Calotype negative

The calotype is where development, and with it the latent image, enters paper photography. Talbot’s own words for it are the ones to keep, because he was reaching for a vocabulary that did not exist yet: “the impression is latent and invisible, and its existence would not be suspected by any one who was not forewarned of it by previous experiments.” The Part I lesson establishes the history and the dispute with Reade.

The chemistry

Four chemicals and no more: silver nitrate, potassium iodide, acetic acid and gallic acid.

Iodising converts the sheet to silver iodide, which Talbot says is “scarcely sensitive to light” and keeps indefinitely in the dark. Exciting brushes on gallo-nitrate of silver — equal volumes of an acetic silver nitrate stock and a saturated gallic acid solution, mixed fresh because the mixture does not keep. Exposure leaves an invisible change. Bringing out washes the sheet again with gallo-nitrate and warms it gently, and in a few seconds the exposed part darkens and finally grows entirely black while the rest stays white.

⚗Physical development, and the repair it makes possible

Ware classifies the development precisely, and the distinction runs through the rest of the course. Gallic acid alone can develop the image using only the silver already in the sheet: that is chemical development. Talbot’s gallo-nitrate carries silver nitrate as well, so the developer deposits additional silver from solution onto the sites light created: that is physical development.

The consequence is one Talbot exploited and no modern printer can. A negative worn out by repeated printing could be revived by another wash of gallo-nitrate and a gentle warming, and he reports that reviving one sometimes brought out details never seen before, “having been latent all the time”. A negative you can put more silver into is a negative you can repair. A modern developer has only what is already in the crystal.

The gain is the point. Ware puts first camera exposures at about half a minute at f/4 in bright sun against the hour or more photogenic drawing needed, and Talbot claimed a sensitivity transcending “a hundred times or more” any photographic paper described before. Print-out on paper had reached its ceiling in 1835 and stayed there; development moved it in a fortnight.

Historical workflow

Talbot’s own sequence, published in the Proceedings of the Royal Society in 1841 and recorded here as history: iodise, dry, keep; excite by candlelight shortly before use; expose — he gives one minute in summer at an aperture of one inch on a fifteen-inch lens, and one second at f/3 for a white plaster bust; bring out with gallo-nitrate and gentle warmth; then wash with water, blot, wash with potassium bromide, dip in water and dry. The negative stays transparent, “and that, therefore, there is no difficulty in obtaining a copy from it”.

Then the second half, which is the reason the process mattered. Ware dates Talbot’s hot thiosulfate fixation and the waxing of paper negatives to patent No. 9,753 of 1 June 1843. Waxing makes the sheet translucent so more light gets through and the fibres show less. The negative goes face down on sensitive paper in a screwed-down frame and prints out in the sun, three to thirty minutes, judged by eye, as often as you like.

🕰Le Gray's waxed paper is a different process

Talbot’s waxing is applied to the finished negative, and it is dated by his 1843 patent. Le Gray’s waxed-paper process of 1851 waxes the paper before sensitising, which is a different and later thing. This course does not conflate them.

The image

A calotype is a negative, and the object a viewer usually meets is the salted paper positive printed from it. The negative itself is brown-black in the fibres, translucent, waxed, and often carries a yellow cast where silver iodide was not fully removed. That yellow is not cosmetic: it absorbs exactly the blue and ultraviolet the printing paper needs, so it slows every print pulled from the sheet.

Ware states the fixing problem directly. Silver iodide “is only soluble with difficulty in thiosulphate solution, which needs to be heated, in consequence” — which is why Talbot’s 1843 patent specifies a hot bath, and why the bath was for removing the tint rather than for making the negative safe. The order of difficulty is the order of insolubility in water: chloride, bromide, iodide.

The prints are softer than a plate process, and the Getty atlas gives the reason under the salt print: the fibres of the paper negative scatter the light passing through them. Set a Hill and Adamson salt print beside a daguerreotype of the same decade and the difference in detail is not subtle — and neither is the difference that matters more, which is that one of those objects existed in a single copy and the other in as many as anybody wanted.

Permanence

Two failure modes, and they are not the same failure.

Residual iodine bleaches the negative from the inside. Talbot’s own proviso in The Pencil of Nature is chemistry: the number of copies “appears to be almost unlimited, provided that every portion of iodine has been removed from the picture before the copies are made”, because solar light and a trace of iodine acting together, though neither alone, decompose silver and form a colourless iodide. Every print you pull costs the negative some image.

Residual thiosulfate destroys the print. This is the positive’s problem rather than the negative’s, and Ware’s account of the Scottish exception is the best evidence in the course that it is procedural rather than inherent. The St Andrews group gave up Talbot’s halides for Herschel’s hypo; Cundell’s 1844 account recommends a 2.5 per cent fixing bath where the manuals of the 1850s specify 10 to 30 per cent; Hill and Adamson’s salt prints are notable for rich colour, high density and good stability. At Reading, Henneman washed each batch of twenty-five prints for ten minutes in each of three baths of hot water, and Schaaf traces the severe sulphide fading of The Pencil of Nature prints mainly to residual thiosulfate and to Reading’s water supply. The same chemical, used two ways, gives you Hill and Adamson or gives you a faded book.

Hazards

Silver nitrate at strength, handled wet, which the rubric names under Level B; gallic acid and acetic acid; potassium iodide and potassium bromide as the halide baths; and, in Talbot’s own fixing patent, a hot thiosulfate bath, which is a scald hazard as much as a chemical one. The waste is silver-bearing.

Nothing here is Level D. What keeps the process out of the course’s practical curriculum is not hazard but the fact that no written part yet owns it.

Where the course teaches it

As chemistry and history, in Part I and Part IV. The distinction between printing out and developing out is introduced on Talbot’s page, and Part IV takes the latent image apart properly: the calotype is the first evidence in the course that a change too small to see can be found and amplified.

As a procedure, nowhere yet. No part of the written course makes a calotype negative, and this entry does not pretend otherwise. Part XXII owns salted paper, which is the positive half of the pair, and its lab on salting and sensitising a sheet is the nearest thing the curriculum plans. Whether the negative half is ever taught is not a decision this atlas can make.

What the course does inherit from it, everywhere, is the negative-positive principle: one original, many prints. Herschel’s own 1840 paper treats “indefinitely multiplied fac-similes” as the more important line of inquiry precisely because it alone makes publication possible, and every printing technology the rest of this course touches descends from it.