Salt printing: Exposing the print

An excerpt from Ellie Young’s book The salt print manual on how to expose a salt print and more about salt printing.

Writer and photography / Ellie Young

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This is the challenging and exciting part of the salt printing process. While exposing negatives on the sensitized paper you can observe your image as it appears. You have control over the intensity of the light and the length of exposure which affects the colour and density of the final print.

The salt print requires ultraviolet (UV) light to form an image. The sun, or alternatives such as lamps or tubes emitting UV light can be used. The ligh sources essential for printing-out processes are a million times stronger than those for developed-out processes.[40] Photographic enlargers cannot produce enough light for printing-out processes.

The colour and contrast of the print is influenced by the intensity and colour of the exposing light. A UV light box offers stability and repeatability while sunlight’s UV exposures are variable and complex.

Sunlight will affect the colour and contrast of the print while it is exposing depending on:

• Time of the day.
• Time of the year.
• Humidity.
• Cloud cover.
• Direct sunlight. This gives less contrast as the exposure is faster.
• Indirect sunlight. This increases contrast as the exposure is slower.
• Ambient temperature will also affect sensitivity of the exposing print with little reduction of silver taking place below 5°C.
• Different levels of UV and visible light levels will generate differences.

Although the effect of light on the salt print maximises in the region of 380nm, visible light up to approximately 650nm will reduce silver chloride to metallic silver. The spectrum below 380nm also reduces the reduction of silver but normal picture frame glass does not transmit light under 340nm.

An exposure frame, also called a contact print frame is made up from 3 parts; the frame, glass and a removable hinged back. The frame holds the glass in place and the hinged back applies pressure on the paper and negative to stop movement, ensuring they remain in contact and registration while exposing. The following step must be carried out in subdued light.

Make sure the sensitized paper is dry or the negative will be destroyed with brown silver nitrate stains. The glass must be clean and free from dust before placing the negative, followed by the paper, onto the glass.

Position the film emulsion to paper emulsion. Place the hinged back on top and secure. Turn the frame over and check before exposing. Make sure no marks or dust have appeared and the negative is in the correct position on the paper. If you have difficulty positioning the film and paper, removable tape can be used to attach corners of the film onto the paper; this can be removed once positioned. The hinged back allows you to inspect your print with little risk of deregistering the film.

When placing the exposure frame in the sunlight, the difference in temperature between the cool glass and the encased sensitized paper creates a humidity chamber. This may deregister the image as the film and paper expand at different rates. It will give the impression of a soft or out of focus image.[41]

Print exposures should be checked under subdued light. This should be bright enough to make a decision on the exposure but not so bright that it continues to expose the print. Always view under the same lighting conditions. This keeps a consistency when judging exposures.

Exposure times of salt prints

Judging the right exposure can be difficult. The print must be exposed beyond what appears to be the correct exposure by about two stops. The fixer removes approximately two to three stops of density, but as the image dries it regains one to two stops in density. Personal experience is the best guide to understanding the correct exposure. Dodging can be carried out by cutting shapes to cover areas on the negative where less exposure is required. These are laid on top of the glass in the exposure frame and moved frequently to avoid distinct outlines of the dodging tool shapes.

*Based on a bank of 17 Philips Actinic BL 40 watt mercury vapour fluorescence UV tubes at a distance of 20 cm from the print. These tubes emit UVA from 400 to 320nm peaking round 360 to 380nm.

The exposure times are dependent on a number of variables:

1. Negative density
2. Binder type.
3. Additives such as potassium dichromate will increase exposure time but sodium citrate will decrease exposure time.

Reciprocity failure

Slower exposures increase the contrast of the image, whereas fast exposures decrease the contrast. The extremes of either of these will cause reciprocity failure.

High intensity reciprocity failure can occur if the image is exposed directly to high UV content in bright sunlight. The photolysis activity decreases and the shadows will not print as dark or as dense as in the longer exposures. The loss of the density in the shadows flattens the tonal range and decreases the contrast.

