William Henry Fox Talbot’s Calotype Process of 1840 (British Patent 10627) represents a pivotal moment in the history of photography, marking a significant leap from early photogenic drawing to a reproducible negative-positive process.
The calotype (also known as the Talbotype) employed paper sensitized with silver salts to create latent images, which could then be developed chemically and used to produce multiple positive prints. Unlike earlier processes like the daguerreotype, which created unique, non-reproducible images, Talbot’s method laid the foundation for modern photographic printing.
At its core, the calotype process involves precipitating silver iodide within the fibers of the paper, sensitizing it with an excess of silver ions, imprinting a latent image through exposure to natural light, and then developing the image using gallic acid.
I’m currently writing a series of in-depth articles exploring the calotype paper negative process. The articles will focus on detailed chemical analysis and the evolution of workflows from Talbot’s early experiments in the late 1830s to Alexander Greenlaw’s advancements in 1869. If you’re interested in this process’s history and technical aspects, subscribe to my blog to be notified when new articles and videos are published.
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High-Level Summary of Calotype Evolution
The calotype paper negative process, invented by William Henry Fox Talbot in 1840, revolutionized early photography by introducing a method to create reproducible negatives on paper. Over the next decade, prominent European figures refined the process, each contributing significant advancements that addressed its limitations. Below is a timeline highlighting key innovations and contributors.
1840 – William Henry Fox Talbot
- Invention of the Calotype Process: Talbot introduced a three-step process: iodizing, exciting, and developing paper negatives using silver nitrate, potassium iodide, acetic acid, and gallic acid.
- While groundbreaking, Talbot’s method was complex, requiring precise timing and preparation. The unstable chemistry also posed challenges for photographers.
1847 – Dr. Guillot-Saguez
- Single-Solution Iodizing: Guillot-Saguez simplified Talbot’s two-step iodizing method, using only potassium iodide to prepare the paper. Silver nitrate was added later during the exciting stage, offering greater control.
- Indicator Color Change: His method produced a tell-tale violet or blue tint when potassium iodide oxidized, signaling when the exciting solution had fully penetrated the paper.
- This innovation laid the groundwork for subsequent advancements, including Gustave Le Gray’s pre-waxed paper technique.
1851 – Gustave Le Gray
- Pre-Waxed Paper Process: Building on Guillot-Saguez’s simplifications, Le Gray introduced pre-waxing the paper to improve its strength and reduce light scattering.
- Enhanced Practicality for Travel: Pre-waxed paper could be prepared in advance, transported without degradation, and developed later, making it ideal for field photography.
- Le Gray’s process was more stable and versatile than plain calotypes, bridging the gap between the calotype and the collodion processes.
1856 – Alexander Greenlaw
- Adapting to Hot Climates: Greenlaw found that Guillot-Saguez’s method excelled in tropical conditions, where other processes often failed. His name became associated with this variation, which persisted into the 20th century.
Key Contributions and Legacy
Each contributor addressed specific limitations of Talbot’s original method:
- Guillot-Saguez simplified and stabilized the iodizing stage.
- Le Gray made the process more practical for photographers on the move.
- Greenlaw ensured the method’s viability in diverse climates.
- By the 1850s, the calotype process reached a point of chemical refinement, balancing sensitivity, stability, and ease of use.
These innovations highlight the collaborative and iterative nature of photographic development, with each figure building on the work of their predecessors. The calotype’s evolution demonstrates how technical ingenuity shaped early photography, making it more accessible and practical for artists and scientists.
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Featured Guest on Studio Q Show
I’m excited to announce that I’ll be the featured guest on the Studio Q show this coming Saturday, October 5, 2024. During the show, I’ll be sharing my passion for creating calotype paper negatives, offering a glimpse into the timeless art of photography as pioneered by Fox Talbot. I’ll walk you through the original calotype chemistry and workflow, alongside showcasing some of my most recent works.
In addition to the technical aspects, I’ll delve into the creative narrative behind my projects and explain why I chose the calotype process as my medium to bring these stories to life. I’d love for you to join me and participate in this engaging discussion.
To watch the show live or a recording, go to https://www.youtube.com/watch?v=hySgChnIZNM
Modern Calotype Chemistry Inventory & Explanation
I created a list of the chemical components used in the original Fox Talbot calotype process, along with explanations of their purpose and how they work.
Summary of Process:
- Iodization: The paper is soaked in a potassium iodide solution, and then silver nitrate is applied, forming silver iodide within the paper fibers.
- Sensitization: The iodized paper is then sensitized with silver nitrate, providing excess silver ions needed for image formation.
- Exposure: The sensitized paper is exposed to natural light in a camera, where the light breaks down the silver iodide in the exposed areas, forming a latent image.
- Development: Gallic acid is used to reduce the silver ions in the exposed areas, making the latent image visible by forming metallic silver.
- Fixing: The image is fixed with sodium thiosulfate, removing unreacted silver salts and making the image permanent.
- Waxing (Optional): The negative is waxed to increase transparency and improve the quality of the final prints.
Chemistry
Silver Nitrate (AgNO₃)
- Purpose: Silver nitrate is used to sensitize the paper by introducing silver ions that will react with iodide to form silver iodide within the paper fibers.
- How it works: When silver nitrate solution is applied to the paper, it interacts with potassium iodide to precipitate silver iodide, which is light-sensitive. This silver salt is crucial for capturing the latent image. The excess silver nitrate is also crucial for sensitization after the formation of silver iodide, making the paper responsive to light.
Potassium Iodide (KI)
- Purpose: Potassium iodide is used to iodize the paper, preparing it for the formation of silver iodide.
