Although the process is basically simple, getting strong prints of reasonable contrast is difficult. The image needs to be held on the top of the paper and not allowed to sink into the paper fibres. Various organic binders are used in the salting solution to produce a surface coating. Most of the organic binders used also aid in the reduction of the silver salts under the action of light, and may also produce light sensitive silver salts.
Salting solutions
Although some 'salting solutions' were no more than simple solutions of rock salt or sea salt (largely sodium chloride) in water, most photographers found more complex formulas more useful. The simple solutions relied on the presence on the paper of organic sizes added in manufacture, usually starch or gelatine, both to keep the salts on the surface and to aid in the reduction.
Different salts, such as ammonium chloride, potassium citrate, potassium tartrate and potassium bromide are often used and may have a slight effect on brilliance and image colour. Typical salt concentrations are around 25-50 g per litre. Starch, gelatine and albumen were commonly added. Papers were soaked in the salting solution for around 5-10 minutes and then hung to dry.
Sensitising
The paper then has to be made sensitive to light. This step needs to be carried out away from sunlight or other UV light sources, but can be done in dim tungsten lighting. The sensitising solution used is silver nitrate, often containing a small amount of citric acid (most silver nitrate in the nineteenth century was also acidic.)
The silver nitrate solution is usually brushed onto the paper fairly liberally and evenly, and then allowed to dry. The process requires the use of an excess of silver nitrate, and it is best to use a measured amount and brush it over the sheet relatively evenly. The paper can also be coated by 'floating' on a dish of silver nitrate solution, as was the normal practice with albumen paper.
The paper is then allowed to dry, and exposed without lengthy delay.
Exposure
Although prints can be made from paper negatives, exposure times are longer, and film is easier to use. Negatives for use with the process can be shot using a large format camera, enlarged to the size required from smaller negatives or transparencies, or produced on film using an image-setter or ink jet printer.
The best results are obtained from continuous tone film developed to a considerably higher than normal contrast - negatives that would print perhaps on a Grade O bromide paper, or slightly more contrasty than this.
The film emulsion is placed in contact with the sensitised surface of the printing paper and held firmly together under glass in a sprung or vacuum printing frame. Exposure is with a UV light source or the sun. A typical exposure using summer sunlight and a film negative is around 20 minutes. The progress of the exposure can be checked at intervals out of direct sun, as printing frames usually allow one side of the print to be pulled away from the negative for inspection while holding the two firmly together over the rest of the image. Exposure is continued until highlight detail is rather bolder than required.
Contrast is largely controlled by the negative contrast, although different light sources have a slight effect. Negatives can be 'reduced' or 'intensified' chemically. With inkjet (or image setter) negatives, the contrast (and 'exposure') depend almost entirely on the densities in the image printed to give the negative.
Toning
No development is used for salt prints. Immediately after exposure the prints are usually washed for around ten minutes and then toned. Washing removes soluble silver salts that otherwise react with the toner solution. Toning is normally in a gold toner and results in a slight darkening and richer deep brown and purple-brown tones.
Early photographers used toners based on 'old hypo' which were essentially sulphide toners. When they worked well, these produced stable prints with image silver converted to silver sulphide, but they were unreliable and often a cause of prints that rapidly stained and faded. Mixed gold and hypo toners (sel d'or) introduced in the 1850s had similar problems. Improved alkaline gold toners and later thiocyante gold toners from the 1860s on were more reliable. Thiocyanate toners use more gold and are thus more expensive, and were not widespread, but they gave more neutral results and also greater permanence.
Platinum toning was used in the later period of salt printing (ca 1895-1915) both for increased permanence and to give a more neutral black tone, similar to that of platinum prints.
Fixing
After toning, the prints were thoroughly washed and then fixed in a simple bath of sodium thiosulphate (hypo) made alkaline with a little sodium carbonate. Fixing makes the prints yellower and paler (some of this change is reversed on drying.)
Much of the impermanence and fading seen in salted paper prints arises from fixing baths becoming exhausted as too many prints are put through them. Salted paper prints have relatively large amounts of excess silver chloride that needs to be removed, and thoroughly washed out.
Modern practice includes the use of hypo clearing agents, largely sodium sulphite, to improve washing efficiency. Even so, lengthy wash times are needed, especially with thicker papers.
Drying
Prints are usually pegged on a line to dry. When using thinner papers, it is best to mount the prints while still slightly damp.
Mounting
Mounting of older prints was probably most often carried out using starch pastes. Salted paper prints are often on thicker papers that do not require mounting.
