Simple Silver Gelatin Emulsion
A teaching-oriented hand-coated silver-gelatin emulsion for coating onto glass plates, baryta-coated paper, or film base. Produces a low-speed (ISO 1–6) blue-sensitive emulsion suitable for contact printing under enlarger or sun, or for pinhole and large-format plate work. Batch size is 100 ml. Coat under red safelight, warm the substrate, and dry in a dust-free cabinet for 24 hours before exposure.
Contrast is moderate (around grade 2 equivalent). Grain is coarse by modern standards — suitable for contact printing and pinhole / large-format work, not for enlarging small negatives. Tones are warm-neutral; shadows can be extended with longer ripening. Plates must be exposed within a few weeks of coating for best results. The ISO 1–6 ceiling is a consequence of the simplified unwashed workflow — the classical chill-set / shred / wash steps that would remove excess halide (and raise speed into the ISO 12–25 range) are deliberately omitted for teaching.
Mixing Instructions
Work under red safelight from the moment silver nitrate is added onwards. All glassware must be scrupulously clean — any metal contamination ruins the emulsion.
Part A (salted gelatin, 60 ml):
- Soak 10 g photographic gelatin in 40 ml cold distilled water for 15 minutes to swell.
- Warm to 45 °C until fully dissolved; do not boil.
- Dissolve 5 g potassium bromide and 0.1 g potassium iodide in 20 ml distilled water; add to the gelatin solution. Hold at 45 °C.
Part B (silver solution, 40 ml):
- Dissolve 5 g silver nitrate in 40 ml distilled water. Warm to 45 °C.
Precipitation (silver addition):
- Under red safelight, add Part B to Part A slowly over about 5 minutes with constant stirring. The mixture will turn opalescent as silver bromide/iodide crystals form in the gelatin.
Digestion (ripening):
- Hold the combined emulsion at 50 °C for 30–60 minutes to ripen the crystals. Longer ripening → higher speed and grain.
Preservative:
- Prepare a 1 % thymol stock by dissolving 1 g thymol in 100 ml ethanol. Add 2–4 drops of this stock to the finished emulsion.
Setting and coating:
- Cool to 35–40 °C before coating. Pour or brush onto pre-warmed substrate in safelight conditions. Allow to set at room temperature, then dry in a cool dust-free cabinet for 24 hours.
Finished emulsion keeps in a refrigerator for 2–6 weeks (thanks to the thymol); re-melt at 40 °C before each coating session.
Ingredients for 1L of Stock Solution
| # | Chemical | Role | Qty (1L) | Unit | Note |
|---|---|---|---|---|---|
| 1 | Gelatin | Other | 100.0 | g | (Part A gelatin) |
| 2 | Distilled Water | Solvent | 600 | ml | (Part A + salts) |
| 3 | Potassium Bromide | Restrainer | 50.0 | g | (Part A salts) |
| 4 | Potassium Iodide | Other | 1.0 | g | (Part A salts) |
| 5 | Silver Nitrate | Sensitizer | 50.0 | g | (Part B silver) |
| 6 | Distilled Water | Solvent | 400 | ml | (Part B carrier) |
| 7 | Thymol | Other | 20 | drops | (of 1% ethanol stock) |
| 8 | Ethanol | Solvent | 2 | ml | (thymol carrier) |