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melting pyrex for blanks

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#1 danjones

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Posted 22 December 2012 - 12:48 AM

I'm using a high fire ceramic kiln for making blanks. the set up is not finished, as I am making a ramp/soak controller to control the temp thru the stages of melting the glass and the annealing process, which is all worked out and in testing, but looking successful. In a trial run with plate glass, what i'm primarily using, the blank was not bad for a first attempt. useable as a tool. my question is about pyrex. I understand that pyrex cookware today is not exactly true pyrex, but does contain boron, which plays a big role in the thermal expansion, which is why it's used for baking. what about melting it down into a mirror blank. i can do up to 17" in my kiln and thick as i want. i see a ton of good clear "pyrex" cookware at garage sales all the time.

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#2 Alan A.

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Posted 22 December 2012 - 12:54 AM

Very cool Dan, how hot does the kiln need to get for the melting?

#3 danjones

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Posted 22 December 2012 - 01:43 AM

the kiln will reach just over the 2300 degree mark. plate plass will melt at just under 1800. so called pyrex will melt at just a bit higher, but well within what the kiln will do. the glass needs to heat thru certain stages, soaking along the way, then reheated for the annealing process. thats where the controller comes into play, to control all this. (thank goodness for ebay). my first test wasnt really to make a blank, but more for testing the heat capacity of the kiln, which is awesome. i thought since i'm going to fire it high, why not throw in a mold and some glass and see what happens. i for sure got a blank, little rough around the edges, and not annealed at all. just a test piece or use for a tool. controller will do it. i just read that the company that bought 'pyrex' hasn't changed its formula in like 45 years. it's a type of soda lime glass chemically enhanced to have the same properties of the very old borosilicate glass. the pyrex brand is everywhere. wondering if this stuff will work having a better thermal quality than ordinary plate.

#4 Mark Harry

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Posted 22 December 2012 - 08:00 AM

Just read about the soda-lime glass pyrex 'replacement'. and there have been quite a few more episodes of exploding bakeware. It shows that the expansion characteristics aren't as good as the old borosilicate stuff.
My 2 cents, the replacement may not quite be as easy to melt or slump into a blank??? You'll have to tell us how it works out.
I remember one of the names making that 'blue' pyrex is Anchor-Hocking or similar, but can't recall what the other was that is a major supplier.
M.

#5 Achernar

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Posted 22 December 2012 - 11:06 AM

Laboratory glassware is also made of Pyrex, if you can find some it would be borosilicate glass that would make an excellent mirror blank.

Taras

#6 kfrederick

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Posted 22 December 2012 - 11:06 AM

http://www.mdpub.com...orks/index.html might be some use full info

#7 DAVIDG

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Posted 22 December 2012 - 01:46 PM

Most modern "Pyrex" is not borosilicate glass at all. What is done is the glass is thermally stressed on the outer skin in the manufacturing processes to make it shatter resistance from thermal shock. This is why you hear of exploding cookware when it fails. The trade name of "Pyrex" has been licensed by Corning and no longer means one is buying borosilicate glass of the same chemical make up as true Pyrex.

- Dave

#8 KenScharf

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Posted 22 December 2012 - 02:01 PM

In the sizes that you are thinking there is nothing really wrong with common "Plate" glass for mirrors. The 60" and 100" telescopes on Mt Wilson worked very well for years, they are both thick plate glass. True, those scopes do take time to adjust to a rapid change in temperature so sometimes they couldn't be put into service for several hours after sundown. But those mirrors are MUCH larger and THICKER than what are used in today's Dob's. The worst thing about plate glass is that you might need to wait several hours after a figuring session before you can get a good test "reading".

I saw Mike's web pages and want to look for a Kiln and try his method some day. BTW a while back I found some large candle holder glass at Bed Bath and Beyond. These are a bit larger than 8" in dia. and 3/4" thick in the middle, with a 1/2" high wall around the sides to hold the (large!) candle. I wondered if two of these could be melted to make a 9-10" mirror blank.

#9 seryddwr

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Posted 22 December 2012 - 02:16 PM

Do they make kilns large enough for a 30" blank? Preferably old and cheap ones? The controller would need to cool the kiln for much longer, but...

#10 KenScharf

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Posted 22 December 2012 - 02:34 PM

Probably. Someone near me has a kiln for sale that has a chamber 2'wide by 40" tall. I've seen Kilns that are made for firing plates that are very wide but not very tall. That's what you'd want. I'll probably pass on the kiln since it's too big for what I want, and I'd have to re-wire the garage outlet from 20A to 30A to handle it. BTW, it's on Craig's list in Hollywood Fla if anyone is interested.

