Reilly, James M. The Albumen & Salted Paper Book: The history and practice of photographic printing, 1840-1895. Light Impressions Corporation. Rochester, 1980.

Chapter Eight
Toning

How simple this all is, the novice will exclaim; where are the difficulties of photographic printing? But stay, let us look at our print; we were very pleased with its tint when it came from the printing frame, but alas! the rich violet has given place [in the fixing bath] to a dull brick-red rusty colour, and our print is for that reason valueless.

--Fitt, 1856¹

Very early in the history of photographic papers the desire arose to modify the color and density of the images produced on printing-out papers. The effect of "hypo" on salted papers--however beneficial it might have been as a fixative--was to produce an unpleasant yellowish-brown image, and one that had suffered a serious loss in overall density as a result of the fixing process. Eventually a number of different approaches came to be used to intensify and modify the image color in albumen and salted papers. Many of these treatments had an effect on the permanence of the image as well as its appearance. The most important and significant to the lifespan of printing-out paper prints is the technique of noble-metal toning, in which the silver image is partially replaced by a deposit of metallic gold or platinum. This chapter presents the history of toning methods and the theory and practice of noble-metal toning.

History of Toning

HEAT TONING

Probably the oldest toning method was the use of heat to darken the image on salted papers. This method was used by Talbot and most other early printers; it no doubt evolved after a hot iron was applied to a finished print to flatten it, whereupon a distinct strengthening of image occurred. Heat toning with a hot iron must be carefully done in order to avoid scorching the paper, and the hot iron must never be applied to the face of the print. Recently a large number of Talbot's original negatives were printed on salted paper at the Kodak Museum in Harrow, England, and heat toning of the prints was accomplished by drying the prints in a modern heated print dryer.² Heat toning has more effect with plain salted papers and arrowroot paper than with other kinds of paper.

SULFUR TONING

The other type of toning used extensively in the first two decades of photography was sulfur toning. Known to photographers as the "old hypo" method, it involved the purposeful decomposition of the sodium thiosulfate fixing solution to the point where sulfur was set free to react with the silver image and form brownish silver sulfide, thus accomplishing a kind of toning. Presumably this effect was first noticed when fixing baths were allowed to become exhausted through continued use. To the surprise of photographers the older the fixing bath got, the more intense was the toning action. It soon became apparent, however, that prints fixed in a totally exhausted fixing bath speedily faded, sometimes within a few weeks. To overcome the difficulties of a completely exhausted fixing bath some photographers actually added old "hypo" to newly made up fixing solutions, in hopes of a compromise between toning and fixing. Some of the literature of the period 1840-1850 suggests that a fixing solution was ready only when it smelled of sulfur and the sides of the tray were encrusted with brown scum, or when the fixing solution itself had been black for several days!

Blanquart-Evrard suggested in 1847 that the "instant aging" of fixing solutions could be obtained by adding concentrated silver nitrate solutions.³ An extension of the same idea was the addition of silver chloride, an ingredient called for in many early "toning-fixing" baths. The same liberation of sulfur from sodium thiosulfate could also be accomplished by acid additions to the fixing bath, and Blanquart-Evrard was also the first to propose the use of acetic acid additions to promote the toning effect.⁴ Although the potentially dangerous effects of "old hypo" to the permanence of the prints (whether the hypo was truly "old" or artificially decomposed) were well known by 1850, the evidence against the practice was not totally conclusive. Even today many sulfur toned albumen and salted paper prints survive in good condition--for example many of the prints of Hill and Adamson. Apparently under some circumstances a stable image of silver sulfide could be produced by the sulfur toning methods used, though in general the odds in favor of permanence in the use of the practice were slim. One factor which undoubtedly helped was the recommendation made by many authors that "old hypo" be followed by a fresh, strong bath of "new hypo."

Gold Toning

The idea of gold toning paper prints was borrowed from daguerreotypists, who used a mixture of gold chloride and sodium thiosulfate called sel d'or (literally, salt of gold) to intensify and tone their daguerreotypes. This important modification of the original daguerreotype process was discovered in 1840 by the famous French physicist Hippolyte Fizeau, and it soon became standard practice.' Not until 1847 was the same idea applied to paper photographs; in that year P. F. Mathieu suggested the technique in a pamphlet entitled Auto-Photographie.⁶ Unlike the enthusiastic adoption of "gilding" by daguerreotypists, gold toning of paper prints did not immediately catch on with photographers. Only when publicized after 1850 in the influential writings on photographic technique by Gustav LeGray did gold toning gain popularity, and then only slowly. LeGray was one of the best known and most successful photographers in France, and he did much to popularize all three of the great technical innovations in photography that appeared in the early 1850's--the wet collodion negative, the albumen print and gold toning of printing paper.

