Iodizing Paper For My Next Pictorial Whispers Plate

Today I am back in the darkroom preparing paper for my next Pictorial Whispers plate.

Before I can put a sheet of paper into the camera, there are several chemical steps that have to happen first. Today I am working on one of the earliest stages: iodizing the paper.

This is one of those parts of the calotype process that I find fascinating because you can actually watch the chemistry change the appearance of the paper before a photograph has ever been made.

What Does It Mean to Iodize the Paper?

I am working with a version of the calotype process published by Colonel Alexander Greenlaw in the 1850s.

During the iodizing stage, the paper is immersed in a solution containing potassium iodide and potassium bromide. These salts penetrate into the paper and leave iodide and bromide ions distributed throughout its fibers.

At this point, the paper is not yet fully sensitized for photography.

There is no finished photographic image hiding in the sheet. I am simply preparing the chemical foundation that will become light-sensitive during the next stage.

That distinction is important.

Later, when the iodized paper is placed into a silver-nitrate sensitizing bath, the silver ions will react with the iodide and bromide:

Silver + iodide → silver iodide

Silver + bromide → silver bromide

Those silver halides are the materials that will eventually respond to light inside the camera.

Why Does the Paper Turn Purple?

The color change is one of the most interesting parts of Greenlaw’s process.

In his 1869 instructions, Greenlaw added a small amount of elemental iodine to the potassium-iodide and potassium-bromide bath until the solution reached what he described as a dark claret color.

He then observed that the treated paper could appear dark purple and later change toward a light brown color as it dried and aged.

The chemistry behind that color is associated with iodine and polyiodide species formed when iodine is present with iodide.

Iodine can form species such as triiodide and larger polyiodides. These absorb visible light strongly and can produce brown, violet, blue, or nearly black colors depending on their concentration and what they are interacting with.

If starch or similar polysaccharide sizing is present in a paper, iodine can produce an especially strong blue-violet complex with the amylose component of starch. This iodine-starch reaction is a well-known piece of chemistry.

The exact color therefore depends partly on the particular paper.

As the sheet dries, the distribution and concentration of iodine species changes, and some free iodine can also leave the sheet. The intense purple appearance gradually weakens, leaving the much warmer brown or tan paper that I see after drying.

[Photo: Paper after iodizing, beginning to settle into its tan color as it dries.]

One important detail is that potassium iodide and potassium bromide themselves are essentially colorless. The strong purple/brown colors historically described in Greenlaw’s process are associated with the iodine chemistry and its interaction with the paper rather than the iodide and bromide salts alone.

Waiting Is Part of the Process

The sheets I prepared today will now hang and dry for at least 24 hours.

There is nothing useful I can do to hurry this part.

That is one of the things I continue to appreciate about working with the calotype. The process has its own pace.

Mix the chemistry.

Prepare the paper.

Wait.

Sensitize it.

Wait again.

Expose it in the camera.

Develop it.

Wax the finished paper negative.

And finally place that negative in direct contact with another handmade sheet of photographic paper to make the salt print.

A single finished photograph is the result of many small steps spread across several days.

What Happens Next

The next stage will be much more consequential.

Once these iodized sheets are completely dry, I will take one into the darkroom and immerse it in my silver-nitrate and glacial-acetic-acid sensitizing solution.

That is the moment when the iodide and bromide already inside the paper encounter silver and the light-sensitive silver-halide layer begins to form.

After that, the paper can finally go into the large-format camera.

I will share that part of the process in my next update, along with what happens to the paper during sensitization and why Greenlaw used such a substantial amount of acetic acid in his silver solution.

This is how I want to share Pictorial Whispers going forward—not only the finished photographs, but the slow sequence of chemistry, paper, light, failure, testing, and handwork that makes each plate possible.

If you would like to follow the next step, subscribe to my Free Darkroom Diary Newsletter below.

I share new photographs, darkroom notes, experiments, and updates from Pictorial Whispers as the work develops.

The next sheet is already hanging in the darkroom.

Now I wait.