Sidero-Cupreatype / DMDTC turn

REF #56

Ferric Ammonium Citrate [NH4]{y}[Fe{x}(C6H4O7)] sol. 20%

Potassium Ferricyanid K3[Fe(CN)6] sol. 8%

Ammonium copper citrate (NH₄)₂Cu(C₆H₅O₇)₂ sol. 20%

Sodium Dimethyl Dithiocarbamate (DMDTC) C3H6NNaS2 sol. 0.1%

Make a normal sidero-cupreatype (sheet #3) with:
- an exposure time of 8 minutes
- a rinse time of 30+30 seconds
DMDTC bath for 30 seconds