Uncoated Paper and Salt Prints
This chapter describes anthotypes, Breyertypes, calotypes, salt prints, catalysotypes, ceroleins, chromatypes, crysotypes, cyanotypes, energiatypes, Feertypes, fluorotypes, kallitypes, platinotypes, Printing Out Papers (POP), and Developing Out Papers (DOP).
The phenomenon of darkening of silver salts in the presence
of light was known in the 1600's. Silver nitrate is soluble in
water, while silver chloride is water-insoluble. The chloride
occurs in nature as a soft mineral called horn silver, while
silver nitrate does not occur naturally.
Since silver nitrate is water soluble and was observed to darken when exposed to light either in solution or dried, it would seem to be the simplest of experiments to dip paper into a solution and make shadow pictures in the sun. This may have been done in the 1700's, but the first documented experiment was performed by Thomas Wedgwood, son of the English potter Josiah Wedgwood, and was reported in 1802 by the chemist Humphrey Davy. Wedgwood and Davy also experimented with silver chloride, called silver muriate, prepared with muriatic (hydrochloric) acid. They observed that silver chloride was considerably more light sensitive than silver nitrate, which we now know to be true. But simple silver nitrate photography was a technological dead end; photography was to encounter many such dead ends in the next century.
Because Wedgwood and Davy failed to solve the problem of fixing the image, all their pictures have faded, and they are not credited with the invention of photography. Their work is remembered as the forerunner of Talbot's success.
calotypes (salt prints): 1841 (patent) to the 1860's.
William Henry Fox Talbot patented the positive/negative salted paper process in 1841 after a public announcement in 1839. He soaked paper in a solution of common salt (sodium chloride), then applied a water solution of silver nitrate, thus achieving a mixed coating of silver nitrate and silver chloride on one side. He fixed the image by again soaking the light-exposed paper in a salt solution. The process produced a printing-out image during exposure, but Talbot also found that the image could be considerably intensified by developing in a mixture of silver nitrate and gallic acid. During the same year Sir John Herschel suggested the use of sodium hyposulfite as a fixer instead of sodium chloride, and "hypo" has remained to this day the basis of photographic fixers.
Talbot gave the name 'calotype' to paper negatives or prints made from them. The term 'salt print' refers to prints that were made by the salt/nitrate process from various negatives including calotype negatives and glass negatives (the latter being superior because of the absence of paper fiber). According to Lassam [89} he later gave the name 'Talbotype' to the calotype process at the urging of friends.
The calotype is described in many historical books; a particularly concise description is found in reference  by Stapp. DuBose  has an excellent history and process description that gives a perception of the results of process variations, particularly on color. This is discussed in Appendix III on the Fotofind program.
Calotype paper negatives were translucent, not transparent. When the negatives were printed, the paper fiber was imaged along with the silver image, to the detriment of resolution. As early as 1841 Talbot had applied melted wax to his negatives with a hot iron after they were processed and dried; he included it in one of his patent claims. In 1851 Gustave LeGray demonstrated better results by waxing the paper before it was sensitized and processed. Negatives made by his process were called ceroleines, a more convenient name than "LeGray's Process". Positives made from good paper negatives showed excellent resolution and tonal range, but they were soon superseded by wet-plate glass negatives.
The index of refraction of waxes is closer to that of paper fibers than is the refractive index of air. Therefore if wax fills the spaces between fibers, light scattering by the fibers is significantly reduced. Paper consists of a mixed population of fiber compositions, some of which are not even transparent, so a perfect match cannot be attained. Waxes, too, are complex mixtures, and white wax was recommended over yellow. Towler [108, 178] gives a procedure for separating the cerolein, or white component, from bees' wax.
To achieve best results, wax should wet the fibers and completely displace the air. Application was done with heat in most cases, with care to avoid scorching. The sizing materials used in some papers prevented good penetration and wetting, as did silver salts and processing residues. Talbot's negatives often showed blotchy and uneven light transmission. LeGray's process of waxing before processing was inherently better, provided the most suitable paper and wax was used.
Interesting sidelights on the waxed paper process as practiced by Roger Fenton are given by Hannavy , particularly regarding pre-exposure and post-exposure waxing.
Oils were tried, since they penetrate and wet without heat. Unfortunately they tended to soak into storage envelopes and anything else the negatives contacted, necessitating periodic reoiling. The process was messy enough without that.
The quality of the paper base was important because trace impurities caused spots, discoloration, and fading. Individual photographers tried the available artists' and drawing papers and usually settled on a favorite. They could buy presized papers or prepare their own from various recipes.
Reilly  has emphasized an important distinction in nomenclature. Salt prints are made by a two-step process: salting and sensitizing. The paper may or may not be coated with an emulsion or binder. Albumen prints are also made by sensitizing a pre-salted paper and therefore technically are salt prints, even though they have an albumen coating, unlike Talbot's earliest prints. Plain salt prints have a surface of exposed paper fibers; albumen prints are always glossy, but paper fibers are visible in the highlights through the transparent albumen because there was no undercoating of white baryta as in later bromide paper.
