Historical Context

Invented by Frederick Scott Archer in 1851, wet plate collodion dominated photography from the 1850s through the 1880s. The process produces negatives or direct positive images (ambrotypes on glass, tintypes on metal) with extraordinary detail and tonal range. The "wet" designation refers to the requirement that the plate must be exposed and processed while the collodion remains moist.

Chemistry

Salted Collodion

Almost everyone buys ready-made collodion (pyroxylin dissolved in ether and alcohol, which is highly flammable) and a ready-made iodiser rather than mixing from gun cotton. Bostick & Sullivan's kit salts USP collodion with their "Old Workhorse" bromo-iodizer, which contains cadmium bromide, ammonium bromide and potassium iodide in a 50/50 alcohol and ether solution:

Collodion, USP	Bromo-iodizer
25 ml	40 ml
50 ml	80 ml
125 ml	200 ml
250 ml	400 ml

For glass plates, 3-5 ml of amino silane subbing per 100 ml of salted collodion improves adhesion. Freshly mixed kit collodion is ready to use at once. As salted collodion ages it darkens from light yellow towards burgundy red, and exposure times and contrast increase. Store it in amber glass, tightly capped (it absorbs water from the air), below 38 °C.

Silver Bath

Component	Amount
Silver nitrate	45-50 g
Distilled water	500 ml

A fresh bath is about 10% silver nitrate (specific gravity 1.08). Bostick & Sullivan add 16 drops of 7% nitric acid to a litre of bath; a new bath typically reads pH 3 to 5, and they aim to keep it around pH 4 to help prevent fog. Check the pH after every 30-40 8 x 10 in plates. Filter the bath regularly and add silver nitrate crystals if the specific gravity falls below 1.04 (stop at 1.06 to 1.07). In the UK, nitric acid above 3% w/w is a regulated explosives precursor: members of the public need an EPP licence to buy or possess it.

Developer

Component	Amount
Ferrous sulfate developer concentrate (ready-made)	1 part
Water (tintypes and ambrotypes)	3 parts

For glass negatives dilute 1 part concentrate with 5 parts water (Bostick & Sullivan). The traditional developer is acidified ferrous sulfate in water with alcohol; exact quantities differ between workers, so use a published recipe or kit rather than guessing.

Fixer

Component	Amount
Rapid fixer concentrate (ammonium thiosulfate)	250 ml
Water	750 ml

This is the 1:3 dilution Bostick & Sullivan supply; 1 litre fixes roughly 75-100 plates of 8 x 10 in. Sodium thiosulfate also works. Potassium cyanide fixer, used historically, is not recommended: it can kill in small doses and releases hydrogen cyanide gas on contact with acid (see safety notes).

Procedure

Clean plate: Glass must be absolutely clean. Clean with whiting or rottenstone, then rinse with alcohol.
Pour collodion: Collodion is not light-sensitive until it has been in the silver bath, so it can be poured in normal light, well away from any flame. Pour salted collodion onto plate, tilting to coat evenly. Allow excess to drain back into bottle. Coating should appear even with no holes or thin spots.
Sensitise: Let the solvents evaporate (Bostick & Sullivan: about 30 seconds for 4 x 5 in, 60 seconds for 8 x 10 in), then, while the collodion is still tacky, lower the plate smoothly into the silver bath. Sensitise for 3-5 minutes.
Load and expose: In safelight, remove plate from silver bath, drain briefly, and load into plate holder. Expose immediately: plates lose sensitivity rapidly. In bright sunlight Bostick & Sullivan report about 1-2 seconds at f/8 or f/11.
Develop: Pour developer over the plate in one even sweep. The image appears within seconds. At 1:3, Bostick & Sullivan give 20-30 seconds for tintypes and ambrotypes (40-60 seconds at 20 °C in their summary; longer pulls up shadow detail). Glass negatives: 1:5, exposed about 3 stops more, developed 2 minutes in a tray.
Fix: Wash for 45-60 seconds in steady cold water, then fix until the milky appearance clears. Bostick & Sullivan's sheet gives 4-5 minutes in one place and 5-7 in another for the 1:3 rapid fixer, then a 3-7 minute wash.
Wash: Thorough water wash.
Varnish: Once dry, plates should be varnished to protect the delicate collodion surface. The traditional varnish is gum sandarac in alcohol with oil of lavender, flowed onto a plate warmed over an alcohol lamp. The varnish is highly flammable: pour it away from the flame and keep the wet surface out of the flame while baking it. It takes about a week to harden fully.

Ambrotypes vs Tintypes

Each now has its own guide: ambrotype and tintype.

