CALOTYPE process, The

The Calotype was a positive/negative process introduced in 1841 by Fox Talbot, and popular for the next ten years or so. Strictly speaking the term refers only to the negative image, but it is commonly taken to mean both.

A piece of paper was brushed with weak salt solution, dried, then brushed with a weak silver nitrate solution, dried, making silver chloride in the paper. This made it sensitive to light, and the paper was now ready for exposure. This might take half an hour, giving a print-out image. It was fixed in strong salt solution - potassium iodide of hypo.

Fox Talbot, who devised the process, showed his results at the Royal Institution on 25 January 1839, delivering a paper on the last day of that month.

The following year Fox Talbot succeeded in improving the "photogenic drawing" process, renaming it the calotype. He discovered that if he added gallic acid, the paper became more sensitive to light, and it was no longer necessary to expose until the image became visible. With further treatment of gallic acid and silver nitrate, the latent image would be developed.

In 1844 Fox Talbot opened a photography establishment in Reading in order to mass produce prints.

To make a print, the negative was placed on top of more photo paper, laid flat in a glass frame, and allowed to develop in sunlight.

The Calotype process was not as popular as its rival one, the Daguerreotype. There were various reasons for this:

One might also suggest that the fact paper was used as a negative lessened the detail of the picture, though from an artistic point of view some would regard this as a desirable feature.

However, the calotype also had its advantages compared with the daguerreotype:

When the Collodion process was introduced in 1851, the calotype became obsolete. However, the negative-positive process was one day to become the standard photographic one, which is still used today.

© Robert Leggat, 1999.