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I have several Cuprotypes several years old with no blue stains.
If any iron is left in the paper, a Cyanotype blue stain will result. After developing in the thiocyanate/citric acid solution, wash well in at least 3 trays of citric acid water (1/2 tsp/liter) to remove all residual iron. Then tone with the potassium Ferricyanide. You can skip the Ferricyanide toner and use the Selenium/Sulfide toner at this point for a dark brown.
If printing a Cyanotype/Cuprotype combo, print the Cyanotype first to avoid changing the Cuprotype to a Cyanotype.
You can selectively change parts of a Cuprotype to a Cyanotype by using a small brush with Ferric Ammonium Sulfate solution.
On Jun 1, 2021, at 7:22 AM, Niranjan Patel via groups.io <nirpat89@...> wrote: