The single-bath “double-iodide” iodizing method, as described by William Holland Furlong and read aloud by John Adamson on April 3, 1843, made calotypes more reliable and repeatable.
That stability opened the door for the Adamsons (and Hill & Adamson) to work at scale, and it changed everything for the calotypists in 1843. One hundred eighty-two years later, in 2025, it changed the game for me as well.
This single improvement by Furlong enables me to sensitize, develop, and fix my calotypes on demand in the field while traveling, without the need for a complete darkroom, just as Hill and Adamson did when they created 3,000 calotypes and salt prints in the early 1840s in Scotland.
Why Furlong’s modification matters:
- It replaced Talbot’s fiddly two-solution iodizing with one predictable bath.
- It solved two chronic iodizing problems in one stroke, giving makers confidence to batch-prepare paper.
- That reliability helped enable the massive Hill & Adamson output in the mid-1840s in Scotland, and now it is allowing me to do the same with Pictorial Whispers – Window Studies in the Ozarks.
I make fully handmade 1840s Adamson-era calotype paper negatives and ammonio-nitrate of silver (ANS) salt prints at a single vintage window, using only natural light and 19th-century chemistry to speak about grief and endurance.
Who was William Holland Furlong?
Furlong was an Irish-born chemist-photographer who came to St. Andrews from Dublin around 1841 to assist Professor Arthur Connell (chemistry). He moved in Sir David Brewster’s circle and soon corresponded directly with W. H. F. Talbot.
We have Talbot’s own mail: in March 1842, Furlong wrote from St Andrews, noting Brewster had sent Talbot a negative he (Furlong) made in County Wicklow, and asking how Talbot achieved the “beautiful lilac” tone in his chloride positives. That letter pins Furlong’s active role and Irish connection.
What happened on April 3, 1843?
At a meeting of the St Andrews Literary and Philosophical Society on April 3, 1843, John Adamson read a letter from Furlong describing “a new mode of preparing iodized paper for the calotype.” This is documented in the Society minutes and a contemporary newspaper report.
Furlong’s proposal: dissolve silver iodide in a strong excess of potassium iodide to make a single, clear solution—the “double iodide” of potassium and silver—so the paper could be imbibed once, then the silver iodide reprecipitates in the fibers when later washed in water. Modern chemistry calls the complex potassium triiodoargentate(I).
Why it’s a big deal: With one elegant bath, this method removed the guesswork and two major iodizing failure points in Talbot’s two-solution approach. It gave calotypists a way to iodize with “a degree of certainty” rather than trial-and-error.
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John Adamson’s role—clarified.
Adamson did not claim the chemistry; he read Furlong’s letter to the Society and helped spread the method in St Andrews.
He also contributed practical tips (e.g., “sunning” iodized paper) that Furlong later credited as a valuable way to boost sensitivity.
In short, Furlong brought the chemical fix; Adamson helped make it work in practice and taught it.
How this unlocked the Hill & Adamson achievement
Scotland was outside Talbot’s patent reach, allowing innovation to be shared locally. With Furlong’s predictable iodizing and the Adamsons’ mastery of procedure, the process stopped being “skittish”.
It became manageable enough for Hill & Adamson to produce roughly 3,000 negatives between 1843 and 1847. Let that sink in for a moment. Three thousand handmade calotypes and salt prints at a time when there was no concept of running water, as we have today, or electricity, air conditioning, or heat. Our level of personal comfort and utilities is beyond anything they could have imagined.
Sources you can verify
- Minutes and reports for April 3, 1843; technical explanation of Furlong’s double-iodide bath; context on its impact: Taylor & Ware, “Pilgrims of the Sun: The Chemical Evolution of the Calotype, 1840–1852.”
- Talbot Correspondence letter from W. H. Furlong(e) to Talbot, March 1842 (St Andrews; County Wicklow negative; “beautiful lilac”): British Library MS, transcribed online.
- Biographical background (arrived from Dublin; assistant to Connell; Brewster connection): Getty’s Disciples of Light and St Andrews research portal.
In a previous article, as of November 1, 2025, I now work exclusively with the St Andrews-era calotype: iodized paper, excited with aceto-nitrate of silver (without gallic acid in the exciting bath), and developed in gallic acid. This is the chemistry John Adamson employed in 1843 to stabilize and make the process repeatable. John Adamson was a medical doctor and clearly was an intelligent and critical thinker.
I follow John Adamson’s 1843 St Andrews-era calotype and ammonio-nitrate-of-silver (ANS) salt printing method. Each sheet of paper is first acidified, then treated with potassium iodide and silver nitrate to form silver iodide, dried, and later sensitized with a silver aceto-nitrate solution just before exposure. I load this paper in the camera and expose it, then develop the calotype in a dilute gallic acid developer and again in a gallo-nitrate solution to build the final density, before fixing and washing the negative. From this calotype paper negative, I make contact prints on salted paper sensitized with Adamson’s ammonio-nitrate of silver solution—100% analog, no digital steps, only light and chemistry. Every piece in Pictorial Whispers is made by hand, start to finish, just like it was in the 1840s.
Art Collector Resources
- Collector and Student Testimonials [read]
- Collector’s Guide [read]
- Why Analog Photography is Essential to Fine Art Creation [read]
- Why I Create [read]
- Aura – What is it, and why does it matter? [read]
- Why Analog Photography Is a Smart Investment [read]
- Analog photography in the Digital Age: Examining transformation, alienation and authenticity in modern photographic practice. https://doi.org/10.55927/ijads.v2i3.11019
