Calotype

Part of Photography

The calotype process creates a paper negative from which unlimited positive prints can be made — the first true negative-positive photographic system.

Why This Matters

Before the calotype, each photograph was a unique object. The daguerreotype produced a single exquisite image on a silver-plated copper sheet, but it could not be copied. The calotype, developed by William Henry Fox Talbot in 1841, changed this permanently. A calotype is a paper negative — a translucent image that can be laid on top of sensitive paper and printed repeatedly, producing dozens or hundreds of identical positive photographs from a single exposure.

This reproducibility is what makes photography genuinely useful for civilization. A single negative of a land survey, a machine design, a medical illustration, or an educational diagram can be printed as many times as needed and distributed throughout a community. The calotype is not the sharpest process — paper fibers produce softer images than glass plates — but it requires no glass, no silver-plated metal, and no mercury, making it accessible wherever you can obtain silver nitrate, potassium iodide, and gallic acid.

For a society rebuilding information infrastructure, the calotype’s ability to multiply images is more valuable than the daguerreotype’s superior resolution. One person makes the negative; the community reads the prints.

Materials Required

The calotype uses three chemical stages, each requiring specific compounds:

Stage 1 — Iodizing the paper:

• Silver nitrate: 60 g per liter of water
• Potassium iodide: 80 g per liter of water
• Smooth, close-grained writing or drawing paper

Stage 2 — Sensitizing:

• Silver nitrate: 100 g per liter of water
• Gallic acid: 25 g per liter of water (or acetic acid — diluted vinegar)

Stage 3 — Development:

• Silver nitrate solution: same as stage 2
• Gallic acid solution: same as stage 2

Fixing:

• Sodium thiosulfate (hypo): 200-300 g per liter of water
• Alternatively: strong hot salt water (sodium chloride, 100+ g per liter)

Paper: Use thick, smooth paper without texture. Watercolor paper works well. Thin newspaper-grade paper tears wet. Rough-textured paper produces grainy, soft images. Try several papers and select the one that holds liquid without buckling and has the finest surface grain visible.

Preparing Calotype Paper

The process has several stages that must proceed in order, with drying between each.

Step 1: First silver nitrate wash

1. Dissolve 60 g silver nitrate in 1 liter distilled water
2. Brush a smooth, even coat onto one side of the paper in ordinary light
3. Brush in one direction only — do not scrub back and forth
4. Allow to dry completely (hang or lay flat on glass)

Step 2: Iodizing

1. Dissolve 80 g potassium iodide in 1 liter water
2. Float the paper face-down on the surface of this solution for 2-3 minutes
3. The silver nitrate in the paper reacts with iodide to form silver iodide, turning the paper pale yellow
4. Lift, blot the back gently, and dry in subdued light
5. The paper is now moderately light-sensitive — work quickly in dim conditions

Step 3: Gallo-nitrate sensitizing (immediately before use) This step must be done just before loading the paper into the camera, as the sensitized paper is highly light-sensitive and degrades within hours.

1. Mix equal volumes of your silver nitrate solution (100 g/L) and gallic acid solution (25 g/L)
2. The mixture turns brown immediately and must be used within 30 minutes
3. Brush evenly over the iodized paper surface under safelight
4. The paper turns from pale yellow to deeper yellow-orange
5. Dry lightly near a gentle heat source — just dry to touch, not baked
6. Load immediately into the camera

Batching

Prepare iodized paper (steps 1-2) in large batches and store in a light-tight box. Apply the gallo-nitrate sensitizing only when ready to use. Iodized paper keeps for weeks; sensitized paper expires within hours.

Exposing and Developing

Exposure: Calotype paper is considerably less sensitive than modern films or even gelatin dry plates. Expect exposures of 1-10 minutes in bright sunlight with a lens camera, or 15-60 minutes with a pinhole camera.

The negative effect is the reverse of the scene: bright sky will appear dark in the negative, dark foreground will appear pale.

Development: The calotype is a physical development process — you continue developing after exposure using the same gallo-nitrate solution.

1. Remove the exposed paper from the camera in total darkness or under deep red safelight
2. Brush the gallo-nitrate sensitizing mixture over the paper surface
3. Lay flat and watch — the image appears gradually over 1-5 minutes
4. The image emerges from nothing to full strength as metallic silver deposits where light struck
5. Stop development by rinsing immediately in clean water when shadow areas show adequate density
6. Rinse for 2 minutes

Fixing:

1. Immerse the developed negative in sodium thiosulfate solution for 5-10 minutes
2. The pale background clears to translucent — you can now hold the paper up to a light and see the image clearly
3. Wash in running water for 20-30 minutes

Waxing the negative (optional but recommended):

1. Gently heat the dried negative paper over a candle or lamp
2. Rub beeswax or paraffin over the warm surface until the wax soaks in
3. Wipe off excess wax
4. The waxed paper becomes translucent, showing finer detail when printing, and is more durable
5. Waxed calotype negatives were used successfully for years

Making Positive Prints

The calotype negative is used to contact print on separate sensitized paper.

Printing paper preparation: Use the same first two stages as the calotype negative (silver nitrate wash + iodize with potassium iodide), but skip the gallo-nitrate step. The simpler silver iodide paper prints well in strong sunlight.

Printing process:

1. In the darkroom, place the printing paper face-up on a flat board
2. Lay the calotype negative on top, face-down (emulsion to emulsion)
3. Cover with a glass plate to press them flat
4. Carry to bright sunlight or position under a bright lamp
5. Expose until the print darkens to a visible, rich tone — watch the edges of the glass to monitor progress. This typically takes 5-30 minutes in direct sunlight
6. Return to darkroom, separate the negative and print
7. Fix the print in sodium thiosulfate, wash, dry

Print-Out vs. Develop-Out Paper

When printing in sunlight, the image forms visible during exposure — this is called printing-out. The image will appear over-dark before fixing because fixing lightens it. Make the print slightly darker than you want the final result.

Comparing Calotype to Other Processes

Feature	Calotype	Daguerreotype	Gelatin Dry Plate
Image support	Paper	Silver-plated copper	Glass
Sharpness	Low (paper grain)	Highest	High
Print copies	Unlimited	None (unique)	Unlimited
Exposure time	Long (minutes)	Moderate	Short (seconds)
Chemical difficulty	Moderate	High (mercury)	Moderate
Portability	Excellent	Moderate	Fragile
Cost	Low	High (silver plate)	Moderate

The calotype’s weakness — paper grain producing soft images — can be reduced significantly by waxing the negative, which makes the paper fibers less visible. Even so, for maximum sharpness, glass-plate negatives are superior. For maximum portability and minimum material cost, paper negatives are unbeatable.

Troubleshooting

Image refuses to appear during development: Exposure was insufficient, or the gallo-nitrate sensitizing was too old. Re-sensitize a new sheet and increase exposure time.

Image appears but is very low in contrast: Gallic acid concentration too low, or development stopped too early. Try developing longer, or increase gallic acid concentration.

Dark overall fog on the negative: Light leak during exposure or handling, or sensitizing solution too old (silver nitrate mixture oxidizes and fogsfilm). Always mix sensitizer fresh.

Negative too thin (pale, low density): Underexposed or underdeveloped. Double exposure time first; if still thin, increase development duration.

Print too light: Negative too thin (see above), or printing exposure too short. In sunlight, try 15-minute increments.

Prints fade after weeks: Fixing was incomplete, or washing was insufficient. Fix for at least 10 minutes and wash for at least 30 minutes in running water. Sodium thiosulfate residue left in the paper causes long-term yellowing and fading.