Low intensity reciprocity failure occurs with an extremely weak light source as the longer printing time creates very small, vulnerable silver particles, the size of the particles relating to the length of the exposure. [42]

It has been found that only about 2% of the original applied silver remains on the paper. With particles sizes at about 1/100 of a silver gelatin print, the salted paper print is at risk to any impurities.[43]

Self masking

The shadow region of the print, (the more transparent region of the negative) ‘self-masks’. The reduction of silver salts to metallic silver blocks the darker shadow reducing the amount of light reaching that part of the image. The lighter areas continue to expose normally. It is this self-masking that produces the extended tonal range of the salt prints.44

Bronzing in salt prints

Bronzing means the particles of reduced metallic silver have migrated to the surface forming a layer on the surface of the print. This is the saturation point where the print is over-exposed. During exposure a bronze greenish, brassy brown sheen appears on prints. 45 Very light bronzing in the image will disappear during processing. But continuing the exposure creates a deep bronzing which will remain in the image shadow regions after processing. This spoils the print. If a negative has correct density and bronzing has formed in the margins, the print is at its maximum density. With suitable negatives, exposure can be monitored by viewing the margins outside the negative area so there is little need to open the exposure frames.

A 21-step transmission wedge is useful to include with your print while exposing your negative. Checking the exposed step wedge against the print allows informed judgments to be made about future exposures and the needs of the negative.

What occurs when the sensitized paper is exposed to light

When the print is exposed to light

• Light acting on the silver chloride dissociates the silver and the chloride and in so doing releases small particles of silver metal.
• Free chlorine molecules are released simultaneously.
• The chlorine absorbers are moisture in the paper fibres, and the excess silver nitrate reacts with the released chlorine forming more silver chloride.
• The light continues to break down this new silver chloride creating more metallic silver and the cycle continues.[46]

Without light, spontaneous reduction of metallic silver creates a yellow colour. This consists of reduced metallic silver in a finely divided state (slow exposures increase contrast by creating finer silver) accelerated by warmth and humidity.47 If the reduction of silver continues the colour will change to red/brown and finally after a period of months, the print may actually have a bronzed appearance.

Tiny particles combine forming larger particles chemists call colloidal. They absorb some wavelengths but are not large enough to absorb all wavelengths of light. This is dependant on the index of refraction of the binder in which they are dispersed. For example, albumen has a different refraction to gelatin creating a different colour. Printing-out papers after fixing become yellow/red/brown in colour. Silver particles pack closer together as the fixer dissolves unreduced silver chloride. The index of refraction has completely changed, in so doing dramatically changing the colour. When the print is dry it appears darker and colder due to the particles packing closer together on the surface.[48]

A certain amount of moisture is required for sensitivity and contrast. Over-drying coated paper before exposure will reduce sensitivity. Excess dampness will destroy the negative by transferring the silver nitrate on the film. Mylar™ sheet (10 to 20µm microns or less) between the paper and negative will act as protection with minimal loss of image sharpness.

Do not allow wet or damp fingers to touch the surface of the exposed print before placing it into the water baths. This will result in brown stains.

References on the Salt print process:

[40] Ware, M. J. (1994) Mechanism of Image Deterioration in Early Photographs – the sensitivity to Light of WHF Talbot’s halide-fixed Images 1834 -1844, Science Museum and National Museum of Photography, Film & Television, Bradford, London. p17
[41] Reilly, J. M. (1980) The Albumen & Salted Paper Book: The History and practice of photographic printing, 1840-1895., Light Impressions Corporation., Rochester, USA.p73
[42] Ware, M. J. (1994) Mechanism of Image Deterioration in Early Photographs – the sensitivity to Light of WHF Talbot’s halide-fixed Images 1834 -1844, Science Museum and National Museum of Photography, Film & Television, Bradford, London., p24
[43] Ibid.p25
[44] Crawford, W. (1979) Keepers of the Light, Morgan & Morgan, Inc., New York, USA.p151
[45] Reilly, J. M. (1980) The Albumen & Salted Paper Book: The History and practice of photographic printing, 1840-1895., Light Impressions Corporation., Rochester, USA.p73
[46] Ware, M. J. (1994) Mechanism of Image Deterioration in Early Photographs – the sensitivity to Light of WHF Talbot’s halide-fixed Images 1834 -1844, Science Museum and National Museum of Photography, Film & Television, Bradford, London. p17
[47] Reilly, J. M. (1980) The Albumen & Salted Paper Book: The History and practice of photographic printing, 1840-1895., Light Impressions Corporation., Rochester, USA.p63
[48] Ibid. p3

Ellie Young teaches alternative photographic processes in Australia and is an author of The Salt Print Manual. Also take a look at Ellie Young’s gallery.