Hopefully those reading this have seen my previous post about my Bicolor Cyanotype technique. Calling this next step "Tricolor Cyanotype" is a little misleading, because one of the color layers is in fact not cyanotype at all, but rather cuprotype. I've found that other attempts at creating a red layer don't satisfy my "purist" view on creating a fully analog process, or at least one that can be recreated using easily-obtained raw materials.
The chemistry of cuprotypes is similar to that of cyanotypes, where a solution of copper ions reacts to ultraviolet light similar to how an iron solution is used for cyanotypes. The chemistry is remarkably similar too, with ferric ammonium citrate comprising one part of the cuprotype mixture, the other part being copper sulfate. I use a third component, thiourea, but this is not strictly necessary when making cuprotypes. There are many resources available online to learn about cuprotypes, so I won't describe the process here.
Exposure for this technique is a little tricky, as the magenta/red (cuprotype) layer can very easily overpower the other two layers. Additionally, too much exposure on the final cyanotype layer can completely destroy the pretty green tones from the mix with the yellow layer. In general, exposure for the cyan layer is roughly one-third the exposure for the yellow layer. I'm experimenting with exposure for the magenta layer, the exposure above I believe was roughly equal to the exposure for the yellow layer.
Another post will follow soon where I use this technique to make a trichrome print from negatives.
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