The 10" thick American-made piece is very slightly stressed, so that's why it will probably never make a premium mirror. Someone polished it out, but then gave up...maybe the chip on the back discouraged him? But who knows? If you figure it to a smooth sphere and test it under changing temps on the bench, you might see the impact of strain..or maybe it will be too slight to make a difference.
Once you can see that strain, by polarization methods, you'll have a good measure for the Zhumell. Also the figure on that practice piece is so horrible that you will learn a lot by making a 6-8" lap and smoothing and working interior zones, etc.
Like you say, the figure on the Zhumell is not too bad. I know of a 12" that has a good figure, but wicked rough. So by practicing on the interior zones on the practice disk, you'll get a good handle on how to control and smooth a surface. Then the Zhumell will respond well to your work.
Right, maybe the Zhumell is strained? You'll know how to determine that once you see what strain looks like. If the Zhumell is not worse than the practice piece, you're good.
Too bad I didn't think to send you some Barnesite.
I attach a stolen pic from Lockwood's site showing the type of strain pattern that I saw on the practice blank, though your practice blank is much LESS severe than that shown in Lockwoods example.
Edited by ccaissie, 17 April 2018 - 08:46 AM.