- Calotype or talbotype is an early photographic process introduced in 1841 by William Henry Fox Talbot, using paper coated with silver iodide.
Iodise the Paper
1. Find a suitable paper.
Many of today's papers contain hypo or other chemicals that make them unsuitable for the calotype process. A white paper, with a good wet strength, not made from wood pulp, and free from watermarks is required. [Talbot favoured a rag/gelatin paper from Whatman]
2. Brush onto the paper an 8% solution of silver nitrate in distilled water. Allow the paper to become matt dry.
3. Immerse the paper in a solution of potassium iodide 2 to 3 minutes. [Should this be done under a red light?]
Take care to avoid bubbles. Then wash the paper for several hours under running water.
4. Dry the paper, then hang it for up to two hours in sunlight. This will help the paper to produce a more contrasty image.
At this stage, the paper has a coating of silver iodide, which is insoluble and insensitive to light. This paper should be stored in an acid-free box until ready to use in the camera.
Sensitise the Paper
When ready to take the photo, the paper needs to be sensitised as follows, under a red light, using a Buckle Brush (named after Samuel Buckle) or cotton wool held in a glass tube, in order to get a good even coating on the paper.
5. Under a red light, coat the iodised paper with an silver nitrate in distilled water. Other chemicals are also required at this stage. I believe that these include glacial acetic acid and gallic acid
Blot the paper to remove excess liquid, then cut to size for the camera.
Expose the Paper
6. While the sensitised paper is still wet, expose it in the camera. A two minute exposure at f8 may be required on a sunny day.
7. Brush a solution of silver nitrate (similar to the sensitising solution but stronger) over the paper, followed by gallic acid. [Possibly also use acetic acid.]
The image should appear, perhaps in half an hour, or 3 to 4 hours on a cold day.
The image is ready when it shows a good contrast, viewed in red light.
8. Wash the paper, then fix in potassium bromide or hypo to create the calotype negative.
Initially, iodides were used for fixing. Talbot initially fixed his image using a solution of potassium bromide.
Herschell recommended the use of hypo (sodium hyposulphite - now known as sodium thiosulphate) as a fixing agent but Talbot was not keen to use this because it tended to bleach his negatives.
[Hypo, is still used today for fixing.]
The action of hypo was known before the calotype process was discovered. But hypo was expensive and difficult to use successfully, so it was not universally used in the 1840s. In fact, in the early days, it may have destroyed more photos than it helped.
Wash the paper, then dry.
The Calotype Paper Negative
9. The result of the process above is a negative image. This is the calotype.
For Talbot, the negative image was an end in itself.
He never retouched his negatives, and it was only with encouragement from others that he went on to make prints from them
For Hill & Adamson and others, it was the final print that was important. They frequently touched up their negatives by hand, strengthening lines and adding sky detail.
Several of the early photographers had no difficulty in making the calotype negative, but had trouble converting it into a good salt print.