Glossary

The words of alternative photography

A short, honest dictionary of the processes, chemistry and gestures behind non-toxic alternative printing.

Aquaprint: Vision Picturale’s reformulation of gum bichromate, dichromate-free (N°03 sensitizer). Same Pictorialist look — matte surface, pigment grain, layers — without any CMR salt.
Bichromate / Dichromate: Salt (potassium or ammonium) that makes gelatin or gum light-sensitive. The hexavalent chromium it contains is classified CMR (carcinogenic, mutagenic). It is the shared toxic point of historical gum, carbon, bromoil and resinotype.
Bromoil: Process (1907) in which a gelatin matrix is inked by hand, with a brush, using greasy ink. In its classic form the matrix is obtained by bleaching a silver print in a bichromate bath.
Carbon print: Pigment process (Poitevin, 1855) in which a gelatin loaded with carbon pigment is sensitized with dichromate, exposed and developed in water. It is the most permanent print in photography.
CMR: Carcinogenic, Mutagenic, Reprotoxic — the classification of the most hazardous substances for health. The dichromate used in historical pigment processes is classified CMR.
Cyanotype: Iron-based process (Herschel, 1842) producing a Prussian blue, developed in water. Low-toxicity from the start, it is the gateway to alternative photography.
Water development: Step where the image appears through plain washing: areas not hardened by light dissolve, exposed areas remain. Specific to pigment processes (gum, carbon).
Gelatin: Binder that carries the pigment and hardens under light once sensitized. Vision Picturale uses its N°05 gelatin for carbon, bromoil and resinotype.
Gum bichromate: Pigment process (Poitevin 1855, revealed by Demachy in the 1890s) mixing pigment, gum arabic and dichromate. A drawing-like, painterly look. Reformulated dichromate-free by Vision Picturale as Aquaprint.
Exposure: Exposing the sensitized paper to UV, through a negative, until the latent image forms. In sunlight or under a UV unit such as the Luminograph.
Digital negative: A negative printed on transparency from a file, placed in contact with the paper for exposure. It replaces the large-format film negative.
Permanence (archival): A print’s ability to resist degradation over time. Pigment processes (carbon, gum) are among the most permanent, because the pigment is chemically inert.
Pictorialism: Movement (late 19th–early 20th c.) that claimed photography as art through hand processes (gum, carbon, bromoil) yielding images close to drawing or painting.
Pigment: Powdered colouring matter that forms the image in pigment processes. Inert and stable, it ensures the print’s permanence (unlike silver or dyes).
Pigment process: Family of processes where the image is made of pigment held in a binder hardened by light (gum, carbon, bromoil, resinotype). Historically sensitized with dichromate.
Four-colour: Colour gum obtained by superimposing several pigmented passes (cyan, magenta, yellow, black). Vision Picturale practises it dichromate-free.
Resinotype: Pigment process (Namias, 1920s) that seals the pigment in a hardened resin: a vitrified print, halfway between photograph and enamel. Vision Picturale version without dichromate or solvent.
Sanguine: A gum variant in red-brown pigment, evoking the red chalk of Old Master drawings. Made famous by Demachy’s red gums.
Sensitizer: Agent that makes a support light-sensitive. Where historical processes used dichromate (CMR), Vision Picturale uses its N°03, dichromate-free.
UV (radiation): Ultraviolet radiation that drives the exposure of alternative processes. Provided by the sun or a dedicated UV unit (Luminograph).
Contact printing: The printing mode of every alternative process: the negative, printed at the final size of the image, sits directly on the sensitized paper during UV exposure, with no enlargement. Contact must be perfect — a glass sheet, a hinged contact printing frame, or the Luminograph's vacuum system — because any gap between film and sensitized coating creates diffusion blur that softens detail.
Coating: Brushing the light-sensitive chemistry onto the paper: a flat, hake or foam brush lays a uniform, matte, even coat, smoothed with a foam roller for gum processes. For an A4 cyanotype, coating takes about five minutes, followed by twenty to forty-five minutes of drying in the dark depending on the process. Nitrile gloves are recommended: the chemistry leaves lasting stains on skin.
