Re: [ATM] The results of the glass casting run.

["Donald Good" <donald.good@comcast.net>]

What size blanks are they?  Are they for a project you are working on?

To protect your pump, pump the air out before the heat is turned on and put
a simple water cooled copper coil between the chamber and pump to handle the
rest.  You may need a water trap, too, to handle all that extracted water.

Also, due to the viscosity of molten glass, all the voids may not collapse
while the kiln is under vacuum, especially near the top surface (no pressure
to force the collapse).  You might consider releasing the vacuum some time
after the glass has melted and most bubbles eliminated, so that atmospheric
pressure can force the collapse of the "vacuum" bubbles.  And since the kiln
is in a chamber, you have the choice of what gas to introduce.  I don't know
if there is an optimum gas for glass, but nitrogen would be inexpensive and
better than air.  Cooldown would also be aided by allowing convection heat
transfer.  You could also circulate the nitrogen through an old steam
radiator to aid and control the cooldown more precisely.

-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf Of
Thomas Janstrom
Sent: Wednesday, July 12, 2006 8:44 AM
To: 'James Lerch'; 'VacuumX'; 'ATM list'; 'atm_free'
Subject: Re: [ATM] The results of the glass casting run.

Thank you.

Yes they are secondary mirror blanks, although these aren't quite up to
snuff, they will do for proof of concept.

I have discussed building a "vacuum kiln" with my refractory supplier and he
(no surprises there) is game, but I think that some of the problems are
(well not insurmountable) going to be difficult to deal with, one thing is
that vacuum (rotary vane type anyway) pumps tend not to like high temp air
going through them. However like I said not an insurmountable problem, just
a mater of engineering really.

I am considering it for my next gen' kiln, as I think it will make a big
difference, at least in dealing with trapped water in the castable mold
materials, most of the ramping up period is given over to exhausting this
water.... As an example this casting run had to get rid of some 2.8L of
water before letting the molds get to more than 200'C.

Cheers, Thomas.

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