To flatten gold and silver sheet, I use a method taught to me when I was an apprentice, it works perfectly and was the system used by silversmiths who made boxes and photo frames at the company where I was trained.
First anneal the sheet, then grip the sheet between two sheets of 1/8 " thick flat stock steel sheet, gripped tightly using steel G clamps, then anneal the steel sheets until they are dull red and let it cool without quenching. Then when you undo the clamps you will have a nice flat sheet of precious metal. This system was also used when the silversmiths wanted to large flatten silver trays. As i only flatten small pieces I bought what is called gauge plate here in the UK, like this product. And these are the G clamps I use.
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