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

[Tominator <ohara@azstarnet.com>]

Hey Jonny,

Not "Glassify," but VITRIFY!

Tom

Jon Bertrand wrote:

> 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:
>
> ------
>
> 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
>
> ----
>
> That's about it.
>
> Jon Bertrand
> jonb@cirris.com