Step One: Platinotype
In Photoshop, the darker and midtones are separated out and a digital negative for the platinotype is created.
Each digital negative layer must be the exact same size and the size of the resulting print. Each layer, therefore, needs to be aligned/registerd some way to keep the layers exactly in alignment.
This layer was a lightened version of the “black” channel of an image made from the CYMK model. It was printed in Na2 Platinum/Palladium on Hahnemuhle Platinum Rag mounted on thin aluminum sheeting.
Step Two: Cyanotype
Next up, the cyan tones are laid down using the cyanotype process. There are other ways of doing this, like using gum bichromate like the next layers, but cyanotype is straight-forward and sits well over Platinum/Palladium.
A second negative for the cyan channel was used.
Step Three: Yellow Gum Bichromate
On this layer, a third negative is used for the yellow tones. The yellow is provided by watercolor paint (Mayan Yellow) mixed with gum arabic and potassium dichromate. gum arabic hardens, but washes away, so the potassium dichromate is a light sensitive chemical that hardens where the light hits the pigmented gum arabic, allowing it to selectively wash away depending on how much light gets through different sections of the negative.
Of course, yellow and cyan make green.
Step Four: Magenta Gum Bichromate
The final layer is the same approach as the yellow, but this time using a different magenta negative and “Opera” magenta paint. Of course magenta and blue make purple and magenta and yellow make orange.
Potassium dichromate is a serious chemical, handle with care and proper protective gear and ventilation.