Ambrotype

Underexposed negative on clear glass
Appears positive when backed with black velvet, paper, or varnish
Glass must be clean and properly prepared
More fragile

Tintype (Ferrotype)

Collodion on black japanned iron plate
Direct positive image
More durable than ambrotype but surface still delicate
Characteristic dark appearance

Working with Wet Plate

The "wet" in wet plate is critical:

Plates must be exposed while still damp
Working time is short and depends on temperature and humidity (the MICA lab puts it at about 10 minutes)
Hot, dry conditions shorten working time dramatically
Plates cannot be stored—each must be prepared immediately before exposure

Troubleshooting

Problem	Likely Cause	Solution
Fogging	Light leak; contaminated silver bath	Check darkroom; test/filter silver bath
Oyster marks	Plate began to dry before development	Work faster after the silver bath; increase humidity
Pinholes	Contamination in collodion	Filter collodion; clean pour bottle
Low sensitivity	Exhausted silver bath; old collodion	Keep bath specific gravity between 1.04 and 1.08; use fresh collodion

Safety

Chemical	Hazards
Diethyl etherDanger	Extremely flammable liquid and vapour.; Harmful if swallowed.; May cause drowsiness or dizziness.; May form explosive peroxides.; Repeated exposure may cause skin dryness or cracking.
Silver nitrateDanger	May intensify fire; oxidiser.; Causes severe skin burns and eye damage.; Very toxic to aquatic life with long lasting effects.
Nitric acidDanger	May intensify fire; oxidiser.; Fatal if inhaled.; Causes severe skin burns and eye damage.; Corrosive to the respiratory tract.
Potassium cyanideDanger	Fatal if inhaled.; Fatal in contact with skin.; Fatal if swallowed.; Very toxic to aquatic life.; Very toxic to aquatic life with long lasting effects.; Contact with acids liberates very toxic gas.
Cadmium bromideWarning	Harmful if inhaled.; Harmful in contact with skin.; Harmful if swallowed.; Very toxic to aquatic life.; Very toxic to aquatic life with long lasting effects.

Classifications from the European Chemicals Agency (EU harmonised). Read the safety data sheet that comes with your chemicals, and dispose of them in line with local regulations.

Chemical	Hazard	Precautions
Diethyl ether	Extremely flammable; vapour is heavier than air and can reach a distant flame; can form explosive peroxides in storage; harmful if swallowed; causes drowsiness; repeated exposure may dry and crack skin	No flames, pilot lights or sparks; excellent ventilation; store sealed, cool and dated; fire extinguisher nearby
Collodion	Extremely flammable (boils at about 35°C)	Same precautions as ether; store below 38°C
Silver nitrate	Causes severe skin burns and eye damage; oxidiser; very toxic to aquatic life; stains	Gloves and eye protection essential
Ferrous sulfate	Harmful if swallowed; skin and eye irritant	Gloves; keep away from children
Glacial acetic acid	Flammable; causes severe skin burns and eye damage	Gloves and eye protection; ventilation
Nitric acid	Corrosive; oxidiser; fumes are corrosive to the respiratory tract and extremely irritating to eyes and lungs (fatal if inhaled when concentrated)	Gloves, eye protection and ventilation; UK EPP licence needed above 3% w/w
Potassium cyanide	Fatal if swallowed, inhaled or in contact with skin; releases deadly hydrogen cyanide gas with any acid, including the acetic acid in the developer	Do not use; use a thiosulfate fixer. UK regulated poison (EPP licence needed)
Cadmium bromide	Harmful if swallowed, inhaled or in contact with skin; very toxic to aquatic life (EU harmonised classification, which does not list it as a carcinogen; a few notifiers do)	Gloves; avoid inhalation and skin contact; read the SDS for your kit

Fire Warning

Ether and collodion are extremely flammable, and ether vapour is heavier than air, so it can collect at floor level and reach a flame or pilot light some distance away. Never pour or mix near open flames, sparks, or heat sources. Keep the alcohol lamp used for varnishing well away from open collodion and ether. Have appropriate fire extinguisher nearby. Work in well-ventilated areas to prevent ether accumulation. Ether that has been stored for a long time can form explosive peroxides; do not use old, opened ether.

Cyanide Warning

Potassium cyanide was used historically as a wet plate fixer. It is acutely lethal: it is fatal if swallowed, inhaled as dust, or absorbed through the skin, and a small amount is enough to kill. Contact with any acid, including the acetic acid in wet plate developer, releases hydrogen cyanide gas. Do not use it. Sodium or ammonium thiosulfate fixers work well for tintypes, ambrotypes and negatives. In the UK, metal cyanides are regulated poisons under the Poisons Act 1972: members of the public need an EPP licence to buy or possess them.

Sources

Wet Plate Collodion Kit instructions, Bostick & Sullivan
International Chemical Safety Card 0671: Potassium cyanide, IPCS / WHO-ILO
International Chemical Safety Card 0355: Diethyl ether, IPCS / WHO-ILO
ECHA CHEM substance record: Silver nitrate, European Chemicals Agency
ECHA CHEM substance record: Cadmium bromide, European Chemicals Agency (via the Silverlog chemicals database)
Supplying explosives precursors and poison substances: list of chemicals, Home Office (GOV.UK)
Collodion Wet Plates, MICA Graduate Photo Lab

This guide combines established practice with community experience. Always follow safety precautions and cross-reference with primary sources before mixing chemicals.

Wet Plate Collodion

At a glance

Image characteristics

Key chemicals