[alt-photo] Re: On working with hydrochloric acid

[etienne garbaux]

Marek wrote:

>I have tried oxalic acid with FA hot press and was never 100% happy 
>with my workflow. The hydrochloric acid treated sot press was a wow! 
>moment for me.

There is a simple explanation for your poor results with oxalic acid.

Oxalic acid is wonderful for many things, but it is not a good choice 
for removing the calcium or magnesium carbonate from buffered 
paper.  Calcium oxalate is even less soluble in water than the 
calcium carbonate you are trying to remove!  So, to the extent that 
it reacts with the carbonate, it leaves calcium oxalate in the paper 
rather than allowing it to wash out (and to the extent that it 
doesn't react, it leaves the original carbonate in the 
paper).  Magnesium oxalate is slightly more soluble than magnesium 
carbonate, but not enough to make any practical difference.  Oxalates 
are also more than moderately toxic.

On the other hand, calcium (and magnesium) acetate, chloride, 
formate, and nitrate are all hundreds to thousands of times more 
soluble in water than the corresponding carbonates, meaning that 
acetic, hydrochloric, formic, and nitric acids are all effective in 
solubilizing the buffering agents in paper, allowing them to be 
washed out and not leave insoluble products behind.  Citric acid will 
also work, but may require more time and multiple fresh baths if the 
buffering agent is calcium carbonate.

Best regards,

etienne