For a few years after 1850 the sel d'or method of gold toning was the most widely used for both albumen prints as well as salted papers. The effect of such a treatment was to change the image color from a yellowish-brown to either a cool brown, purple or bluish-black. The use of sel d'or constitutes what now would be called a toning-fixing bath. Since it is formed by mixing a solution of gold chloride into an excess of sodium thiosulfate ("hypo"), it retains its power to dissolve silver chloride and thus act as a fixing agent. It can also decompose and liberate sulfur if it is over-used or if it is made acidic by the introduction of acidic gold chloride. In fact, the usual practice of sel d'or toning involved a fairly good chance that sulfur toning and the release of potentially destructive invisible sulfur would take place before gold toning was fully accomplished. As in the case of the older sulfur toning approach, some sel d'or toned prints faded very quickly and others last to this day as vigorous as the day they were made. The difficulty with the method is that control over the outcome is impossible in the routine course of printing. In later years, during the era of emulsion-type printing-out papers (1890-1920), toning-fixing baths were reintroduced. Although more was known of the factors involved in assuring permanence, these new toning-fixing baths were also ultimately repudiated as too difficult to keep under control in the ordinary course of photographic printing.

Fig. 37. "A silvering and toning room," Ca. 1875. Note the double door entrance arrangement.

The approach which came to be used in the late 1850's and which is known to be more favorable to permanence is called separate toning and fixing, in which the toning step precedes the fixing step and no thiosulfate is included in the toning formula. By 1860 the evidence against sel d'or toning was so conclusive that even its most obstinate adherents had to admit its inferiority.⁷ A new and much more effective method, called alkaline gold toning, had been proposed by James Waterhouse sometime around 1855.⁸ This method called for separate toning and fixing, and the main innovation was the use of a toning bath composed of gold chloride and one or two mild alkalies. The alkaline solutions were found to be much more effective in their toning action and made rich browns and purple-browns easily attainable on albumen paper. The use of different combinations of alkaline substances with gold chloride produced different effects in toning, and the number of different formulae for toners quickly multiplied. Many 19th-century manuals contain 10 to 15 different formulae, most of them variations on the alkaline principle.

In 1867 another important class of gold toning baths was discovered, this time based on the combination of gold chloride with thiocyanates.⁹ This type of toner achieves a more complete substitution of gold for silver and thus produces a colder image tone, generally deep purple tending to black. Thiocyanates are solvents of silver chloride, so when a print is introduced into a thiocyanate toner the image at first bleaches, then intensifies as the gold is deposited. More gold is consumed by thiocyanate toners than the alkaline varieties, and although they became fairly popular, they never eclipsed the alkaline toners so beloved by albumen printers. For the later emulsion-type gelatin and collodion printing-out papers, thiocyanate toners were particularly suitable, and became standard practice with these papers. Although thiocyanate (formerly called sulphocyanide in the old style chemical nomenclature) toners do dissolve silver chloride, they definitely are not a substitute for fixer, and subsequent fixation in sodium thiosulfate is still necessary.

Theory of Noble Metal Toning

Toning with noble metals confers two benefits on the albumen and salted paper printing processes. It improves the color and density of the image and it also provides a measure of protection against oxidation and sulfiding of the image silver by partially replacing and enclosing it with metallic gold or platinum. The reason why a toning step is necessary at all has to do with the physical characteristics of the silver image in printing-out papers. As discussed in Chapter 1, the image is composed of very small, very highly dispersed particles of metallic silver. Although metallic silver is considered to be fairly unreactive, it will still react with some substances, notably sulfur and oxidizing agents, especially when the silver is in a finely divided condition. The small particle size means that the silver has a very large surface area relative to its mass; hence a large portion of its total mass is on the surface and readily accessible to destructive chemical agents. Gold and platinum react much less readily with sulfur and are much more difficult to oxidize. A layer of gold or platinum on a silver particle will tend to shield the silver inside from attack, especially from oxidizing agents.

The factors which affect the ultimate color of a print are the size¹⁰ and shape of the image particles, the distance between them¹¹ and the index of refraction of the medium in which they are dispersed.¹² In the toning process with noble metals the color of the image is modified by changing the size and shape of the silver particles through replacement of silver atoms by gold or platinum atoms. This enlargement of the metal aggregates which comprise the image causes the print to appear colder in tone, i.e., more neutral in color.

