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A photographic print with an image of pigment bound to paper in a hardened colloid.

It is similar to Gum bichromate, but uses an iron salt instead of dichromate as the light sensitive agent and polyvinyl alcohol as the colloid instead of gum Arabic. I updated the process below.

See the Safety Chapter on this site.

MATERIALS: Ferric Nitrate Nona-hydrate, Ferric Ammonium Oxalate tri-hydrate, Polyvinyl Alcohol, Thymol & alcohol (only if making your own PVOH solution), Pigment, Surfactant (Polysorbate 20), 1,3-propanediol, Paper, Citric Acid, 40% Glyoxal Solution, Sodium Sulfate anhydrous powder, Gamblin PVA Size, Distilled water, Small mortar & pestle

PAPER: Arches Platine, Hahnemuhle Platinum Rag, or hotpress watercolor. Buffered papers interfere and need acidification to work. See below to acidify paper. PAPER SIZING: Gamblin PVA Size, diluted with distilled water 1:2, 1:1, or full strength as needed to prevent pigment stains in the highlights. Brush on and dry. Two coats of 1:1 dilution, hung by a corner to dry 1st coat, and 2nd coat hung by opposite corner to dry gives a more even coating. SAFELIGHT: yellow bulb or 40 watt tungsten. Avoid fluorescent or daylight. PIGMENT: dry pigment (Sennelier, Gamblin, etc), or tube watercolor. Glycerin, sugar, honey and other ingredients in tube watercolor may interfere, depending on the brand. SURFACTANT: Polysorbate 20 (Tween 20) SENSITIZER: dissolve 24 grams of Ferric Nitrate nona-hydrate and 30 grams of Ferric Ammonium Oxalate tri-hydrate in 60 ml distilled water. Bring the volume to 100 ml with distilled water and store in a brown glass bottle. COLLOID: 7% Polyvinyl Alcohol (Elvanol 50-42 or similar) Solution, or commercially prepared Selvol 09-523 solution (Sekisui Chemicals). See below to make a PVA solution. PLASTICIZER: 1,3-propanediol CLEARING/HARDENING SOLUTION: dissolve 120 grams anhydrous Sodium Sulfate (not sulfite), 50 grams of Citric Acid, and 100 ml of 40% Glyoxal solution in 800 ml distilled water. When dissolved bring volume to 1000 ml with distilled water COATING: add 0.2 gram pigment (more or less) and 2 drops Surfactant to a mortar. Add 1 ml 1,3-propanediol very slowly while mixing with the pestle to wet the pigment and prevent dry pockets. Slowly add 10 ml Colloid while mixing. Slowly add 1 ml Sensitizer while mixing. Mix well to suspend the pigment in evenly. Let stand a few minutes for bubbles to rise. Filter through a fine mesh sieve or butter muslin to remove color clumps if needed.

7% POLYVINYL ALCOHOL SOLUTION: suspend 70 grams PVA powder in 900 ml cool distilled water in a glass or metal container by slowly adding the powder to the water while stirring. Put the container in a larger pot of water with a folded cotton kitchen towel in the bottom and heat to barely simmering and stir occasionally with a sturdy spoon to break up the gum on the bottom. It takes a few hours to dissolve. Cool and bring the volume to 1000 ml with distilled water and filter through butter muslin or a fine mesh sieve. Dissolve 1 gram Thymol in 20 ml 95% ethyl alcohol and add as a preservative.

ACIDIFYING PAPER: soak paper in white distilled vinegar, 2% citric acid, or 2% sulfamic acid for 5 minutes with occasional agitation. Wash in water 20 minutes to clear residual acid and dry.


Coat sized paper with a wire wound coating rod or a flat brush and dry with a fan about 30 minutes. A hairdryer on low heat can be used to remove any residual dampness. Avoid high heat. Expose through a negative to UV light in a print frame. My exposure with a homemade BL tube UV box is about 12 minutes. Develop by floating face down in a tray of water. Change to a new tray of water every 5 minutes or so and agitate gently occasionally. The last tray of water should be clear. It usually takes about 10-15 minutes to develop. At this point you can let prints dry and Clear & Harden as a batch later or go directly to the next step. CLEARING/HARDENING: Put the developed print in a flat bottom tray of Clearing/Hardening solution with occasional rocking for 30 minutes or so until highlights are white. Change to a new tray of solution every 10 minutes or so. The last tray should be clear. Hang up to dry until the next day. Wash in water to remove residual chemicals 30 minutes. The Clearing/Hardening Solution removes the yellow iron stains in the highlights and hardens the colloid so that additional layers can be added without softening layers beneath.

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