Dry Plate Process

Part of Photography

The gelatin dry plate replaced messy wet-plate processes in the 1870s — plates are prepared days in advance, stored dry, and give much shorter exposure times than any earlier photographic method.

Why This Matters

Before the dry plate, the most practical glass-plate photography used the wet collodion process, which required the photographer to coat the plate, expose it in the camera, and develop it — all within about 10 minutes, while the collodion was still wet and sticky. This chained the photographer to a portable darkroom tent, made instantaneous photography nearly impossible, and required extraordinary coordination in the field.

The gelatin dry plate changed everything. Plates coated with gelatin silver bromide emulsion could be prepared weeks in advance, stored in boxes, carried anywhere, exposed at any time, and developed at your convenience. Exposure times dropped from minutes to seconds. The same type of plate taken out in the morning could be developed that evening.

For a rebuilding civilization, the dry plate’s advantages are decisive: you can establish a coating operation in one location, distribute plates to photographers in the field, and collect them for centralized development. One skilled emulsion-maker supports an entire photographic network. The process is more demanding to make than simple salted paper, but once mastered, it is vastly more useful and practical for systematic documentation, surveying, and medical work.

The Gelatin Emulsion

The key to the dry plate is gelatin. Gelatin is a protein derived from boiling animal bones, connective tissue, and skin. It forms a clear gel when warm and solid when cool. For photography, it serves as:

1. A binder that holds silver halide crystals in suspension
2. A protective matrix that prevents crystals from aggregating
3. A permeable layer that allows developer and fixer solutions to penetrate
4. A physical support for the image after processing

Obtaining gelatin: Boil beef or pork bones, hooves, and connective tissue in water for 12-24 hours. The resulting liquid sets to a firm gel on cooling. This crude gelatin contains some impurities but works adequately for photography. For higher-quality results, dissolve the crude gelatin in hot water and filter through fine cloth twice.

Gelatin quality: Better quality gelatin (purer, lower in sulfur-containing amino acids) produces emulsions with lower spontaneous fog and longer shelf life. Bone and hide from young animals typically yield better gelatin than old ones. Experiment with batches from different sources and select the best.

Emulsion Preparation

This is the most technically demanding part of the dry plate process. Temperature control and timing are critical.

Formula:

• Gelatin: 20 g
• Water (distilled): 500 mL
• Potassium bromide: 15 g
• Silver nitrate: 25 g

Preparation (must be done in subdued or safelight conditions):

Step 1: Soak gelatin

1. Soak 20 g gelatin in 300 mL cold water for 10-15 minutes until it swells and softens
2. Warm gently to 50°C to dissolve — do not boil
3. The gelatin should be clear and lump-free

Step 2: Dissolve bromide

1. Dissolve 15 g potassium bromide in 100 mL water

Step 3: Dissolve silver nitrate (in safelight or near-darkness)

1. Dissolve 25 g silver nitrate in 100 mL distilled water

Step 4: Precipitation (dim or safelight conditions)

1. Add the potassium bromide solution slowly to the warm gelatin solution while stirring
2. Then pour the silver nitrate solution slowly into the gelatin-bromide mixture while stirring constantly
3. A milky white-yellow precipitate forms immediately — this is silver bromide suspended in gelatin
4. Continue stirring for 5 minutes

Step 5: Digestion (critical for sensitivity)

1. Keep the emulsion at 50-55°C for 30-60 minutes, stirring occasionally
2. This “ripening” period allows silver bromide crystals to grow slightly, which dramatically increases sensitivity
3. Longer ripening (up to 2 hours) produces higher sensitivity but also more fog
4. For first batches, use 30 minutes and test the results

Step 6: Cooling and washing (optional but improves quality)

1. Cool the emulsion to below 20°C — it solidifies to a gel
2. Chop the gel into small pieces
3. Place in a cloth bag and wash in cold water for 30 minutes, squeezing gently
4. This removes soluble bromide salts that would slow development and cause fog
5. Rewarm the washed gel to 45°C to re-liquefy

Step 7: Final gelatin addition

1. Dissolve another 10 g gelatin in 100 mL warm water
2. Add to the emulsion to increase coating viscosity
3. The emulsion is now ready for coating

Coating the Plates

See the full coating article for detailed technique. In brief:

1. Clean glass plates thoroughly (grease-free)
2. Warm emulsion to 38-40°C in a water bath — fluid but not watery
3. Under safelight: flood each plate, tilt to distribute evenly, drain excess
4. Set plates level; allow gelatin to gel (5-10 minutes)
5. Dry completely in cool, dust-free air for 1-4 hours
6. Stack face-to-face with paper interleaves in light-tight boxes

Properly coated and dried plates are stable for 2-4 weeks stored cool and dark.

Sensitometry: Testing Your Emulsion

Before relying on hand-coated plates for important work, calibrate their sensitivity.

Step test:

1. In daylight, set up a simple scene — a white wall, a gray card, a dark shadow
2. Load a plate and expose it for 1, 2, 4, 8, and 16 seconds by covering different portions with a card for each increment
3. Develop for your standard time
4. Examine the negatives: find the shortest exposure that shows full shadow detail without the highlight area going completely black
5. This is your “correct exposure” for this emulsion batch in this light

Different batches of your emulsion will differ in sensitivity. If the correct exposure changes by a factor of 2 or more between batches, adjust your field exposures accordingly.

Field Use

Loading plates: In the darkroom, slide one plate per holder, emulsion side facing the lens. Close the dark slide. Number each holder so you know which plates are exposed.

Recording exposures: Keep a written log: plate number, location, subject, estimated light level, aperture, exposure time. Without this, you cannot correlate results with conditions and improve.

Return to darkroom: Exposed plates can wait days before development if stored cool and dark. Do not subject exposed but undeveloped plates to heat (above 30°C) — this causes the latent image to regress.

Development: Use pyrogallol developer for maximum quality, or gallic acid developer for simplicity. Standard times: 6-10 minutes at 20°C. See the development article for full procedure.

Advantages of Dry Plate Over Earlier Methods

Factor	Calotype (paper)	Wet Collodion	Gelatin Dry Plate
Plate preparation time	Minutes (fresh each time)	Minutes (fresh each time)	Days in advance
Storage of prepared plates	Hours	Minutes only	Weeks
Field portability	Good	Requires darkroom tent	Excellent
Exposure time	Minutes	15-120 seconds	0.5-10 seconds
Image sharpness	Low (paper grain)	Very high	High
Copies from negative	Yes	Yes	Yes
Chemistry per session	High	High	Low
Skill required per session	Moderate	Very high	Low

Troubleshooting Dry Plate Emulsions

Emulsion too slow (requires very long exposure):

• Digestion time was too short — ripening increases sensitivity
• Gelatin quality poor — try different source
• Solution: extend digestion to 60-90 minutes in next batch

Excessive fog (overall darkening of unexposed areas):

• Digestion too long or temperature too high
• Old or contaminated gelatin
• Light leak during coating or storage
• Solution: reduce digestion time; add potassium bromide 1 g/L as fog restrainer

Emulsion peeling from glass:

• Glass not properly cleaned or subbed
• Gelatin concentration too low
• Over-wet plate — drying was incomplete before storage
• Solution: improve glass preparation; increase gelatin in final batch addition

Uneven coating:

• Emulsion too thick (too viscous) — too much gelatin or too cool
• Plate not level during drying
• Solution: warm emulsion to 40°C; use a leveled coating table

Poor image sharpness:

• Emulsion layer too thick — light scatters sideways
• Solution: reduce gelatin concentration or coating quantity per plate