According to chemical theory it is only necessary to put a print into a solution which contains gold or platinum ions in order to have the substitution process take place. The rate of this substitution is greater for gold ions than platinum ions, although the rate may be modified by the presence of other substances in the toning solution.¹³ Depending in part on whether the solution is acidic or alkaline, gold may take one of several ionic forms. In an acid toning bath such as a simple gold chloride solution, one atom of gold replaces three atoms of silver, a case in which toning action would lag far behind bleaching of the silver. The result would be a flat, lifeless image with a reddish color. In an alkaline toner the gold exists in a different ionic form and there is a more favorable substitution of one atom of gold for one atom of silver. Platinum toning, on the other hand, goes on much more effectively in an acid environment. In either gold or platinum toning, the substitution process creates silver chloride as a by-product. Toning must always be followed by a fixing step to remove this silver chloride.

The Practice of Gold Toning

CAUTION: It is recommended that rubber gloves be used during toning operations.

Toning is an inexact process. Standardized, repeatable results come only with experience and the attainment of repeatability in all other parts of the printing process. Among the factors which influence the outcome of the toning operation are the pH of the albumen or other binder materials used the pH of the silver solution the amount of silver deposited to form the image the thoroughness of the initial wash in

processing the pH of the toning solution the presence of other substances in the toning solution the strength of the gold solution its temperature its age and the time of immersion of the print. Edward L. Wilson, one of the best known American writers on photography in the late 19th century, understated the case when he wrote, "The prints are not acted upon just alike."¹⁴ For beginning printers a very helpful exercise in evaluating toning is to leave one or several prints completely untoned to form a basis of comparison with toned prints.

GOLD CHLORIDE

The main ingredient in all the gold toning formulae is gold chloride. The gold chloride which can be bought from photographic or chemical suppliers is always an acidic substance (technically called chlorauric acid), made by dissolving gold metal in a mixture of nitric and hydrochloric acids. True gold chloride is an unstable substance made by passing chlorine gas over gold leaf at elevated temperatures, and is not usually commercially available. Gold chloride of commerce is usually sold as either an amorphous orange mass or as a 1% solution in distilled water. Gold chloride in the dry state is sold in 15 grain (approx. 1 gram) amounts. Because the dry chemical is very deliquescent, it is packed in small hermetically sealed glass tubes.

Solutions of gold chloride are fairly stable if kept out of light and out of contact with organic materials. Stock solutions made up from the dry chemical should always be made with distilled water. A stock solution of 1% strength (one gram of the dry chemical to each 100 ml of water) is called for by most toner formulae. In the 19th century it was quite common for photographers to make their own gold chloride by dissolving gold coins in the mixed acids and then evaporating the solution to dryness or neutralizing the excess acid with chalk. Because of the fumes evolved and the dangers of the concentrated acids, this is not a recommended procedure for the home laboratory.

As stated earlier the pH of the toner solution has a definite effect on the toning process It is not clear whether or not it is simply pH difference of differences caused by the presence of other ions--or both--which account for the different results obtained by the numerous alkaline toning formulae found in the literature of albumen and salted paper printing A search of that literature reveals two different approaches to alkaline toning practice In the first approach the gold chloride stock solution is neutralized with calcium carbonate (i.e., chalk) before the toner is compounded. Calcium carbonate will not make the gold chloride alkaline, but will neutralize any free acid. In the other approach only the various alkaline substances called for in the toner formula are used to overcome the acidity of the gold stock solution.

For the modern practice of alkaline gold toning it is most convenient to use the gold stock solution in its acid condition, without pre-neutralizing it as much of the older literature recommends. Care must be taken, however, to see that approximately the same mildly alkaline condition is maintained in each batch of made-up toner. The stock gold solution retains a yellow color while still in the relatively inactive acidic state; when it has passed into the more active state through contact with alkaline substances, it becomes colorless. This decolorization is the best guide to the state of the toning bath. Some baths found in the older literature may take hours or even days to decolorize and become usable. The most common of these is the sodium acetate toner, which generally requires 24 hours to "ripen" before it can be used. The printer who wishes to standardize the toning process as much as possible can monitor the pH of the toning solution, but for most applications sufficient control is obtainable simply by making up the toner according to a proven formula and visually observing the progress of toning.

The toning solution should not be made too alkaline because although it tones more quickly in that condition, it also loses its activity much more rapidly. Too active toners, whether from too much gold content or too high pH, do not produce pleasant tones and are more difficult to control. A leisurely pace in toning allows each print to receive individual attention. Making the toner too alkaline will result in baths that still contain a great deal of gold but no longer will tone prints. Most alkaline baths are intended for "one-use" toning, and become inactive spontaneously after a few hours. There is no clue other than the cessation of toning action to indicate the point at which a toning bath has become inactive or exhausted. Baths made with sodium acetate can be used repeatedly if strengthened with additions of gold stock solution.

