Sidero-Cupreatype / ZnSO4 toning

REF #73

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%

Inconnu sol. 2%

Make a normal sidero-cupreatype (sheet #3) with:
- 6 minutes exposure
- 30+30 seconds rinsing
ZnCl2 bath for 2 minutes
Washing 30+30+30 seconds