Simple Silver Gelatin Emulsion

Simple Silver Gelatin Emulsion is a teaching formula for making your own light-sensitive coating from four chemicals: gelatin, silver nitrate, potassium bromide and a trace of potassium iodide. It is the same chemistry that has coated every film and paper since the gelatin dry plate replaced wet collodion in the 1880s, cut down to a 100 ml batch you can make on a kitchen hotplate under a red safelight. Coated on glass, on baryta paper, or on clear film base, it gives a blue-sensitive plate of about ISO 1 to 6, fast enough for pinhole and tripod work, and a slow contact-printing speed suited to exposure under an enlarger.^[1]

The formula is deliberately short. Commercial emulsions go through five stages; this one stops after two and skips the washing step that raises speed. That is why it is slow, and also why it works first time for most people. Once you have made it twice, the rest of this page explains what each extra stage buys you.

When to choose a hand-made emulsion over the alternatives

• Over a bottled liquid emulsion (Liquid Light, Foma Emulsion, Rollei Black Magic). Bottled emulsions are faster and more consistent. Make your own when you want to understand what the bottle contains, when you want a slower speed for pinhole work, or when you want to coat glass plates for a period camera and control the halide ratio yourself.
• Over wet-plate collodion. Wet plate must be coated, sensitized, exposed and developed within about ten minutes, in the field, with an ether-based collodion and a halide salt (historically cadmium bromide; potassium bromide today). A gelatin dry plate is coated at home, dried, and exposed weeks later. The dry plate is the reason the wet plate died.
• Over a washed, ripened workflow. Start here. Move on to washing and digestion (stages 3 and 4 below; a full recipe page is a planned follow-on) when you need ISO 12 to 25, finer grain, or a cleaner shadow tone.

What happens in the beaker

Every silver halide emulsion, from this one to a modern film, is made by the same sequence. The teaching formula performs the first two stages and then coats.

1. Precipitation. Silver nitrate solution is run into a warm gelatin solution that already holds the bromide and iodide. Silver bromide is insoluble, so crystals form the instant the two meet, and the gelatin keeps them apart as a suspension rather than letting them settle. The speed of addition, the temperature, and the stirring decide the crystal size. Five minutes at 45 °C with steady stirring gives a coarse, mixed crop of grains, which is what a slow contact-printing plate wants.^[2]

This formula runs bromide-rich. Five grams of potassium bromide is about 42 millimoles against 29 millimoles of silver, so a third of the bromide is left in solution after all the silver has reacted. Excess bromide is deliberate: it keeps free silver ions out of the emulsion, which would fog it, and it drives the next stage. The 0.1 g of iodide is about 2 mol % of the silver. That small dose of iodide is enough to raise speed and shoulder the curve; more than a few per cent and the emulsion gets slow and hard to fix.

2. Physical ripening. Holding the emulsion at 50 °C for 30 to 60 minutes lets the small crystals dissolve in the excess bromide and redeposit on the large ones. Grains grow, the size range widens, and speed rises, because a larger grain catches more light. Ripen too long and the grains get so large that the plate fogs and contrast falls. Thirty minutes is the safe end for a first batch.

3. Washing (skipped here). The emulsion still contains all the potassium nitrate the reaction made and the bromide that was in excess. In the classic process the emulsion is chilled to a jelly, shredded into noodles or pressed through a coarse sieve, and washed in cold water until the salts are gone, then remelted. Washing is what takes a plate from ISO 1 to 6 up to ISO 12 to 25: the leftover salts crystallize in the dried coating, scatter light, and restrain development. The teaching formula skips washing because it doubles the work and is where most beginners lose the batch.

4. Chemical ripening, or digestion (skipped here). After washing, commercial emulsions are held warm again with a few micrograms of a sulfur compound, usually sodium thiosulfate, so that specks of silver sulfide form on the grain surfaces. Those specks are the sensitivity centres that make a grain developable after fewer photons. Old gelatins did this on their own, because they carried sulfur compounds from the animal; modern photographic gelatin is inert and needs the sulfur added on purpose.^[2] The gain is large, often a further two to four stops.

5. Finals. Just before coating, the maker adds what the coating needs to survive: a hardener such as chrome alum so the gelatin does not frill and dissolve in warm baths, a wetting agent so the emulsion flows level, a preservative so it keeps, and, if wanted, a sensitizing dye. The teaching formula adds only the preservative, thymol in ethanol.

Why it is blue-sensitive, and what changes that

Plain silver bromide absorbs only blue and ultraviolet light. Red, yellow and green pass through without effect, which is why you can coat and handle the plate under a red safelight, and why a landscape on this plate shows a white sky and dark foliage, the look of every nineteenth-century photograph. Adding a dye that adsorbs to the grain surface extends the sensitivity. A few drops of dilute erythrosine make the emulsion orthochromatic, sensitive to green as well; panchromatic dyes extend it to red and would force you to work in total darkness. Without a dye, ask for "blue-sensitive"; "orthochromatic" means a dye was added.