STRENGTH OF GOLD TONING SOLUTIONS

Papers with porous surfaces, such as arrowroot, plain salted paper, etc., require toners with much less gold content than those intended for glossy albumen paper. These porous papers tone more quickly, and would rapidly become overtoned in the strong baths employed for albumen paper. Toning baths for matte salted papers should contain .1 to .2 g gold chloride per liter of toning solution, while glossy albumen paper toners should contain between .4 and .5 g gold chloride per liter of toning solution. Toning of both albumen and salted papers is generally done by inspection, and should take from 3 to 15 minutes depending on conditions and the kind of paper being toned. Toning should generally be carried on well past the point when a visible change takes place in the image color. The toning solution should be used at temperatures of 17-20�C, and prints require constant agitation in the toning solution. The toner solution is ruined by even a trace of fixer, so cleanliness and care are requited. The toning operation is best carried on in weak incandescent light so that the color of the prints may be accurately determined. In the case of glossy albumen paper the toning should be continued until only the shadows of the print retain their original warm color by transmitted light. Judging toning by looking at prints lying flat in a tray may be deceptive. After toning is completed the prints should be placed in running water and given a 5-minute wash before fixing them.

GOLD TONER FORMULAE

The following are alkaline gold toning formulae:

The following is a thiocyanate-based toner:

Platinum Toning

HISTORY OF PLATINUM TONING

The first published mention of platinum toning occurred in 1856, when a photographer in Istanbul, M. De Caranza, recommended toning photographs with an acidified platinic chloride solution.¹⁵ Very little notice was taken of De Caranza's suggestion because such a toner has only a very slight toning action and possesses a strong tendency to bleach the silver image. In fact all platinic chloride formulae have so little toning energy that at their best they are restricted to use with matte papers, and are totally ineffective with albumen paper. On the other hand, the platinous chloride salts, chiefly potassium chloroplatinite, are very active toners when combined with acids,¹⁶ and have a smaller tendency to attack the silver image.

Potassium chloroplatinite was a fairly obscure substance certainly not one generally available to photographers--until 1879, the year in which Willis began to market the platinotype process. In this process potassium chloroplatinite is an indispensable ingredient. Many workers of the platinotype process also used silver papers, and in 1886 J. Reynolds discovered that potassium chloroplatinite was a very energetic toner of silver prints,¹⁷ yielding brown and brownish-black tones instead of the purplish and bluish-black tones obtained with gold toners. In 1889 Alfred Stieglitz also published one of the pioneering platinum toner formulae for silver papers, consisting simply of nitric acid, potassium chloroplatinite and water.¹⁸

Platinum toning of silver prints became quite a popular practice, especially with matte-surfaced papers. Its most widespread use occurred during the period 1895-1925 when matte collodion and matte gelatin emulsion type printing-out papers were so popular. Matte gelatin papers were often toned only with platinum and therefore have a brown hue, while matte collodion papers were generally toned with both gold and platinum. This produced the familiar olive-black color so often seen in the studio portraits of the era 1895-1920. Platinum toning was also preferred for the various salted papers in use during this time, such as the many kinds of arrowroot and matte albumen paper.

THE PRACTICE OF PLATINUM TONING

The effects obtainable with platinum toning do seem to harmonize especially well with a matte surface, although platinum toning can be used with albumen and other kinds of glossy paper. The main technical difference between gold and platinum toning is that gold toning goes on best in an alkaline environment, while platinum toning requires a neutral or acidic condition. One difficulty in platinum toning is the tendency to produce yellowish highlights if the toning is carried on too long, or if the solution is too strong or too acidic. Another is the tendency of any impurities such as silver nitrate or sodium thiosulfate to rob the toning bath of its activity by altering the platinum to an irreducible condition. For this reason prints to be toned in a platinum toner must be well washed and treated in a 5% sodium chloride solution for 2 minutes and then washed again for 5 minutes in running water before being placed in the toning solution. Still another precaution to be observed is to wash the prints well after toning, to be sure that none of the acidic platinum toner is carried into the fixing bath.

A useful platinum toning formula is as follows:

Combined gold and platinum toning may be used to obtain neutral blacks; tone the prints in the Borax gold bath until they are lilac in color, and long enough in the platinum toner to achieve a neutral color. It is not possible to consistently attain the absolutely neutral black characteristic of the platinum print itself, however.