POP and DOP Processes
Printing-out papers (POP) are those in which the silver image, called photolytic silver, appears spontaneously during light exposure without chemical development (subsequent fixing is still necessary). There is no negative image produced by the POP reaction. The production of photolytic silver under the action of light quanta is related to the simultaneous formation of a latent image, but the exact relationship is not fully understood. Photolytic images must be gold-toned because they are inherently unstable even if fixed in hypo. Photolytic silver is accompanied by the release of an equivalent amount of free halogen gas (chlorine, bromine, or iodine), which may then recombine and reduce the effective rate of darkening. If recombination is prevented or slowed, a faster rate of darkening results; one way of accomplishing this is the inclusion of reducing agents in the emulsion or binder. All silver papers will eventually darken in daylight; POP papers are simply those in which the change is fairly rapid and the tonal range is useful. POP processes, including albumen, dominated 19th century photography.
In developing-out papers (DOP) light exposure produces an invisible latent image requiring chemical development to become visible. The colloidal particles of reduced silver in POP images are very much smaller than the filamentary particles in DOP images. Comparison electron micrographs are presented in Eastman Kodak [47-28], and Reilly ; a transmission electron microscope is necessary because individual particles in POP images are too small to see in light microscopes. The small size of these clumped particles is the principal reason for the characteristic reddish color of POP prints, though processing variations and toning alter the color. This is discussed in more detail in Chapters 3 and 11, and in Reilly [123-3]. According to Reilly [122, 6], the largest class of DOP prints from 1840 to 1885 were crayon portraits, which continued to be made into the 20th century (see Appendix II).
These surface characteristics are summarized below because they are important recognition clues for dating:
1. Exposed paper fibers over entire surface:
POP salt prints 1841-c1860
kallitypes & platinotypes 1870's-c1890's
2. Glossy surface, paper fibers visible:
POP albumen c1850-c1890
3. No fibers visible, glossy or matte:
POP or DOP, silver chloride or bromide c1890-present
Cyanotypes: 1842 - present
The cyanotype has not been taken seriously by professional photographers because the tonal range is poor and the images are bright blue, an unrealistic color for both portraits and landscapes. On the other hand cyanotype paper is cheap and easy to make and process, and the image has good permanence. However, cyanotypes should not be stored in contact with buffered or alkaline paper, sometimes called non-acidified paper and used in archival applications: such paper will fade cyanotypes. Exposure to light will also fade the images.
Specimens showing family groups and buildings are fairly common, but the greatest use was in copying text and line drawings, as blueprints. It has been in continuous use perhaps longer than any other photographic process.
The original process was discovered in 1842 by the astronomer Sir John Herschel, motivated by a need to copy his scientific calculations before the era of copy machines. Herschel's first process was based on ferric ammonium citrate and potassium cyanide, which produces a blue image where light strikes it. The image is fixed by simply washing in water. A positive print with blue shadows and white highlights is made by printing from a negative. A contact print against a line drawing makes the familiar blueprint with white lines on a blue background. Crawford [38, 163-166] describes the process in detail for home experimenters. Reference C is a valuable source of process information.
Less well known is Henri Pellet's patented (U.S.) process for making direct positives, described in Lietze [84, 65-69]. Poitevin also made direct positives in a violet color (Lietze [92, 75-78]). In both processes line drawings could be made in one step with dark lines on a white background, without reversal.
In all these processes the paper fibers are exposed in all parts of the image. Often the paper was sized with glue or starch to reduce penetration into the surface. Because of toning and process variations the colors were not always bright blue; they may range from blue-black to purple.
Valuable insight into the cyanotype process is in Ref. C by Mike Ware.
Platinotypes, Palladiotypes: 1873 (patent) - 1937
The platinum process is classed as a ferric process related to the cyanotype. William Willis patented platinotypes in 1873 in England. Crawford describes the full working process [38, 76-78, 167-175]. The chemistry is also discussed in Eder [48, 543-546] and in Lietze [92, 79-90]. According to Crawford, platinum paper went off the market in 1937, but there have been revivals, and palladium paper is again commercially available as of this writing. Platinum paper shows a weak image during exposure (POP), but developing is necessary for the final image. Colors range from silver gray (neutral black) to warm brown, depending on processing and toning. Paper fibers are visible throughout the picture, and the image appears embedded in the fiber texture. The image consists of reduced metallic platinum and is more stable than the underlying paper.
A variation, multiple platinum printing, is described briefly by Struss .
Palladium is chemically similar to platinum but cheaper and more plentiful and was used both alone and in mixed chemistry. Willis' patent claims the use of salts of palladium, gold, iridium, platinum, and mixtures thereof.