#11 seryddwr

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Posted 22 December 2012 - 04:03 PM

Well, it's a ways down on my list, but I've got the structure for a 30" telescope. That I wasn't able to get optics for. (I was cheated out of said optics.)

#12 Norm Meyer

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Posted 22 December 2012 - 04:36 PM

This topic is very interesting I'd like to try it some day.
One thing to consider is the economic feasibility. Would it
cost me more to make my own than it would be to buy one.Other
than the fun factor which you can't put a price on. Maybe as a
group project where you could make more than you could use yourself thus lowering the per unit cost. I like the possibility of making cellular mirrors making them lighter.
good plate glass is fairly plentiful at reasonable cost.
The price of the kiln wouldn't be as big a factor as the cost of the electricity to run it.
The 100" mirror was basically green wine bottles cause that's what St Gobain was making. They had to pour in
several batches cause they didn't have the capacity to melt
it all at once. It is full of bubbles and what looks like waves. In fact Ritchey wanted to refuse the blank but his
superior said no. In fact I don't think he showed up for first light because he was sure it wouldn't work, but it did
and very well at that.
That was probably more than 2 cents worth.
Norm

#13 KenScharf

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Posted 22 December 2012 - 05:27 PM

I've read an estimate of the cost of fusing a mirror blank of about 10-12" at about $25 for the cost of the electric power. That will vary depending on what you pay for a KWH. The cost of the kiln? Well you can find used ones for about $100-500, new they are around $1000 but that depends on size. Most kilns don't have a suitable temperature controller for the slow ramp up/down needed for glass though you can babysit the thing and run it by hand if you have a good pyrometer in the thing. An automated temperature controller would add a few hundred to the cost. BUT the hw costs would be figured over making more than one mirror, otherwise the cost of buying a blank would be cheaper than making your own. If I were in the business of making telescope mirrors It's something I'd consider having in the shop (as well as an Aluminumizing chamber). I'm currently in the process of trying to make my own 7" mirror from thin plate glass. If the project comes out usable I would want to try making a slightly larger second mirror. The hobby could lend itself toward a nice retirement business for someone, who knows?

#14 orlyandico

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Posted 22 December 2012 - 09:31 PM

I saw this one and it's very interesting...

http://www.instructa...scope-Mirror...

Basically a cellular mirror (and not the glued-on-posts thing that Hubble Optics makes).

The labor of making all the little hexes must be substantial, but there's an almost 50% weight savings. Might be a business model there... since I don't know of anyone selling cellular blanks.

#15 danjones

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Posted 23 December 2012 - 12:40 AM

thanks for all the comments. mike is exactly where i got the idea at mdpub.com. i even trepanned glass at 1" to start with, but it's so hard to come by. i have the kiln, got it cheap from an art studio, and can do up to 17" in it, needing an inch for the mold. trial was okay, but really just testing the kiln out. there's a ton of plate glass around here of all thicknesses, so easy to come by. as far as the cost of the kiln to run, shouldn't be too bad here. using the ramp/soak controller im building now to control the temp, ebay is keeping the cost to just a few dollars actually. the controller is in process now. the world kitchen site that bought out corning states it hasnt changed the formula in 45 years, so this is where the controversy begins. if one was buying pyrex 20 or so years ago, then it should be the same formula according to world kitchen. this is my question. i have no problem using plate, just was wondering about the pyrex if i could get better quality. i'm actually putting together a site for fun about the whole experience, from trepanning, to the kiln blanks, grinding, the aluminizing. I got my diffusion pump this week and also working on the vac chamber, which will have to be another thread, also viewable on my site. thanks for the feedback and let's come to a conclusion on the modern pyrex. Read it from the world kitchen site, quality or not?

#16 glennnnnnn

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Posted 23 December 2012 - 01:11 AM

You'd probably have better luck making your own glass from scratch rather than trying to melt and recycle glass that may or may not be the exact, same formula. If it is, good. If it isn't, you produce an expensive paperweight that warps and bends with temperature variations.
The glass should be of a uniform type throughout the structure of the blank. The only way to guarantee this is to make the glass, which you won't be able to do with your kiln. You need large crucibles and furnaces for much higher temperatures. You have to melt and stir the mix(es) in the right proportions, and even though you might know the ingredients there might well be proprietary steps that are still trade secrets. This would be something that has "Caution, May Cause Death" written all over it.

