Why No BVC??

I'm very, very curious about this.

Why do glass pushers not use Black Vitrified Ceramic substrate for fabricating mirrors?

It seems to have very desirable characteristics, particularly if you are an amateur who is making your own optic. BVC has very low (almost 0)coefficient of expansion, it can be worked(ground) much faster than pyrex or even more exotic substrates, and it is considerably less expensive than these more commonly used glasses.
It would seem to me that amateurs would be clamouring to purchase and use BVC in thier optic making efforts,
Even Texereau makes note of it in his seminal text.

Can someone please enlighten me as to why more mirror makers do not use BVC?

Thanks in Advance - Rob

I asked this on the Zambuto Mirror Yahoo Group. Part of it is, as Herbert said, being conservative. If the current glass is working well, it will take a convincing reason to change. Carl did comment that he had seen one BVC mirror that had problems with primary ripple and astigmatism, and that they seemed to be due to the softness. He thought (although he can't be sure) that the polishing lap squares may have been biting too deep and causing the ripple. He wasn't sure about the astigmatism, but if the softness isn't uniform, that could cause it. Another ATM also reported issues with difficulty polishing due to the softness (he was getting a rougher finish and more scratches than with glass).

You can read the discussion here:
http://groups.yahoo.com/group/zambutomirrorgroup/

Although it wasn't mentioned, I would also think that a softer mirror substrate would require more support points in the mirror cell as well. If you can look up the Poisson, Modulus, and density of BVC, you can enter them into PLOP and try a few runs to see.

Jarad

I asked the local optician about BVC a while ago. If I remember correctly, there are issues that arise due to the way the material is composed layered sheets of plate glass (vs. a polycrystalline structure in most other substrates) that makes it difficult to work with. I don't remember the precise details of the conversation, but I would guess that it has to do with grinding/polishing through one layer to the next. In addition to it being soft, this makes for difficulty in finishing out a good surface. You can work with it, and it has great thermal and mechanical properties when it's done, but it takes very careful effort (read: lots more time) to do so.

Cheers,
Jason