Cyanotype

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Dictyota dichotoma by Anna Atkins
Sir John FW Herschel: Lady with a harp , 1842

The cyanotype , also known as iron blue printing , is an old photographic fine printing process with blue tones.

history

The English scientist and astronomer John Herschel developed this method in 1842 . The cyanotype was the third method after the daguerreotype and calotype for producing stable photographic images. It is an iron , not silver- based, process that is otherwise used in traditional photographic printing (and the processes previously invented).

Anna Atkins , a British naturalist, made this photographic technique known through her books in which she documented ferns and other plants with cyanotypes. This early application made her the first female photographer.

The cyanotype had been widely used for reproducing plans since 1870. The reproduction was done in-house, as was the sensitization of the paper, before light-sensitive papers came on the market in Paris in 1876 (Marion Cie.). The exposure is done with sunlight. Electric exposure devices were introduced around 1895. It was not until the 1920s that fully automatic machines were available that carried out a complete process (exposure, fixing , drying). The cyanotype was then replaced by the dry working diazotype (Ozalid® copy) as a method of drawing copying before the Second World War .

Current artists also use the cyanotype for their works, such as Barbara Kasten and Kirsten Heuschen.

Procedure

Absorbent paper, canvas, nettle or cotton fabric is photosensitized with a solution of green ammonium iron (III) citrate (20 grams to 100 ml of distilled water ) and potassium hexacyanidoferrate (III) (8 grams to 100 ml of distilled water) and then dried. Even the mixed solutions are sensitive to light.

The exposure takes place under a shadow-casting object as a photogram (object, plant, etc.) or with a contact negative through UV or sunlight. Construction spotlights, facial tanners, solariums or daylight projectors are also suitable for exposure. The decisive factor is a high proportion of UV in the light source. On a sunny day, the exposure time with daylight is around five to 30 minutes - depending on the time of day and time of year. When projecting with an overhead projector, exposure times of 10–15 hours are necessary. In the exposed areas, the iron compound becomes bivalent and insoluble in water - the dye Prussian blue , Fe 4 [Fe (CN) 6 ] 3 is formed :

Water-insoluble Prussian blue forms on the exposed parts, the unexposed parts remain water-soluble and can be washed out under running water (no development takes place).

The contrast can be increased by bathing in 0.3% hydrogen peroxide solution or 1% potassium dichromate solution . The color can be changed ( toned ) by bathing in tannin , oolong tea or pyrogallol , for example .

Cyanotype, 50 × 70 cm, projection exposure on canvas, Wolfgang Autenrieth 2019

variants

In the original Herschel process, the photosensitive paper layer consisted of potassium hexacyanidoferrate (II) (yellow blood liquor salt) and ammonium iron (III) citrate. Potassium hexacyanidoferrate (III) (red blood liquor salt) was later used.

A more modern variant of the cyanotype comes from the British chemist Mike Ware. He uses potassium hexacyanidoferrate (III) and ammonium trioxalatoferrate (III) instead of ammonium iron (III) citrate.

The cyanotype is sometimes confused with the diazotype, which was used in architectural offices.

See also

literature

  • John FW Herschel: On the Action of the Rays of the Solar Spectrum on Vegetable Colors, and on Some New Photographic Processes. In: Philosophical Transactions of the Royal Society of London, Vol. 132 (1842), pp. 181-214.
  • Tony Worobiec, Ray Spence: Workshop Monochrome and Other Art Printing Techniques. Augustus-Verlag, Munich 2000, ISBN 3-8043-5140-9 .
  • Wolfgang Autenrieth: New and old techniques of etching and fine printing. From witch's meal and dragon's blood to the photopolymer layer. Tips, tricks, instructions and recipes from five centuries - An alchemistic workshop book 6th edition, Krauchenwies 2010, ISBN 978-3-00-035619-3 ( table of contents , (→ excerpts online) )

Web links

Individual evidence

  1. ↑ Barbara Kasten - Amphora.
  2. ↑ Kirsten Heuschen.
  3. ^ The New Cyanotype Process.