dear mark
For better effect you ought to use ferric ammonium citrate (green)
I use this formula :
Solution A
ferric ammonium citrate (green) 68 gr
oxalic acid 1.3 gr
destilled water to make 250 cc
Solution B
pottasium ferricyanide 23 gr
oxalic acid 1,3 gr
dichromate ammonium 0.5 gr
destilled water to make 250 cc
for a 13X18 sheet mix 4cc solution A + 4cc solution B
Juan Carlos Vecino Quiza
C/ miguel de Unamuno
33010 Oviedo
Spain
quiza@lander.es