Photographic Processes Represented in the Exhibit

Each of these processes is sensitive to ultraviolet light.  For all but the tintype (wet plate collodion) process, the sensitized paper is placed under a negative, pressed under glass, and exposed.  The processes differ in the preparation of the sensitized paper and in the processing after exposure.

Carbon 

The first carbon process was patented in 1855 and was favored by the Pictorialist movement (~1860s – early 20th century).  The carbon process relies on the light sensitivity of gelatin when mixed with ammonium or potassium dichromate.  Carbon prints can render an extraordinary tonal range with detail in the deepest shadows and brightest highlights. Carbon tissue is made by applying a gelatin/pigment mixture on paper.  Once dried it is sensitized with a dichromate solution which must again dry before being exposed under UV light.  The exposed tissue is briefly soaked in water and sandwiched with a final support paper.  After ~20 minutes the sandwich is placed in warm water, the tissue is removed and the final image remains on the final support paper.

Cyanotype

Invented in 1842 by Sir John Herschel, this process produces blue images based on the light sensitivity of certain iron salts.  This is the simplest of the processes to learn, and produces delicate images in Prussian blue that can be toned to other colors (e.g. brown) and printed on a variety of media including paper, cloth, glass, ceramics, etc. Cyanotype solution is made with ferric ammonium citrate and potassium ferricyanide.  This solution is brushed onto paper to sensitize.  After exposing under UV light, it is washed for 20 minutes in water.

Gum Bichromate

Developed in 1858, this is another ‘dichromate’ process, which uses gum arabic instead of the gelatin used in the carbon process.  Gum prints can be made in a single or in multiple colors and offer exceptional control of the final image.  A dichromate solution is mixed with a watercolor and gum arabic which is then applied to paper.  Once dried it is exposed under UV light and the areas where light passes through the negative become hardened.  The print is developed in water, which dissolves the unhardened areas but leaves those that are hardened, producing the image.

Oil

Introduced in 1904, this is yet another ‘dichromate’ process this one again using gelatin, in this case without any pigment added.  The Oil process can produce a range of tones and moods, from lithographic accuracy to  atmospheric, painterly images. The paper is coated with gelatin, dried, then sensitized with a dichromate.  Once dry it is exposed under a UV light which hardens the areas where light passes through the negative.  The ‘matrix’ is then washed to remove the dichromate and dried. The matrix is then re-wet.  Image areas that received light – i.e. the hardened areas – can no longer absorb water.  However unexposed areas do absorb water.  An oil-based ink is then applied.  The ink is repelled by the wet areas, but sticks to the hardened areas, producing an image. 

Platinum/Palladium

Introduced in 1873, this is a ‘ferric’ process related to cyanotype which relies on the light sensitivity of iron salts.  In this case the final image is rendered in platinum or palladium.  The process gives the artist considerable control over image hue, contrast, and depth.  The platinum print is created by UV light converting Ferric oxalate it to its ferrous state. Platinum salts bond to the iron during this process.  When developed out with sodium or ammonium citrate, platinum salts are reduced to metallic platinum to create the image.  Residual iron is removed in a series of fixing baths leaving an image of metallic platinum embedded in paper.  Finally the print is washed for 30 minutes.

Salted Paper

Announced by Henry Fox Talbot in 1839, the salted paper process vies with the daguerreotype for the title of the first photographic process.  Salted paper prints are distinguished by a softer tonal range and delicate rendering of the image.  In modern practice, paper is ‘salted’ with sodium citrate and ammonium chloride and, once dry, sensitized with silver nitrate.  Once exposed under a UV light, the print is rinsed in water, toned (usually), fixed in a sodium thiosulfate solution, and finally washed.

Tintype (Wet Plate Collodion)

This was the dominant photographic process from the 1850s-70s, which includes the U.S. Civil War.  Tintype studios were among the first to offer affordable photographic portraits, setting in motion photography’s relentless drive to transform society.  To make a tintype, a collodion solution is ‘flowed’ onto a plate.  The coated plate is plunged into a bath of silver nitrate, where it becomes light sensitive.  The sensitized plate is put in a plate holder and rushed to the camera, which has previously been setup for the photo.  Following the camera exposure, which can take seconds to minutes, the plate is returned to the darkroom where it is developed before it has a chance to dry.  The resulting image is one-of-a-kind.

Ziatype

Named for the Anasazi sun symbol, the Ziatype is a palladium Printing Out Process (POP) based on the work of Pizzighelli.  The process requires no developer, as the image forms completely during exposure. Introduced by Richard Sullivan in 1997, the Ziatype is suitable for printing a wide range of negatives due to it’s self-masking POP nature. Ziatype allows for a wide variety of colors (pink, purple, blue, some green) and tones (warm or cool) that can be achieved through chemical controls and humidity levels and it is easy to tell by inspection when the exposure is ready.