer beads or repels in patches. Grease on the glass — repellency spots. Circular clear spots instead are air bells or dust.

Step 11, the layer tears as plates come apart. They were pulled rather than cut. Cut every gap, plate to dam bar included, before lifting anything.

Step 13, plates are still tacky after a night. Too cool or too humid. Hold the box near 24 °C with airflow through the dust screen, and read mottled coating from humidity. A plate drying slowly in warm damp air is the one that grows mould.

Later, the layer lifts or rolls back in processing. Adhesion — frilling. Fix in an acid hardening fixer and revisit subbing on the lab page, not the emulsion.

What to record

In the batch record, per bed: the plate size, thickness and supplier; the cleaning method, and whether the plates were subbed and with what; the levelling check; the coating temperature measured in the emulsion; the volume per plate and the coating weight computed from it; the plate count; the drying temperature, humidity and hours; and which corner of the first plate fixed cleared last.

On each sleeve: the version code, coating weight and date. A different coating of the same melt takes the next letter; only a different formula takes a new number.