The Principle of Wet Plate Photography (Ambrotype / Tintype)

Coating with Iodized Collodion

Plain collodion is a transparent solution made primarily of nitrocellulose, diethyl ether, and alcohol. When salts such as iodides or bromides are added, it becomes iodized (or salted) collodion.


The process begins by pouring this iodized collodion onto a glass or metal plate. On its own, the collodion is not light-sensitive. Sensitivity arises in the next step, when it reacts with silver nitrate to form light-sensitive silver halides.

*Various recipes exist using different salts, each producing subtle differences in image tone and density.

Sensitizing (Silver Bath)

The collodion-coated plate is then immersed in a silver nitrate solution—a process called sensitizing. During this step, a replacement reaction occurs between the halide ions in the collodion and the silver nitrate, producing light-sensitive silver halides:

• Silver nitrate + Cadmium bromide → Silver bromide + Cadmium nitrate

• Silver nitrate + Potassium iodide → Silver iodide + Potassium nitrate

*Bromides tend to expand tonal range, while iodides increase contrast.

These silver halides (silver bromide and silver iodide) are the photosensitive compounds that record light. The byproducts—such as cadmium nitrate and potassium nitrate—gradually accumulate in the silver bath and can eventually degrade image quality, so the bath must be cleaned or replenished periodically.

Depending on the temperature, the sensitization process takes about 2–4 minutes and forms a thin, light-sensitive layer of silver halides on the plate.

Exposure

Exposure is done in the camera. When light strikes the sensitized plate, photons reduce some of the silver halide crystals to metallic silver atoms. This occurs primarily in the illuminated areas, forming an invisible latent image.To make this latent image visible, development is required.

Development

The developer typically contains ferrous sulfate, acetic acid, and alcohol. It acts as a reducing agent, supplying electrons to the exposed silver ions.Wherever light has struck the plate, metallic silver grains grow and multiply, forming a visible image.

Using a smaller amount of developer causes the reaction to concentrate in the highlights, resulting in stronger contrast and brighter tones.

Fixing

After development, the unexposed silver halides (those in the shadow areas) must be dissolved away using a fixer. This makes the remaining metallic silver image permanent.
Common fixers include sodium thiosulfate (“hypo”) and, historically, potassium cyanide.

Many 19th-century and some contemporary Western wet plate photographers still use potassium cyanide because it produces a warm, coffee-brown tone and requires a shorter washing time.
(However, cyanide is extremely toxic and must be handled with great care.)

Washing

After fixing, the plate often shows whitish residue—unreacted silver salts—especially near the edges. These should be gently wiped away with a cotton ball to avoid damaging the emulsion.

If sodium thiosulfate is used, the plate should be washed for at least 20 minutes.With potassium cyanide, 5 minutes is usually sufficient.A common rule is to wash for twice the time it takes for the plate to appear clear.

Varnishing

Finally, after thorough washing and drying, the image is sealed with a protective coating—typically gum sandarac varnish or a modern water-based alternative.The varnish preserves the delicate silver surface and ensures that the beauty of the wet plate image endures over time.