There are machines that will flatten a piece of plate if it warps during annealing or rolling.
These are quite expensive.
A cost effective solution is to visit your local garage and ask for some old engine valves.
Be sure to ask for ones that are not sodium filled, which some valves are to make them lighter.
One wants the solid plain valves.
Here are two valves that I use.
The small one comes from a car and the big one came from a tractor engine
I used an angle grinder to cut them shorter.
One thing that MUST be done it to anneal the tip of the valve.
The steel is hardened and it will chip and send out a sliver of metal at high velocity when it is hammered.
Annealing solves this problem.
Simply heat the tip of the valve to bright orange and allow to air cool a you are done.
Also check that the valve has a flat bottom. Some valves have a concave bottom.
This is how to flatten silver plate on a metal block and hitting the valve with a hammer.
Here I am using a valve to make a ring into a domed shape in my Making a Domed Ring tutorial.
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This video shows how to cast silver plate from silver scrap and bench sweeps.
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