The first images are of aluminium sheet, impasto resist with a little magenta paint for pattern recognition when dry; copper sulfate and salt etch for about half an hour. This solution is also suitable for steel and zinc. The last picture is a brass shim sheet etched in ferric cloride, a chemical that can be used for copper too. I have made my marks with a paint brush.
A brilliant website for those interested in etching with the mildest possible chemicals is www.nontoxicprint.com
