ATM [long] Making your own blank [was Palomar Optical Supply]

["Jon Bertrand"<jonb@cirris.com>]

Clive Milne said:

>The reason I ask , is that I have a friend who has 
>recently aquired a laboratory grade precission temperature 
>controlled, gas purged kiln with a rotary base. Does anyone 
>out there have the temperature profiles for annealing 
>pyrex & plate glass.....????

Ask Richard Schwarts - I believe he is annealing plate glass and can anneal
pyrex.

>he is accumulating dozens of beer bottles to use as the base 
>glass stock..... is this material usable in this application....?

If you figure out how let Richard (and me) know.

I'll add to this some of the messages I've sent to Richard - hopefully this list
can find the magic procedure for turning bottles to light buckets:

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Here is what I _think_ I know:

Matter has three phases, gas, liquid, solid.

Solid has two types, crystalline (non-vitreous), glass (vitreous).

Crystalline has definable large and small scale structure to its atom/molecule
arrangement.

Glass has no easily definable large scale structure (lots of talk about the
structure being the same as a liquid but I see lots of Ph.D. that say 'not
quite').

Heat something to the liquid state, as it cools it reaches a temp. zone where it
begins to 'solidify' - it has two options; crystalize or freeze (glassize?).

In that temp. zone crystal formation outpaces crystal destruction - it wants to
crystalize (it wants to devitrify).

To avoid crystalization you need to suck the heat out ASAP to pull the temp
below that zone.  Then things will freeze in a non crystal state.

Anything can be made into glass following this procedure (although some things
require pulse heating with lazers etc to pull it off).

Crystal formation; If the crystal is below a certain size you have to do work to
attach another atom (grow the crystal).  Once the crystal reaches a certain size
(a kind of critical mass) it gives off energy as you add another atom (you have
to do work to remove the atom).  So there is a crystal formation activation
energy.

The activation energy is determined by a ration of surface area to volume (I
think) of the just-forming crystal.  

You want to avoid adding crystal "seeds" that are already beyond the area/volume
break point.

The activation energy is lowered by stresses in the liquid.  Stresses occur at
interfaces between the glass and other things (the chamber floor that you are
melting the glass in).

If you have melted a blob of glass the top surface is an arc of a sphere
(almost).  Note the angle between the chamber floor and the tangent line to
start of the arc.  If the angle is small the stress at that corner is high so
the crystal activation energy is low - expect the surface to start crystalizing
in that situation.  

It sounds like you want to design your chamber to maximize the crystal formation
activation energy (don't let the glass wet to the chamber?), then you want to
pull the temp down as fast as you can.

Most common glass making materials have very slow crystal formation rates
(thought to be related to the fact that they are so viscous).  Things are added
to the glass to further slow crystal formation rates or to color the glass or
both.

It seems most of a glass makers life is spent trying to prevent crystal growth.

It sounds like when you are taking the temp up to melt the glass it's normally
done "full on."  You get the heat up as fast as possible.  Melt it quick.

The fine control in temp is needed on the temp down cycle.

You draw the heat down just past the low end of the crystal formation temp. then
you go slow and easy the rest of the way down.

The goals of good glass making are:

    Chemical homogeneity
    Physical homogeneity
    Chemical purity

I think you get chemical and physical homogeneity for free if you melt bottles. 
The chemical purity will be the tough part.

1)  Wash
    acetone wash
    alcohol wash
    distilled water rinse
    avoid dust and human contact

There was lots of talk about Cab-o-sil being acid washed at 1100C - I hope
you're not supposed to do that.

2)  Dry
    6 hrs at 270C
Lots of talk about soaking glass in water at 80C for 4 hours to increase
devitrification rates - so you'd want to dry it.

3)  Fill mold
    "satin-finish" fused silica mold (he was doing glass rods)
    lots of zirconia and carbon fiber used for supports for the rods
    lots of talk of zirconia grog surround the mold

4)  Melt
    From room to 1850C in < 1hr
    mold drawn to high vacuum (< 40 microns Hg)
    vacuum pump on at all times
    vacuum pump had air filter, oil filter, Hg filter, involving liquid N2 and
acetone
    30 min at full temp

5)  Cool
    Work quickly
    Remove vacuum
    Quench sample in 5 gal bucket of cold water

I have no idea how the glass survives such a rapid cool down without breaking. 
I'm wondering if that is what leaves the stress in the glass, i.e. the reason
you have to anneal.

>From the graphs of devitrification rates vs. temp it looks like the temp range
to avoid is 700C to 1000C for most common glasses.

Soda-lime-magnesia-silica glass is 710 to 965 for example.

Good luck


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That's about it.

Jon Bertrand
jonb@cirris.com