Citric acid Pickle

I hit the delete key too fast after I read the message on Citric acid
pickle. By this did you mean Oxalic acid available at pharmacies?

What mixture ratios do you use?

Does it always have to be heated to be effective?

Any advise about sequential picklings of different metals, copper,
brass, gold, silver?

I appreciate all of you who maintain and contribute to this list. If
you ever wonder if you make a difference, you most certainly,
positively DO. Thanks, Connie