Brief description

The collodion process (specifically the wet-plate collodion process) is an early photographic technique that uses a liquid mixture of nitrocellulose dissolved in ether and alcohol to coat glass plates. When sensitized with silver nitrate, it allows for high-detail photographic negatives and positives to be captured with relatively short exposure times (seconds instead of minutes).

Use / Function

• High-Detail Imagery: Capturing exceptionally sharp photographic negatives on glass plates, which can then be printed on paper.
• Direct Positive Production (Ambrotypes/Tintypes): Creating durable, immediate positive images on glass (ambrotypes) or dark-lacquered metal sheets (tintypes).
• Cartography and Documentation: Rapid and highly accurate reproduction of maps, blueprints, and documents.
• Scale: Small, highly localized laboratory or field setups, requiring a portable darkroom for immediate processing.

Operating principle

The process relies on using a sticky, viscous vehicle (collodion) to hold light-sensitive silver halides in a uniform layer on a hard surface:

1. Vehicle Preparation: Nitrocellulose is dissolved in a co-solvent of Ether and Alcohol to create a sticky liquid (collodion) containing soluble halide salts (typically bromides and iodides).
2. Plate Coating: This syrupy liquid is poured evenly over a clean Glass plate. As the solvents evaporate, they leave a thin, gelatinous film.
3. Sensitization: The coated plate is submerged in a bath of Silver nitrate (made by dissolving silver in Nitric Acid). This reacts with the halide salts to form light-sensitive silver halides within the collodion layer.
4. Exposure: While still wet, the plate is placed in a light-tight camera and exposed to the scene. The moisture is critical; if the plate dries, the collodion hardens, blocking the developer from reaching the silver halides.
5. Development and Fixing: The plate is immediately developed (using ferrous sulfate or pyrogallol) and fixed (using sodium thiosulfate or a salt solution) to make the image permanent.

How to create it

1. Preparing the Collodion

• Dissolve Nitrocellulose in a 1:1 mixture of Ether and Alcohol.
• Add small amounts of potassium iodide and cadmium bromide. Shake until dissolved, then let the mixture sit for several days until it turns a light straw-yellow color.

2. Preparing the Silver Bath

• Dissolve pure Silver in Nitric Acid to create silver nitrate crystals, then dissolve these in distilled water to form a 7% to 9% solution.
• Acidify slightly with a drop of nitric acid to prevent fogging (haze on the plate).

3. Coating and Sensitizing the Plate

• Thoroughly clean a sheet of Glass with calcium carbonate (chalk) and water.
• Pour the collodion onto the center of the plate, tilting it carefully so it flows to all four corners, then pour the excess back into the bottle.
• In a darkroom (using a red safelight), submerge the sticky plate into the silver bath for 3 minutes.

4. Exposure and Processing

• While wet, place the sensitized plate into a light-tight holder, insert it into a camera, and expose it.
• Return to the darkroom immediately. Pour developer (ferrous sulfate dissolved in water with acetic acid) over the plate. Image development will occur within 10 to 30 seconds.
• Wash with water, then submerge the plate in a fixing bath (sodium thiosulfate solution) until the unexposed yellow-white silver halides dissolve, leaving a clear negative image.
• Dry slowly and varnish with a mixture of sandarac resin, alcohol, and lavender oil to protect the fragile collodion emulsion.

Materials needed

• Emulsion Vehicle: Nitrocellulose, Ether, and Alcohol.
• Sensitizing Agent: Silver and Nitric Acid (to synthesize silver nitrate).
• Substrate: Flat Glass plates (or dark iron plates for tintypes).
• Developer: Ferrous sulfate or gallic acid, and acetic acid (vinegar).
• Fixer: Sodium thiosulfate or high-purity Salt.

Variants and improvements

• Ambrotype: A wet-plate negative backed by black paper, velvet, or black varnish, making it appear as a beautiful positive.
• Tintype (Ferrotype): Substituting the glass plate with a thin sheet of iron coated with dark protective lacquer. This made photographs incredibly durable, cheap, and easy to produce for travelers.
• Dry Collodion: Adding substances like honey, beer, or gelatin to the collodion to keep the plate porous when dry. This allowed plates to be prepared beforehand, though at the cost of significantly longer exposure times.

Limits and risks

• The Wet Constraint: The entire process—from coating to sensitizing, exposing, and developing—must be completed within about 10 to 15 minutes before the plate dries out. This requires photographers to carry portable darkroom tents and chemical chests wherever they go.
• Extreme Fire Hazard: Collodion contains ether and alcohol, which are highly volatile and easily ignited by a spark or candle.
• Toxicity: Silver nitrate will permanently stain skin black and cause severe eye injury. Nitrogen oxide fumes from acid work are highly toxic.
• Fragility: Glass plate negatives are highly fragile and can crack or shatter easily.