Introduction to Traditional Classical Photographic Printing Techniques: Cyanotype Preparation, UV Exposure, and Water Development Practice

1. The chemical cornerstone of cyanotype: iron salt photosensitivity mechanism

Cyanotype was invented by British astronomer Sir John Herschel in 1842. It does not rely on expensive and scarce metallic silver, but utilizes the photochemical reaction in which ferric ions are reduced to ferrous ions under ultraviolet irradiation:

• Classic traditional formula (two-liquid method):
• Liquid A: 20% Green Ferric Ammonium Citrate (green crystals are much more sensitive to light than brown), dissolved in 100ml distilled water;
• Liquid B: 8% potassium ferricyanide (hemat salt), dissolved in 100ml distilled water.
• Save in the dark: Liquid A and B are stored separately in the dark, with a shelf life of several years; before use, mix equal parts in a 1:1 ratio in a darkroom with a weak tungsten lamp, and mix them now.

2. Coating and drying techniques for carrier paper

1. Paper selection: You must choose pure cotton, heavy weight (above 300g/m²), neutral acid-free watercolor paper (such as French Arches Platine or Saunders Waterford). If the paper contains alkaline calcium carbonate filler, it will react with iron ions, causing the blue color to quickly fade to gray.
2. Sponge brush for even coating: Use a wide and high-density sponge brush (it is strictly prohibited to use brushes with metal hoops, as metal iron and copper will contaminate the potion). Dip in a small amount of photosensitive liquid, brush it once vertically, and then brush it crosswise horizontally to ensure that the coating is even and smooth without water accumulation.
3. Avoid light and air dry: Clamp the coated wet paper in an absolutely dark, well-ventilated darkroom or wardrobe and let it dry naturally for at least 2 hours (you can use a hair dryer on the cold wind setting to speed it up, and hot air is strictly prohibited to damage the film).

3. Sunlight exposure, water development and hydrogen peroxide for brightening

• Pressure frame exposure: Place the large-size black-and-white reverse positive film or large-scale digital negative film (Digital Negative) emulsion side down against the photosensitive paper, and use heavy-duty thick glass to clamp and lock it (Contact Print).
• Ultraviolet (UV) exposure time: Under direct sunlight at noon, the exposure time is usually 3 to 6 minutes; under professional UV exposure box, it is about 8-12 minutes. Visually observe that the unobstructed area gradually changes from yellow-green to bronze or dark gray-green, which is when the exposure is completed.
• Pure water washing and development: Immerse the exposed paper in flowing room temperature tap water and shake gently. The yellow photosensitive liquid that has not been exposed to light quickly dissolves and runs away, and the light-receiving area instantly generates water-insoluble, high-purity Prussian Blue/ferrocyanide in the water! Rinse under running water for 10-15 minutes until the white highlights are completely free of yellow background.
• Hydrogen peroxide instant oxidation and brightening magic: Add a few drops of 3% medical hydrogen peroxide (hydrogen peroxide H₂O₂) to the last basin of water, and the originally slightly dull cyan will instantly and spontaneously oxidize into a deep and shocking royal ultramarine deep blue within 2 seconds!

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