DIFFERENCES IN IMAGE TONALITY PRODUCED BY DIFFERENT TONING PROTOCOLS FOR MATTE COLLODION PHOTOGRAPHS
9 THE COLOR OF SILVER CHLORIDE PRINTS
As stated above, a wide variety of tones could be obtained with collodio-chloride papers (fig. 7). During his presentation to the London Photographic Society in 1865, Simpson said:
These three photographs printed from the same negative display different tonalities, from light brown to deep purple.
The prints I exhibit tonight have been treated in various ways, and many of them have been toned and fixed in the mixed bath of chloride of gold and sulphocyanide of ammonium, which I first recommended for uranium prints. Other in the acetate bath, others in the lime bath, others with sel d'or, and some have been fixed with hyposulfite of soda, and some with sulphocyanide of ammonium. As you will perceive, every variety of tone is possible, from a warm sepia tint to deep black1 (1865, 122).
According to the literature on silver chloride prints, factors that can influence the final tone of the finished print include the conditions of exposure, the negative quality, the emulsion formula, the metal used for toning bath, the toner concentration, the time in the toning bath, and the temperature of the bath. This seems an overwhelming amount of parameters to control, as E. Wallace rightly noted:
When we consider that the production of a certain tone or color is really a matter that has to be calculated, and that requires thorough knowledge of the different solutions, it is little short of the marvelous how perfectly regular tones can be made upon thousands of prints, even from different negatives, by an experienced man (Wallace 1888, 164).
This comment explains in part why photographers were reluctant to adopt the new emulsion papers in their commercial practice. Detailed descriptions of the effects of each of these factors follow.
9.1 CONDITIONS OF EXPOSURE
Many authors noted that exposing the negative in direct sunlight gave images of lesser contrast: “Printing in the shade will give the pluckiest results, while strong sunlight is apt to give soft prints,” wrote T. T. Baker (1904, 483). If the printing was carried out in intense light, the darkening was quick and the midtones could be destroyed. The tone of the resulting picture was red (Hasluck 1906, 240). When the negative was exposed to well-diffused light, the tones were purple. P. N. Hasluck (1906) proposed that the difference in tone was caused by a more gradual reduction of the silver.
According to L. P. Clerc (1937, 346), the degree of humidity of the emulsion considerably affected the color of the image: “A damp condition increases the sensitivity of the organic salts [citrate of silver for example] without affecting that of the chloride, and so, for a given exposure, the image is redder than that given by a dry film.”
9.2 NEGATIVE QUALITY
“The first factor of tone in the print is the character of the negative,” wrote Wallace (1888, 163). Strong and contrasty negatives gave purple and black tones very quickly in the toning bath. Weak negatives would give a maximum hue of red-brown (Hasluck 1906, 240).
9.3 EMULSION FORMULA
The type of acid and chloride selected for the making of the emulsion influenced the final tone of the print. The final tone also depended on the alcohol, ether, and water content of the collodion. For example, a higher alcohol content would give a more reddish tone, while higher proportions of water and ether would provide a bluer tone (Wentzel 1960, 70). Table 1 recapitulates the possible tone variations directly related to the emulsion's formula.
TABLE 1. TONE VARIATIONS RELATED TO EMULSION'S FORMULAE
The image's overall tone depended not only on the composition of the final image material but also on the color of the highlights. To achieve desired effects, baryta, emulsion, and/or paper support were sometimes tinted. The addition of a red dye (cochineal, for instance) to the baryta layer resulted in a warmer tone, especially desirable for portraits, while a blue dye gave pictures a colder appearance, more appropriate for landscape (Wentzel 1960). “Dyestuffs such as methylviolet [a dye consisting essentially of pentamethyl-pararosaniline chloride] are added in very small amounts [into the emulsion] to cover a slight tendency to yellowing of the paper caused sometimes by the collodion or the tint of the baryta layer” (Wentzel 1960, 70).
9.4 METALS USED FOR TONING BATH
Toning was a standard procedure in the processing of POPs. A. Davanne and A. Girard gave in 1864 the following definition for toning:
The operation to which the name of toning is given in photography, has for its object the changing of the hue of the positive proof, so as to place it in the best possible conditions of stability; and, at the same time, to impart to it an agreeable tint (Harrison 1892, 356).
