Gum Bichromate and Carbon Processes
This chapter discusses bichromated gelatin, carbon, carbro, and gum prints, gum platinum, Mariotypes, Ozotypes, Ozobromes, oil and bromoil prints.
Silver has occupied center stage in photography almost since
the beginning, with the successful Daguerreotype and Talbot's
calotype, and before that the early experiments of Thomas
Wedgwood. But a surprising number of non-silver processes also
surfaced, starting with the asphaltum picture of the Frenchman
Nicephore Niepce in about 1824. Some of these processes
flourished commercially and are still in use in one form or
An important group of light sensitive compounds are the chromates: sodium, potassium, ammonium, silver. In 1839 Mongo Ponton (Scotland) discovered that paper soaked in a water solution of potassium dichromate darkened when exposed to sunlight. Fixing was accomplished by simply washing in water. This effect could have produced pictures of a sort, but no practical use seems to have resulted until 1852 when Fox Talbot patented Photoglyphic Drawing, the light sensitivity of gelatin sensitized with potassium dichromate (usually called bichromate in the older literature). This was nearly as momentous a discovery as his paper negative calotypes because he incorporated the separate concept of the halftone screen, which can be observed in any modern newspaper picture; it is discussed in Chapter 5.
When dichromate-sensitized gelatin is exposed to light, it becomes insoluble in water; sections not exposed to light can be washed away in warm water. The gelatin could be dyed various colors when it was initially coated on the base. The thickest areas of gelatin remaining after processing were darkest where the incident light was brightest. Thus the original exposure was a negative; a positive print could be made from any type of negative. John Pouncy (England) produced prints in 1858 incorporating pigments in the gelatin; they were called, not unreasonably, pigment prints, but they had poor tonal range. The chromates had significantly lower light sensitivity than silver compounds.
The reason for poor reproduction of intermediate tones was discovered by Abbe Laborde in a nice bit of clear reasoning, as described by Crawford . When light of intermediate intensity strikes the surface of the gelatin it renders insoluble a thin top layer but does not penetrate further. During warm water washing the unaffected lower layer washes away and takes the top surface with it. Only very heavily exposed areas remained, resulting in excessive contrast. Laborde did not suggest a remedy, but in 1858 J.C. Burnett exposed the paper through the reverse side, which caused exposed regions to adhere to the paper as desired.
However, this was only a partial solution because printing through the paper fibers caused the same texture problem as in the calotype negative. Transparent celluloid was used thirty years later, but a more immediate answer was found in 1864 by Sir Joseph Swan who used glass backing and then transferred the gelatin image to paper.
When carbon black was used as a dye the prints were called carbon prints, and this term came to mean all prints made by transference, not to be confused with carbro prints described below. Sepia or brown were popular as well as black, possibly because they more nearly resembled average flesh tones, or albumen prints. They also resembled silver prints that were gold toned to reduce their rate of fading. Many other pigments were used: the Autotype Company, founded from Swan's patent, at one time listed 55 colors. By using multiple exposures in different pigments Adolph Braun in Alsace made reproductions of famous paintings with great success. Such pictures are not subject to the fading that plagued early silver processes. Accelerated testing as we know it today was not necessary; the time scale for silver fading was short and erratic, as it depended on so many processing and material variables not then understood. But carbon is one of the most stable and unreactive of the chemical elements, and gelatin is also reasonably durable. Some of the pigments in the gelatin were probably organic dyes, and were subject to fading.
The carbon process was cumbersome and carbon prints did not approach albumen prints in popularity. Later, in the 1890's, the process had one of its occasional revivals under the name gum or gum bichromate. Gum bichromate, with gum arabic substituted for gelatin, had been invented by Alphonse Poitevin in the 1850's. Its virtue, besides permanence, was the high degree of artistic latitude permitted by the process through multiple printings, an attraction that continues to this day.
Bichromate prints are always contact prints because of the low light sensitivity; large prints were made from large negatives. Carbon or gum prints can often be recognized by a faint surface relief effect. The gelatin is thickest in the shadows where it has received the greatest light intensity; in the highlights the paper fiber may be all or partially exposed. The shadows therefore tend to be shiny. If there is a sharp shadow against a light background, the difference in thickness can be seen in side illumination with a hand magnifier. Sometimes multiple coats and exposures were used. Woodburytypes also show this relief effect and are often difficult to distinguish from carbon prints.
Carbon prints did not fade, and sometimes for this reason are conspicuous in a mixture of old carbon and silver prints. This evidence is circumstantial, however, since there are some unfaded silver prints, particularly those that were toned. Carbon prints were often imprinted "Permanent" on their mounts, a unique and welcome identifier.
Dichromate-sensitized gelatin is the basis for the modern silk screen process used for many kinds of stencil printing. It has been used for research in color television and in micro-electronic hybrid circuits. It is sufficiently light sensitive to permit enlarging with an intense point-source zirconium arc light. The process has spanned nearly one and a half centuries with considerable success.
References: Crawford [38, 69-75]; Gernsheim [61,338-9]; Harrison, Joan [73, 369-376]; Newhall [105, 60-61]; Bernard  and Holme  contain color reproductions that are helpful identification aids.
Following are some variations on the bichromate process:
This process, introduced in the late 1890's, applied pigmented gum on top of platinum prints and exposed after the platinum was processed. It was noted for special effects such as exposing the two images from different negatives, and using brightly colored pigments in the gum. The technique seems to have evolved from a desire to add deeper blacks to platinum than could otherwise be achieved. Edward Steichen was a well-known practitioner of the technique, examples of which are in Holme . Gum prints could be printed over other prints besides platinum, but gum platinum has acquired a more distinct identity.
This was an image transfer between two bichromated papers, exhibited in 1873 by A. Marion in Paris. It was not viable.
A modification of the Mariotype by Thomas Manly in England in 1898, with no better results. Ozone was fancifully thought to play a role in the process, hence the name.
Manly continued to work on the contact transfer idea, and in 1905 the ozobrome finally worked; it transferred the image from a gelatin silver bromide print to a bichromated gelatin sheet. The name was changed to "carbro" - a composite of carbon and bromide, by H.F. Farmer in 1919, and was commercialized by the Autotype Company. Its evolution thus extended well into the 20th century, but its origin was the Mariotype. Carbro prints are well described by Crawford [38, 187].
Oil Prints and Bromoil
The Frenchman A.L. Poitevin discovered in 1855 that when greasy printers' ink (as opposed to water suspensions of carbon) was applied to a bichromated gelatin image, the ink adhered only to the light-struck shadow regions. The hardened shadows become hydrophobic: they repel water and are wet by oils and greases. It offered a way to darken the image after exposure rather than adding pigment to the gelatin during coating; the prints were called oil prints. In 1907 it was found by C. W. Piper that gelatin bromide prints also showed this effect, hence the name bromoil.
The above description applies to ink-intensified gelatin prints, but it did not take long to observe that duplicate prints could be made by pressing the inked gelatin to a sheet of plain paper. Thus was born the photomechanical process known as collotype, so named because the printing was done directly from the colloid surface rather than from etched plates. Collotypes are discussed in greater detail in Chapter 5.