Sizing: The treatment that controls a paper's absorbency before coating. Over-sized paper prevents the chemistry from soaking in and yields a pale print; under-sized paper absorbs everything and gives a flat, low-contrast image. For Aquaprint, the 640 gsm cotton sheet is scalded then sized with four coats of transparent gesso. Bergger COT 320 offers balanced, ready-to-use sizing for cyanotype.
Clearing (warm-water development): The reveal step of pigment processes: immersed in warm water at 40°C, the print sheds the unexposed gum or gelatin, which dissolves, while the UV-hardened pigment stays in place. Allow five to ten minutes for an Aquaprint layer, fifteen to twenty minutes for a carbon transfer. Clearing water goes down standard household drains, with no prior neutralization.
Optical brightening agents (OBA): Fluorescent whitening compounds added to many papers to make them appear whiter. They react poorly with the iron salts of cyanotype, causing premature yellowing and loss of contrast. For stable prints, Vision Picturale prescribes OBA-free 100% cotton paper — its 640 gsm reference stock, or Bergger COT 320 — keeping the highlights free of yellowing over the decades.
Prussian blue: The insoluble blue pigment that forms the image of a cyanotype. It appears during UV-A exposure through the photochemical reduction of ferric iron (iron III) to ferrous iron (iron II); five minutes of rinsing under running water then removes residual salts. Chemically stable — around 100 to 150 years in normal conditions — it remains sensitive to alkaline environments: non-neutral boards and basic mats fade it, and a bath of water lightly acidified with lemon juice can restore it.
Photogram: An image made without a negative: objects — typically botanical specimens — are placed directly on the sensitized paper, pressed under a sheet of glass, then exposed to UV. Areas shielded by the object dissolve during rinsing and reveal the white of the paper; exposed areas fix the image — a deep Prussian blue in cyanotype. Feasible from your very first print, with no digital negative and no printer: sensitized paper and a UV source are enough.
dMax (maximum density): The maximum optical density of a print's black, measured with a densitometer: the higher the dMax, the deeper the black. The carbon process holds the highest dMax of all alternative processes, above 2.1, thanks to a transferred pigment layer several tens of microns thick; resinotype reaches about 1.7. A high dMax visually saturates subjects with a wide tonal range — chiaroscuro portraits, high-contrast landscapes.
Linearization curve: A correction curve applied to the digital negative to compensate for each process's own tonal response — and each pigment's in four-color work. Without it, the grays of the file do not translate faithfully into the print. It can be built without a densitometer using Calibration Flow: a 25-patch test target is exposed, measured with a smartphone camera, and the corrective curve exports as .acv (Photoshop) or .quad (QuadToneRIP).
Gum arabic: The binder that carries the pigment in gum bichromate and Aquaprint: mixed with pigment and a sensitizer, it hardens under UV and becomes insoluble, while unexposed areas dissolve during clearing in warm water at 40°C. It was never the toxic point of the historical process — the dichromate was. Vision Picturale supplies it pre-sensitized as VP N°04, to be melted in a bain-marie.
365 nm UV-A: The reference wavelength of alternative-process exposure. Cyanotype, gum, Van Dyke and carbon chemistries sensitize in UV-A (320 to 400 nm), with peak efficiency around 365 to 375 nm. 365 nm UV-A LEDs are today's reference artificial source: stable spectrum, low heat, long lifespan. Luminograph exposure units emit at 365 nm — 5 to 15 minutes of exposure for a cyanotype at 30 cm.
Registration: The precise alignment of a color print's successive layers on their marks, so that each separation negative lands exactly on the previous one. Four-color CMYK Aquaprint demands millimetre-level registration across four negatives; Couleur Profonde carbon requires half-millimetre precision between its three transfers, using punched or drawn registration marks. Monochrome variants, in one or two layers, tolerate approximate registration — which is what makes them accessible.