Early investigators used the word “coloring” rather than “toning” to describe the process of changing the image tone (Lee 1987). While the literature offers a wide variety of recipes, gold toners, platinum toners, or a combination of both metals were most commonly used for silver chloride prints. At first, gold toners that gave nearly black tones were recommended. Soon, however, a combination of gold toning followed by platinum toning became standard for the processing of matte collodion printing-out papers (Reilly 1986, 12). Table 2 summarizes the most common toning baths used for printing-out papers and the tone they would provide.
TABLE 2. COMMON TONING BATHS USED FOR PRINTING-OUT PAPERS
Gold toner was the most widely used toning bath. A wide range of tonalities, from sepia to purplish black, could be obtained with a gold toning bath. The action of gold chloride on the silver image can be expressed by the following equation: Ag + Au Cl → Ag Cl + Au. The size and, thereafter, the color of finely divided gold depends very much upon the rate at which gold is deposited, that is, the speed of toning. This rate depends on the alkalinity of the bath. In an alkaline medium, gold (III) chloride (AuCl3) changes progressively into gold (I) chloride (AuCl), which deposits three times as much gold per unit amount of silver as the auric salt (Clerc 1937, 350). However, if the bath is too alkaline, the gold (I) salts change to the inactive aurite state, and the toning does not take place (Clerc 1937, 350). Neutral and slightly alkaline baths tone very quickly because the reduction takes place rapidly. The gold particles appear blue because they are somewhat large. In an acidic bath, reduction takes place very slowly. The gold is deposited in a dispersed form and reddish images are produced because of the extremely fine state of division of the gold. Clerc (1937, 350) noted that alkaline gold toning baths that were recommended for salted and albumen papers could be used for gelatin printing-out papers but that they were “almost without action on collodion papers; these latter should be toned in a bath containing a solvent of silver chloride, e.g. sulfocyanide or thiourea, which can slowly make its way within the collodion.”
The use of platinum as a toning agent was first proposed by Ernest de Caranza (1856) who added hydrochloric acid to his toner, but the acid caused the bleaching of the print. A practical method of platinum toning was proposed in 1890 by Lyonel Clark, who solved this problem by using nitric acid (Burbank  1973). The action of platinum salt can be expressed by the following equation: Ag + KPtCl2 → KCl + AgCl + Pt. Platinum toners gave collodio-chloride papers warm brown to sepia tones. They were seldom used for glossy varieties of papers, and thought to be more suitable for matte surface papers (Hasluck 1906).
Gold toning followed by a platinum toning bath gives a black neutral tone. The platinum tone was to a great extent controlled by the gold toning: the shorter the toning in gold, the warmer the final tone (Photo-Miniature 1910).
This writer does not know when palladium toning was adopted. A. Reynolds suggested the use of palladium as a printing medium around 1890, and palladium paper was commercially introduced as a cheaper substitute to platinum around 1916 (Nadeau 1989, 355). It seems reasonable to assume that palladium toners were introduced at the same time, for the same reasons. Gold and palladium toning gave approximately the same tone as gold and platinum.
Selenium toning was introduced in 1910 (Lee 1987) and replaced the expensive platinum and gold during World War I. Toning was done after fixing to avoid staining of the whites. Gold and selenium gave the same tone as platinum if the photograph was fixed prior to its immersion in the selenium toning bath.
Lead was used in combination with gold in combined fixing-toning baths. It gave purple tones. A combined toning and fixing bath was first suggested in 1850 by Gustave Le Gray, but it was not employed until about 1890 (Clerc 1937, 354). If not used properly, this technique could produce very unstable prints due to incomplete fixing or exhausted bath. Baker (1904, 492) noted that “combined toning and fixing is more applicable to collodion than to gelatin printing-out papers.”
9.5 DURATION, TEMPERATURE, AND CONCENTRATION OF TONING BATH
Most authors agree that different results can be obtained from the same formula if different working methods are employed. The quantity of metal salts in the toning bath also influenced the final color of the print. A greater proportion of gold, for example, gave a bluer and colder tone, while little gold in the bath gave from reddish to warm brown tones (Hasluck 1906, 245).2 The shorter the time in the gold toning bath, the warmer or redder the color of the finished print. The longer the print remained in the toning bath, the bluer or colder it became (Bothamley n/d, 102). Overtoning gave the print a cold and faded appearance (Burbank  1973, 40). If the bath was too cold, the tendency was to get a red print because gold was formed slowly (Hasluck 1906, 245). The recommended proper temperature of the bath was usually set around 60� F.