Many writers are lyrical about the unique beauty of platinum prints. It is perhaps the only process whose intrinsic beauty is a useful (though subjective) identifier; Crawford describes it very well [38, 77].
Kallitypes: 1843 - 1890's
The kallitype process is chemically similar to the platinotype process except that the final image contains metallic silver rather than platinum. Kallitypes resemble platinotypes in their beauty, and in fact kallitype paper was marketed commercially as a substitute for the very expensive platinum paper. Unfortunately people expected the substitute to be as resistant to fading as platinum, but the stability of silver does not compare with that of platinum, and kallitypes acquired a bad name. At about the same time the more convenient gelatin silver chloride papers became available and kallitypes fell from favor.
Crawford describes two kallitype processes in detail [38, 177-180]. Pernicano  gives a detailed modern recipe. Kallitypes are also described in the 1908 Library of Practical Photography , in The Photo , and in Eder [48, 543]. The paper surface shows exposed paper fibers throughout. The image becomes visible during exposure (POP) but darkens during development and fixing. Colors ranged from black to brown; there were many homemade process variations.
Non-commercial Types of Uncoated Prints
A non-commercial listing does not mean that there are no
surviving specimens. The early days of photography were a
do-it-yourself period; amateurs and professionals eagerly tried
every process that was published. Some, but not all, inventors
tried to license or restrict use of their processes. Robert
Hunt of London freely made public three of his processes: the
catalysotype, energiatype, and fluorotype, and many
publications carried instructions and notes on their
application. It would be of interest to find more authenticated
specimens of these lesser-known types.
Sir John Herschel discovered in 1842 that water and alcohol extracts of flower juices coated on paper were light sensitive. Several workers published recipes: Snelling [133, 37-42, 113-116] has considerable detail. Among the list of recommended flowers were the violet, red poppy, and wall flower. The images were "fugitive", and exposures as long as four to five weeks were needed. The light instability of organic dyes was a problem of long standing, and Herschel's early attempt to make a virtue of it is intriguing. Five weeks' exposure time is a little long for practicality, however.
Breyertype: 1839 - ?
An obscure but historically important process invented by Albrecht Breyer of Berlin in 1839. It was a silver chloride facsimile print of text and line drawings made by shining light through the back of sensitive paper placed in contact with a printed page. The print was produced by the light reflected from the page being copied, and was a negative from which positives could be made.
Breyertypes may be recognized by their subject matter of printed text or drawings, either positive or negative, whose color was brownish black with a texture of paper fibers, and with exposed paper fibers over the entire surface. The same subject matter may appear in prints made by the various cyanotype processes described in Lietze , but they were colored blue, purple, or other distinctive colors. Other photographic processes were capable of copying text, but they can often be recognized by characteristics such as coatings.
Catalysotype: 1844 - ?
This printing-out process was invented by Dr. Thomas Woods of Ireland in 1854, and improved by Robert Hunt in London. The paper was coated with iron iodide and sensitized with silver nitrate. The name was derived from catalysis, which was thought to explain image formation, probably an erroneous concept.
Chrysotype: 1842 - ?
Another of Sir John Herschel's iron processes of 1842, in which the paper was first coated with ferric ammonium citrate and dried. After exposure it was developed in gold chloride and fixed in potassium iodide. The image consists of reduced metallic gold that is purple in color. It had a limited tonal range and was used mostly for copying line drawings and text, producing a negative of white lines on a purple background. Chemically it is related to the kallitype and cyanotype. Although the paper was not made commercially, the recipe was published and widely used by amateurs.
Chromatype: 1843 - ?
Another of Robert Hunt's processes in 1843. The sensitive material was a mixture of copper sulfate and potassium dichromate, which produced a direct positive image of an orange or lilac color; Eder [48, 553] lists variations. If the process had been more sensitive it might have been successful, because it produced a direct positive in the camera.
Energiatype: 1840 - ? Also called Ferrotype.
This process of Robert Hunt's had considerable vogue, with lengthy articles appearing in books and periodicals. The paper was coated with a mixture of succinic acid and sodium chloride in a gum arabic binder, then sensitized with silver nitrate. After exposure it was developed in iron sulfate (hence the name ferro-type). According to Snelling [133, 111] the same developer works well with other salts of silver.
Feertype: 1889 - ?
An early diazo print, 'Diazo' refers to a class of light sensitive nitrogen-based organic compounds, which can produce a wide range of colors, mostly broad-band colors low in saturation. Feertypes were not commercially successful when Dr. Adolf Feer patented the process in Germany in 1889, but they were the forerunner of the Ozalid process after World War I that competed with blueprints for copying line drawings. There was no pictorial usage then, though today the process is sometimes used to print on cloth bases, such as tee-shirts.
Fluorotypes: 1844 - ?
Another Robert Hunt process, with little acceptance or documentation. The name was derived from sodium fluoride, which was mixed with potassium bromide; it was developed in iron sulfate.
Summary of Identification of Uncoated Prints