Explaining My Wet Plate Collodion Negative Emulsion Chemistry in Detail

I am making collodion dry plates for my two core projects: Pictorial Whispers and America’s Grist Mills. I am making platinum-toned Kallitype contact prints for Pictorial Whispers, and for America’s Grist Mills, I am making Platinum/Palladium contact prints.

These projects are very different, but it turns out that wet plate collodion is a perfect fit for both and perfectly aligns with each project’s narrative. I encourage you to read the artist’s statements for both projects to gain a deeper understanding of why collodion is such a great fit.

In the section below, I will break down each part of my collodion negative chemistry emulsion and explain the double-decomposition process when the salted collodion plates come into contact with the silver nitrate bath to form a light-sensitive emulsion.

This knowledge is important so you understand each component in chemistry and its related purpose and function. This is key to adjusting your formulas and troubleshooting issues when they arise.

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Components of My Collodion Negative Formula:

• Plain USP Collodion: This is a solution of pyroxylin (nitrocellulose) in a mixture of ether and alcohol. It serves as the film-forming agent.
• Everclear (ethanol): Acts as a solvent in the collodion mixture, reducing viscosity and aiding in the even coating of the plate.
• Ethyl Ether: Also a solvent, ether helps in dissolving the nitrocellulose and controls the drying rate of the collodion film.
• Cadmium Bromide (CdBr₂): A halide used in forming light-sensitive silver halides upon reaction with silver nitrate.
• Ammonium Iodide (NH₄I): Another halide which reacts with silver nitrate to form light-sensitive silver iodide.

Double Decomposition Chemistry:

The photographic properties of the collodion dry plate come from the light-sensitive silver halides, which are formed by a double decomposition reaction (also known as metathesis) when the plate coated with halides is dipped into a solution of silver nitrate.

Reaction with Cadmium Bromide:

In this reaction, silver nitrate (AgNO₃) reacts with cadmium bromide to form silver bromide (AgBr) and cadmium nitrate (Cd(NO₃)₂). Silver bromide is a key light-sensitive material that darkens upon exposure to light, forming the latent image.

Reaction with Ammonium Iodide:

Here, silver nitrate reacts with ammonium iodide to produce silver iodide (AgI) and ammonium nitrate (NH₄NO₃). Silver iodide also forms part of the light-sensitive layer and is even more sensitive to light than silver bromide, enhancing the overall sensitivity of the photographic plate.

In the double decomposition reactions you are using to create your collodion dry plate negatives, cadmium nitrate and ammonium nitrate are byproducts formed alongside the light-sensitive silver halides (silver bromide and silver iodide). Here’s how they factor into the overall process:

Double Decomposition Reactions:

Cadmium Bromide and Silver Nitrate:

• Product: Silver bromide (AgBr) is the light-sensitive agent.
• Byproduct: Cadmium nitrate (Cd(NO₃)₂).

Ammonium Iodide and Silver Nitrate:

• Product: Silver iodide (AgI) is the light-sensitive agent.
• Byproduct: Ammonium nitrate (NH₄NO₃).

Role and Impact of Byproducts:

• Cadmium Nitrate (Cd(NO₃)₂): As a soluble compound, cadmium nitrate remains in the collodion layer but does not directly participate in the image formation process. Its presence does not have a significant impact on the photographic qualities of the emulsion. However, it could potentially influence the physical characteristics of the collodion layer, such as its granularity or the drying speed.
• Ammonium Nitrate (NH₄NO₃): Like cadmium nitrate, ammonium nitrate is also soluble and does not contribute to the light sensitivity or image formation. Its presence in the emulsion could affect the viscosity and drying behavior of the collodion film. In some cases, if the concentration is high, it might slightly influence the stability or shelf life of the prepared emulsion due to its hygroscopic nature, which could attract moisture.

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Practical Considerations:

When processing the plates, especially during washing, these byproducts are typically washed away. They do not interfere with the development and fixing processes where the image is made visible and permanent. In historical photographic processes, much attention is paid to the silver halides as they are crucial for the photochemical reactions upon exposure to light, but it’s good practice to understand the role of all chemicals involved to optimize your results.