#17 seryddwr

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Posted 23 December 2012 - 02:11 AM

That's why the guy on instructables recommended using a single tabletop for a single blank (At least on the hex blank), to try to achieve uniformity. The U of A mirror lab buys chunks of borosilicate glass, from O'Hara.

#18 Jon Isaacs

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Posted 23 December 2012 - 03:52 AM

You'd probably have better luck making your own glass from scratch rather than trying to melt and recycle glass that may or may not be the exact, same formula. If it is, good. If it isn't, you produce an expensive paperweight that warps and bends with temperature variations.
The glass should be of a uniform type throughout the structure of the blank. The only way to guarantee this is to make the glass, which you won't be able to do with your kiln. You need large crucibles and furnaces for much higher temperatures. You have to melt and stir the mix(es) in the right proportions, and even though you might know the ingredients there might well be proprietary steps that are still trade secrets. This would be something that has "Caution, May Cause Death" written all over it.


The homogeneity/uniformity of glass made by remelting glass was what immediately came to my mind. In the remelting process, the glass is probably not well enough mixed that it is satisfactorily homogenized. If the glass is not uniform through out, then there will be small but meaningful variations in the coefficient of thermal expansion which result in unavoidable thermal stresses if the temperature changes. Annealing eliminates the thermal stresses in a uniform material but cannot address those in a inhomogeneous material.

If a batch consisted of 10 pieces of real pyrex and just one piece of fake pyrex, there would be serious variations in the composition throughout the mirror with resulting differences in the CTE, the differences in the CTEs of the two materials differ by a factor of about 2.5x. That would result in large thermal stresses. But I suspect even different between different glass within a type are sufficient to cause problems.

Given that real pyrex seems have been off the market for a good long time, it seems the best bet is to just make the mirror with some well annealed plate glass.

Jon

#19 Mirzam

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Posted 23 December 2012 - 06:58 AM

Might be interesting to try and make some slumped meniscus mirror blanks, either from thin pyrex blanks or from plate glass.

JimC

#20 Pinbout

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Posted 23 December 2012 - 08:48 AM

make some slumped meniscus mirror blanks, either from thin pyrex blanks



or take a 10"x1-3/4" and make a thin 12"

#21 glennnnnnn

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Posted 23 December 2012 - 11:34 AM

I think that's a very good idea! Slumping plate glass is not a difficult project, and the generated curve would be a huge time-saver.
There's something about Pyrex that I don't quite understand: how could a product with such usefulness and value just be pulled out of production?
I think that the answer is more complicated than just business buying and selling, and may involve the EPA and other agencies.

#22 DAVIDG

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Posted 23 December 2012 - 11:56 AM

I think that's a very good idea! Slumping plate glass is not a difficult project, and the generated curve would be a huge time-saver.


A group of 6 of us are doing exactly that with 16" x 3/4" blanks to make 16" f/3 newtonians. We will be grinding and polishing them at the next Delmarva Mirror Making class http://www.delmarvas...mw13/MMM13.html after they have been slumped in one of our members kiln.

- Dave

#23 ccaissie

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Posted 23 December 2012 - 11:58 AM

BTW a while back I found some large candle holder glass at Bed Bath and Beyond. These are a bit larger than 8" in dia. and 3/4" thick in the middle, with a 1/2" high wall around the sides to hold the (large!) candle. I wondered if two of these could be melted to make a 9-10" mirror blank.


Or maybe they could be re-annealed to higher usable specs?

#24 KenScharf

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Posted 23 December 2012 - 03:04 PM

BTW a while back I found some large candle holder glass at Bed Bath and Beyond. These are a bit larger than 8" in dia. and 3/4" thick in the middle, with a 1/2" high wall around the sides to hold the (large!) candle. I wondered if two of these could be melted to make a 9-10" mirror blank.

Or maybe they could be re-annealed to higher usable specs?

The problem is that as these disks are right now they are useless for a mirror blank. One surface is a dish with a 1/2 inch wide, 1/2" high dam around the edge. The bottom is not flat with lots of concentric ridges in it. Remelting the glass into a flat disk would be better than trying to grind both sides flat (and wasting a lot of glass). It would also result in a thicker disk. If remelting is possible, I could make a 9-10" blank out of the two 8" candle holders. I don't see problems in melting pieces of glass to make a larger item if all the glass came from the same source (IE: a single large sheet or several sheets of the same type. Those candle holders are identical). As far as Pyrex goes, well the market is flooded with cheap *BLEEP* from China. That's where the problem glass comes from.

#25 Mirzam

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Posted 23 December 2012 - 04:59 PM

Got any more blanks? :praying:

JimC






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