- How it works: When potassium iodide is applied to the paper, it reacts with silver nitrate to form silver iodide. Silver iodide is a photosensitive compound, and it forms the basis for the calotype’s ability to record light and shadow during exposure.
Potassium Bromide (KBr) (Optional)
- Purpose: Potassium bromide can sometimes be used in calotype preparations to moderate the sensitivity of the paper and improve contrast.
- How it works: Potassium bromide acts as a restrainer, preventing excessive fogging of the paper during exposure. It helps improve the contrast by limiting the action of the light to only the exposed areas.
Gallic Acid (C₇H₆O₅)
- Purpose: Gallic acid is used in the development process to reduce the silver ions on the exposed parts of the paper, bringing out the latent image.
- How it works: Gallic acid acts as a reducing agent. During development, it selectively reduces the silver ions in the exposed regions of the paper, converting them into metallic silver. This results in the visible image appearing on the paper. Gallic acid enhances contrast and helps build up the density of the image.
Acetic Acid (CH₃COOH)
- Purpose: Acetic acid is often used in the calotype process as part of the gallic acid solution to moderate the development process.
- How it works: Acetic acid lowers the pH of the developer solution, slowing down the reduction of silver ions. This gives better control over the development process, allowing for finer gradations of tone and better detail.
Silver Iodide (AgI)
- Purpose: Silver iodide is the photosensitive compound that records the latent image during exposure.
- How it works: When the paper is exposed to light, the silver iodide breaks down in the areas exposed to light, creating a latent image. This image is invisible until development but contains the chemical changes needed to form the final image.
Sodium Thiosulfate (Na₂S₂O₃)
- Purpose: Sodium thiosulfate, also known as hypo, is used as a fixer to dissolve unreacted silver salts, making the image permanent.
- How it works: After the image is developed, the paper still contains unreacted silver iodide that would darken with further exposure to light. The sodium thiosulfate dissolves this unreacted silver iodide, effectively “fixing” the image by making it stable and preventing further exposure from altering it.
Wax
- Purpose: Waxing the paper is an optional final step used to make the paper more translucent, which improves the clarity of the negative.
- How it works: After the paper negative has been fixed and washed, waxing is applied to fill the paper fibers, making the negative more transparent. This allows for sharper prints when the negative is contact printed onto photographic paper, producing clearer images.
This overview should provide a detailed understanding of the calotype process and the chemistry behind it.
Talbot’s 1840 Original Calotype Process
Iodizing
A fine writing paper (Gohn Whatman’s ‘Turkey Mill’) was first coated with a 3.8% solution of silver nitrate using a brush and allowed to dry. Next, the paper was immersed in a 5.7% solution of potassium iodide, ensuring the latter was in excess, which precipitated silver iodide within the paper fibers. After that, the paper was briefly immersed in water to rinse off any excess chemicals and then dried.
This iodized paper was relatively stable to light and could be stored indefinitely. However, the timing of the last two steps was critical: immersing the paper in potassium iodide for too long could re-dissolve the silver iodide, and prolonged rinsing in water could cause the silver iodide to wash away. Either mistake could result in poor-quality negatives.
Exciting
Before exposure, iodized paper was “excited” by treating its surface with a solution containing 4.9% silver nitrate, 7% acetic acid, and 0.5% gallic acid. Talbot referred to this mixture as “gallo-nitrate of silver.” Since this solution was unstable, it had to be freshly prepared by mixing equal volumes of Talbot’s stock solutions of “aceto-nitrate of silver” and saturated gallic acid.
At this stage, the silver iodide in the paper became sensitive to light due to the excess silver ions introduced by the “exciting” solution. The inclusion of gallic acid further enhanced sensitivity, although Talbot noted it was not essential. Acetic acid was added to prevent the excited paper from spontaneously decomposing, but even with this safeguard, the paper remained highly unstable and needed to be exposed within an hour or two to avoid fogging.
Finally, the excited paper was briefly dipped in water, blotted, and optionally dried before exposure.
Exposing
The excited paper could be exposed while either moist or dry, with the moist state being preferred due to its greater sensitivity to light. In hot weather, the negative was often held in the dark slide by a sheet of glass to prevent it from drying out. This same technique was also used to keep large negatives stable, flat, and properly aligned during exposure.
For a typical subject in bright sunlight, Talbot claimed that an exposure time of one minute at f/8 was sufficient. For subjects with white objects, the exposure could be as brief as one second at f/3.
Developing
After exposure, the invisible latent image was, as Talbot described, “brought out” by developing it with more of the gallo-nitrate of silver solution. Talbot knew that gallic acid alone could develop an image (experiments documented in his Q notebook), so the inclusion of silver nitrate and acetic acid was not strictly necessary. However, their presence strengthened the image by selectively depositing silver onto it, increasing its density. This process, now known as physical development, became a crucial method for Talbot to “revive” underexposed or faded negatives.
In contrast, using gallic acid alone—without additional silver nitrate—relied entirely on the silver already present in the sensitized paper to form the final image. This approach is now referred to as chemical development.
Fixing
To stabilize the image, the negative was initially washed with water, leaving the developed calotype in a relatively light-stable condition. Talbot recommended further securing the image by treating it with a 2% potassium bromide solution, which converted any residual free silver ions into insoluble silver bromide. The remaining unchanged silver iodide imparted a slight yellow tinge to the background, but since the negative was fairly “fixed,” its removal was not urgent.
In 1843, Talbot noted that the residual silver iodide could be fully removed by immersing the negative in a bath of hot, concentrated sodium thiosulphate solution—commonly known as hypo, the fixer introduced by Sir John Herschel in 1839.
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