Hi,I really don't know metric.,Here they sell metal by gauge or weight.20 ga is about as thick as 5 or 6 sheets of printer paper .It's hard to bend by hand . It has to be thick enough to take a good beating without cracking.I found this on the following site if it's any help.Thanks .32 Mil/20 Gauge (.032") 20 gauge copper sheetinghttps://basiccopper.com/thicknessguide.html
Copper Wall ArtView Instructable »
Granules is fine.If the copper is real thin and your worried about it bending .Try hammering one side with a rubber mallet first.
They just get darker so if you don"t like them shiny just wait a day.
I really don't know.Most instructions for paper use Ferric chloride which is milder.I use this because for transfer paper you need a printer with toner and I have an inkjet.Give it a try and see what happens.Let me know .
Copper Etched EarringsView Instructable »