Coating

The emulsion is melted to 35 to 40 °C and poured or brushed onto a warmed support, then set and dried flat in the dark.

• Glass. Clean the plate with a scouring powder or a dilute alkali until water sheets off it without beading, then sub it with a thin gelatin and chrome alum solution, or a sodium silicate wash, so the emulsion grips. Pour a pool in the centre, tilt to each corner, drain the excess from one corner, and set the plate on a cold level slab. About 4 ml covers a 4 × 5 inch plate.
• Paper. Baryta-coated or gelatin-sized art paper works; unsized paper drinks the emulsion and gives a thin, mottled coat. Spread with a glass rod, a "puddle pusher", or a soft brush.
• Film base. Clear polyester or subbed acetate takes the coating like glass but needs a subbing layer; without one the dry emulsion peels.

Dry for 24 hours in a cool cabinet with the door shut. Dust is the enemy at every stage: a speck in the wet coat becomes a comet, a speck on the dry plate becomes a pinhole.

Exposing and processing

Rate the plate at ISO 3 for a first test and bracket two stops each way. The emulsion has strong reciprocity failure, so pinhole and long exposures need more time than a meter suggests. For contact prints, seconds under an enlarger are usual.

Develop in a paper developer such as Dektol (D-72) diluted 1+2 for two to three minutes, or in D-76 stock for four to six minutes, at 20 °C. Use a plain hypo bath or Kodak Fixer F-5 for five minutes; the alum in F-5 hardens the soft gelatin and helps against frilling. Wash glass plates and film base for ten minutes in running water at the same temperature as the other baths. Hand-coated baryta or other fibre paper holds fixer in the paper base as well as in the gelatin, so treat it as a fibre print: a hypo clearing agent bath, then a wash of thirty minutes or more, as described under Archival Washing. Keep every solution within two degrees of the others; an unhardened hand-coated emulsion will reticulate or lift at a temperature jump that a factory film would not notice.^[3] See Temperature Control.

Troubleshooting

• Overall fog. Ripened too hot or too long, a safelight too bright or too close, or silver added too fast so that free silver was left in the gelatin. Cut the ripening to 30 minutes first.
• Frilling and lifting at the edges. No hardener, a dirty or unsubbed support, or a warm bath after a cold one. Add chrome alum to the finals, or fix in a hardening fixer.
• Comets and pinholes. Dust in the coating room, or bubbles from pouring too fast. Let the melted emulsion stand ten minutes to let bubbles rise, and skim.
• Thin, uneven density. The support was cold, the emulsion was too cool to flow, or the coat was too thin. Warm both, and pour more than you think you need.
• Streaks and lumps. Gelatin skin from the surface of the melt, or salts undissolved before precipitation. Filter the melted emulsion through washed muslin before coating.
• Low speed. Normal for an unwashed emulsion. Ripen longer, or move to the washed workflow.

Practical notes

• Cleanliness decides the batch. Any trace of metal, especially copper or iron, fogs the emulsion. Use glass and stainless steel, never aluminium or brass, and rinse everything with distilled water.
• A tested safelight from the moment the silver goes in. Use a photographic safelight with a filter rated for blue-sensitive and orthochromatic materials, at the maker's distance, and run a safelight test on your first coated plate before trusting it. Improvised coloured bulbs, red LEDs of unknown spectrum and phone screens all fog the emulsion. Read Mixing Safety for the silver nitrate handling rules; it stains skin black and burns eyes.
• Weigh the iodide, do not estimate it. At 0.1 g per batch a small error is a large percentage. Make a 1 % stock solution and pipette 10 ml if you have no fine balance.
• Thymol keeps the emulsion, not the plate. The preservative stops the gelatin rotting in the refrigerator for two to six weeks. Coated plates lose speed and gain fog over the same period, so expose them within a few weeks and process promptly.
• Remelt gently. Bring the stored emulsion to 40 °C in a water bath; do not microwave it, and do not remelt more than twice.
• Scale the batch, not the times. Double every ingredient for 200 ml and keep the five-minute addition and the ripening time the same.
• Dispose of silver waste properly. Spent fixer and the wash from a shredded emulsion carry silver; collect them for silver recovery rather than pouring them down the drain.

Related recipes

• Kodak Fixer F-5 — hardening fixer that protects a soft hand-coated emulsion
• Dektol (D-72) — the usual developer for plates and hand-coated paper
• D-76 — gentler negative developer for finer grain on plates
• Van Dyke Brown and Classic Cyanotype — iron-based hand-coated processes that need no silver halide and no safelight
• Gum Bichromate — pigment process often printed from negatives made on hand-coated plates
• Contact Printing — how plates and hand-coated paper are printed