While cadmium nitrate and ammonium nitrate are produced in the reactions, their impact on the photographic process is mostly inert in terms of image formation. They primarily influence the physical properties of the collodion layer or the processing steps.

Summary of the Photographic Process:

When the collodion, now containing cadmium bromide and ammonium iodide, is applied to a glass plate and exposed to a silver nitrate solution, the silver nitrate reacts with these halides to form the light-sensitive silver halides (AgBr and AgI). These halides are dispersed in the dried collodion layer. Upon exposure to light, these silver halides decompose, and the silver atoms aggregate to form a latent image. The image is then developed, fixed, and made permanent through photographic processing.

Photographic Qualities:

• Cadmium Bromide: Adds to the contrast and spectral sensitivity of the plates. I especially like this for my negative collodion emulsions.
• Ammonium Iodide increases general sensitivity to light, which is particularly useful in lower-light conditions. I like Ammonium iodide in my negative chemistry because it helps age the collodion faster.

I should note that you must understand the safety issues with handling and using any type of chemical and in particular, wet plate collodion has several chemicals that could be harmful or even kill you if used incorrectly.

Safety & Health Risks

Working with the chemicals listed in the collodion dry plate negatives formula involves handling materials that can pose significant health and safety risks if not managed properly.

Here is an overview of the primary concerns associated with each component:

Plain USP Collodion (Nitrocellulose in Ether and Alcohol)

• Flammability: Both ether and alcohol are highly flammable, and their vapors can form explosive mixtures with air. Nitrocellulose is also flammable and can be unstable under certain conditions.
• Health Risks: Inhalation of vapors can irritate the respiratory system and may cause central nervous system effects such as dizziness or headache. Prolonged skin contact can cause irritation.

Everclear (High-Concentration Ethanol)

• Flammability: As a high-proof alcohol, Everclear is extremely flammable and poses significant fire and explosion risks, especially near open flames or sparks.
• Health Risks: Inhalation or ingestion can lead to intoxication, and prolonged exposure can cause dehydration of skin or mucous membranes.

Ethyl Ether

• Flammability and Explosiveness: Ether is highly volatile and flammable, with a tendency to form peroxides, which are explosive. It must be handled with extreme care, away from any sources of ignition.
• Health Risks: Ether vapors are anesthetic and can cause respiratory irritation, dizziness, nausea, and unconsciousness. Chronic exposure can damage the kidneys and liver.

Cadmium Bromide

• Toxicity: Cadmium compounds are highly toxic and carcinogenic. They can cause severe damage to the kidneys, lungs, and bones (itai-itai disease). Handling requires stringent controls to prevent inhalation and ingestion.
• Environmental Hazard: Cadmium is a persistent environmental pollutant that can accumulate in the food chain.

Ammonium Iodide

• Health Risks: While less hazardous than cadmium compounds, ammonium iodide can still cause irritation to the skin, eyes, and respiratory tract if not handled correctly.
• Stability Concerns: It can decompose upon exposure to light, heat, or moisture, potentially releasing iodine and ammonia gases, which are irritants.

Safety Recommendations:

• Proper Ventilation: Always use these chemicals in a well-ventilated area to avoid accumulation of harmful vapors.
• Protective Equipment: Wear appropriate protective gear, including gloves, goggles, and respiratory protection when necessary.
• Fire Safety: Have suitable fire extinguishing media on hand, such as CO2 or dry chemical extinguishers. Avoid using water, as it can spread some chemical fires.
• Spill Management: Be prepared for spills with appropriate spill containment and cleanup materials.
• Storage: Store chemicals in properly labeled, sealed containers away from light and heat. Ethyl ether should be used and stored in a way that minimizes the formation of explosive peroxides.
• Waste Disposal: Dispose of chemical waste according to local environmental regulations, especially for toxic substances like cadmium compounds.

Handling these materials with due respect to their properties and potential hazards is crucial for your safety and the safety of others in your workspace. Always follow best practices for chemical safety and environmental protection.

If you enjoy slowing down with film, darkroom printing, and meaningful photography, consider subscribing to my YouTube Channel. I share new videos each week focused on simple tools, timeless techniques, and the quiet joy of analog.