An emulsion preparation session

Purpose

To carry one make from the balance to a coated sheet set to dry, at the rates and temperatures the formula gives, so that the sheet can be compared with the last one you made.

When to use it

Any plain-silver make — a chloride, a chlorobromide or a bromo-iodide — coated on paper or on glass.

It assumes a make with no ammonia in it: an ammoniacal make is a different operation with its own prohibition and is not run here. It also assumes no hardener, which is this course’s default; the consequence is a tender wet layer, and the coating lesson holds that ruling.

Before you start

This is Level B work, for one reason only: silver nitrate, weighed dry and handled in solution. Splash goggles, nitrile gloves, apron and closed shoes for both sessions, with eyewash and spill provision within reach, and the silver nitrate procedure read first. Never in a room where there is ammonia — the incompatibility matrix has the pairing. Silver halide on skin darkens in daylight and takes days to wear off.

Two hazards here are physical: a hot bath running for the better part of an hour, twice, and several litres of ice water on a wet floor.

On the bench: the coating station; a dedicated ricer, straining bag and insulated flask, none of them used for anything else; crushed ice and eight to twelve litres of cold water; a sealable bag and a lightproof box that fits the refrigerator; distilled water; the mesh filter; labels; the batch record. Confirm today’s session is inside the safe time from the safelight test.

Procedure

The make

1. Weigh in the light, before anything light-sensitive exists: the halide, the silver nitrate and both gelatins, into labelled containers. Grind hard halide pellets to a powder in a dedicated mortar — an undissolved fragment gives a black slug on the finished sheet.
2. Dissolve the halide fully in distilled water and hold the vessel up to the light to check it.
3. Sprinkle the first gelatin onto that solution, cover it, and let it bloom for the stated time.
4. Stand the covered vessel in the bath at the make temperature and hold it there, checking that the bath has come back up after the cold vessel went in.
5. Bloom the second gelatin in its own water at the same time.
6. Dissolve the silver nitrate in distilled water and stand it in a small, very hot bath immediately before use: warming it cuts down pepper.
7. Under the safelight, with the salted gelatin at temperature and stirring steadily and consistently, add the silver solution at the published rate, and keep stirring to the end of the stated time. The rate is not a convenience: it is what decides growth against nucleation, and halving it makes a different emulsion.
8. Leave the emulsion in the bath for the stated hold, not stirring.
9. Stir in the second gelatin now, before the set and not after the wash.
10. Pour into the bag with as little air as you can, seal it, put it in the lightproof box and refrigerate until it is firm — a few hours to a few days.

The wash

11. Under the safelight, press the cold set emulsion through the ricer straight into ice water inside the straining bag, and tie the bag’s neck above the rim of the flask. Noodles no bigger than a pea.
12. Dunk it for the stated time, squeeze gently, lift it out, and pour the water into the collected waste — never a drain.
13. Refill and repeat for the published number of changes. Cold, small, changed rather than stirred, and dark: the four things at once.
14. Squeeze the bag firmly until little water comes off, pat it dry, and scrape the noodles into the ripening container.

Ripening, finals and coating

15. Add the finals the formula gives and hold at the stated temperature for the stated time without stirring, or until the emulsion itself reaches that temperature.
16. Cool in cool water, stirring very slowly, to the coating temperature the formula gives, and strain through the mesh filter into the coating vessel.
17. Coat at the volume per sheet your comparison depends on, measured rather than judged: a coating weight that moves between batches invalidates every comparison drawn from them.
18. Set the coats undisturbed, then dry them in the dark, in still, dust-free air.
19. Label the refrigerated remainder — what it is, the batch code, the date, the volume — by the labelling procedure, in a closed box, stored apart from anything anybody eats.
20. Collect every stream. The wash water, the make residues, failed coats and the processing fixer and first wash are all silver-bearing and go to their own container, under the disposal caveat. The wash water alone is the largest volume this part produces.

How you know it worked

• The salted gelatin was clear before the silver went in.
• The addition took the time the formula gives, by the clock, at a steady rate.
• The set emulsion was firm enough to press into noodles rather than paste.
• The noodles stayed cold throughout, and the last change came off much less cloudy than the first.
• The coats are even and the volume per sheet is written down.
• Every gram of silver is accounted for as sheet, remainder or collected waste.

If a step fails

Step 2, a fragment of halide will not dissolve. Do not proceed: filter it out or start again. Grinding is what step 1 is for.

Step 4, the bath overshoots the make temperature. Bring it back before the silver goes in. Above the formula’s figure the make ripens faster than the formula assumes, and that is not recoverable.

Step 7, the addition runs fast or the stirring stops. Note the elapsed time and the gap, and carry on. The batch is still an emulsion and it is now a different one, so it gets a new code rather than the formula’s.

Step 10, the emulsion has not set firm. Give it longer, cold. An emulsion shredded warm turns to paste in the ricer and cannot be washed; there is no way to compress this make into one session.

Step 12, the noodles are softening. The water has warmed. Drain and recharge with fresh ice before continuing: warm noodles are a lost batch, not a slow one.

Step 16, 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 17, the coats are uneven, mottled or repel in patches. Three faults, three causes; the atlas separates them, starting at uneven coating thickness.

Step 18, a wet coat is touched or squeegeed. Unhardened gelatin is tender: the mark is permanent, and if the coat frills in processing, fix in an acid hardening fixer.

What to record

The batch record: the code and date; every weighed mass; the water; the bath temperatures and the times actually held; the addition rate measured rather than intended; the number and duration of the wash changes, with the pH after each if you have a meter; the finals; the coating temperature and volume per sheet; and the step-wedge result once dry. The wash curve is worth keeping: almost nobody has published one.