Thank you very much!
Yes, I etched it in ferric chloride, but the result is not very good - it still looks dirty, the contrast is small, it is possible that I kept it in the solution for too long, about a minute.
Then I looked more and found this discussion.
http://www.knifenetwork.com/forum/sh...ad.php?t=57076
That site is fortunately still working
https://www.nontoxicprint.com/etchcopperandbrass.htm
There is generally a very good database on many types of etching
https://www.nontoxicprint.com/etchin...ioncontent.htm
I added the required amount of citric acid to the solution (4 liters ferric chloride solution + 1 liter hot water mixed with 300ml citric acid powder (ratio 3:1) = Edinburgh Etch) and heated it to 70 degrees Celsius, held for about 3 seconds, result on the left, on the right with an ordinary solution of ~ 25 degrees Celsius for 1 minute
