Just remember that Ferric chloride is used for copper, and the
Ferric nitrate for silver. Use the Ferric chloride as you already
it in solution form. No need to dilute.
Ferric nitrate is mixed with water until it is the color of strong
tea. I just pour out the amount of water I will be needing, then keep
adding the ferric nitrate until it gets to the color of strong tea.
If it seems to be too weak, I just add some more crystals.
You might check through the Orchid Archives as there is a wealth of
on using both of these etching mordants.