Attempt at duotoning a cyanotype with gum

Oh hum!  The theory was that I would take one of my cyanotype prints ( see Cyanotype - 28/12/2010 below) and, retaining the cyanotype blue in the shadows, colour the highlights with a purplish red.

So much for theory!  Despite pre-sizing the paper with a coating of uncoloured gum, and then using a very diluted pink and pale blue watercolour gum mix for the colour layer, I still have red staining across the whole image, shadows and highlights.

I suspect that my 'rose' watercolour pigment bites too well on the watercolour paper I am using and gets embedded in the paper during the washing phase.

Gum over Cyanotype

The basic technique is to coat the paper with a coloured gum arabic solution which has been sensitised with potassium dichromate solution.  Dichromates are not themselves sensitive to light but when mixed with some organic substances makes the mixture harden when exposed to ultra-violet light becoming nearly insoluble in water.  Emulsions made from gum arabic, potassium dichromate and colouring pigments can therefore be used to make a photographic positive from a negative by contact printing.

After exposure, the print is floated face-down in water to dissolve away the unhardened gum and wash out the unused dichromate from the paper.  What is left is the positive image exposed by the light.  This process is called by its traditional name of Gum Bichromate printing  (BIchromate was the historical name of what was later called DIchromate, but is now called Potassium Dichromate (VI) )

In this example, the cyanotype was produced from a negative, but the gum layer was exposed through a positive film, as I wanted to expose the highlights.  Ideally, the result should have been lilac/purplish tones through the highlight areas of the print.

Oh well - better luck next time!