Re: [ATM] Glass Blank Question

[Guy Brandenburg <gfbrandenburg@yahoo.com>]

Jason,

You certainly could make a mirror out of it, and it
will grind and polish quite easily, but it will be a
little tough when you get to the figuring part unless
you have a LOT of patience. Why? Because crown glass
has a much higher coefficient of thermal expansion
than Pyrex or its cousins. Why does that matter?
Because every time you do some figuring, you are
creating a lot of friction of the glass against the
pitch lap, and thus you are creating a lot of heat.
And because crown glass has a relatively high CTE,
this means that the parts that got 'worked' will heat
up and will expand a good bit, unlike the parts that
did not get 'worked'. This means that in the last
parts of figuring, which members of this list can
attest often seem to go on even longer than the final
5 minutes of a professional basketball game or the
last 5 minutes of an NFL game, and where you are
trying to remove just a few millionths of an inch of
glass from particular parts of your mirror, you are
simply going to have to wait about 5 hours for the
glass to come to thermal equilibrium with the room you
are working in. Otherwise, when you are examining the
glass via either a Ronchi or a Foucault or a Couder
test or a Ross null or a Dall null or whatever, your
results will lead you seriously astray. If you put the
mirror immediately on the stand after you have done
your 2 to 5 minutes of figuring, you will think that
you have seriously degraded the shape of the mirror,
when that might not be the case at all.

Some folks on this list say that you can immerse the
glass in a bath of clean water that has already
reached thermal equilibrium with the room you are
working in, and that that immersion will speed up the
process. That may be true, but where I work, that's
not an option.

So - can you make a mirror with this glass? Yes, but
you just have to be very PATIENT.

This is not just from my reading of the classics by
Texereau and others, but it's also my personal
experience and that of others on this list.

Now, if you happen to know its index of refraction and
its abbe number (or coefficient of dispersion), AND
you happen to know that it does not have any bubbles
or strain, then somebody on this list might be
seriously interested in taking this possible lens off
your hands so that they could combine it with a flint
element to create a wonderful achromatic refractor.

Oh- concerning strain: get yourself 2 sheets of
polarized plastic (Edmund 'scientific' and Surplus
Shed carry them) and put one on one side of the glass,
and one on the other side, in such a way as the
polarization lines are at 90 degrees to each other.
The glass goes in-between like the meat or cheese or
whatever in a sandwich. If you see a cross figure (+)
appear, then your blank has a serious case of strain,
and you may as well ignore the glass altogether. I saw
one of those crosses appear the other day. Someone had
found some very thick cylinders of glass that are used
on the Metro Rail platforms in Washington DC's subway
system, and had started grinding them to make a
mirror. I had never seen the strain's cross before; on
these blanks, it was as plain as if it were the cross
on the side of a World War 2 American ambulance.

Guy

--- Jason Lehmann <jtravel@bellsouth.net> wrote:

> Hi All,
> 
>  
> 
> I have a 15"  diameter by 1" thick piece of glass
> with a density of
> 2.685g/cm3.  It looks like this disk is in the crown
> glass group.  Any ideas
> on what to do with this blank?  I would like to make
> a mirror out of it but
> I read somewhere that crown is not the best
> substrate for a mirror.  Any
> help would be appreciated.


Guy  Brandenburg
Washington, DC
My home page:
http://home.earthlink.net/~gfbranden/GFB_Home_Page.html
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