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Hissing Glass

03/08/2012 7:49 AM

I regularly melt disks of glass in preparation for making large aperture mirror blanks for telescopes. Some are large; some are small.

The chambers in the kilns used, vary in diameter from as little as 17 inches to as much as 36. Depths are anywhere from 24 to 36 inches.

The kiln "furniture" used to support fired objects are fired, unglazed ceramic.

Molds to contain the molten glass are made from 304 type stainless.

Part of the process involves opening the lid to vent heat and accelerate cooling, while the glass is still in its molten state so as to prevent devitrification.

"You want fries with that?"! Two molds with molten glass. The smaller one 4 inches; the larger one 8.

Every time I close the lid after a venting or inspection, I hear this "hissing" sound for about 3 seconds. After a few hours at 1800 to 2000 degrees F, that's not likely to be steam!

I've been told that glass sucks oxygen out of the air during vitrification but at 1800 degrees F, the glass is too hot to vitrified.

So what is it that makes that sound every time I close the lid?

Thanks.

L.J.

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#1

Re: Hissing glass

03/08/2012 8:01 AM

I'm thinking that it's the opposite of what happens when you shut a refrigerator.

When you shut the reefer, the warm air that was pulled in when the door was opened, cools down, contracts and pulls a partial vacuum.

In the case of your kiln, when you close the lid, the cooler air that was pulled in when it was opened, heats and expands, and leaks out through the lid seal.

That's some really cool work you do!

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#4
In reply to #1

Re: Hissing glass

03/08/2012 8:11 AM

Oops. I was typing when you and AH were posting. So I didn't see your posts.

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#5
In reply to #4

Re: Hissing glass

03/08/2012 8:15 AM

Seems like a consensus is building. OMG. Does that mean the debate is over?

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#6
In reply to #5

Re: Hissing glass

03/08/2012 8:27 AM

No. Give it time. Someone will come along and say it's a vacuum.

Maybe even a new energy source.

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#7
In reply to #4

Re: Hissing glass

03/08/2012 8:44 AM

No problem...

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#2

Re: Hissing glass

03/08/2012 8:09 AM

Pressure differential after closing?

Could be that when opened the ambient temperature drops just a little and when closed again the pressure rises slightly with temperature.

Or... it could be that closing the chamber lid increases the chamber pressure which vents back out over a short time.

Or... maybe a colleague is making a hissing noise behind your back to get your head. Be glad they are not popping a big paper bag like some engineers did to one technician as he was j-u-s-t reaching inside a device to take a measurement.

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#3

Re: Hissing glass

03/08/2012 8:09 AM

I'll take a wild guess.

When you open the lid, you let in atmospheric air, which slightly cools the uppermost layer of molten glass. When you close the lid, that "air" becomes superheated and pressurized. The 3 second hiss is the sound of that air escaping.

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#8

Re: Hissing Glass

03/08/2012 8:55 AM

I know this is not helpful and is OT from what you're asking, but I recently saw a show on PBS about making a telescope and thought it was really cool!! http://www.pbs.org/programs/journey-to-palomar/

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#25
In reply to #8

Re: Hissing Glass

03/08/2012 4:12 PM

it is indeed helpful and appreciated.

http://www.pbs.org/programs/journey-to-palomar/

Not only is it timely but it points to an era in this Country when our biggest achievements in Science were funded by private industry not by reckless beurocrats. Everyone should see this, off-topic or not!

"Those who do not know their history are destined to repeat it".

We should be so lucky!

This history you just pointed to restores our pride in what once was and could be again.

Thanks

Laughing Jaguar!

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#9

Re: Hissing Glass

03/08/2012 9:23 AM

LJ, very kewl hobby you have there!

I'm assuming that you're making Cassegrain telescope mirror blanks, and also assuming you grind/polish/test you're own mirrors?

If so, happy to see another telescope building in our forum! FYI, I've build a few Newtonians in the past, the largest being a 16-inch F4.0.

Love mirror grinding & polishing! It's a good way to relax and release the day's stress!!!

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#10
In reply to #9

Re: Hissing Glass

03/08/2012 9:33 AM

16" is an impressive mirror!

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#12
In reply to #10

Re: Hissing Glass

03/08/2012 9:48 AM

Yeah, it was one heavy monster. It was my 4th Newtonian build. The first one I built was a 8-inch F2.5 when I was 16, many many moons ago!

Anyhow, I now wish I had never sold the 16-incher, but, I need the $$$ to plop down a down payment for the Solar PV system installation on this house!

Someday I'd love to grind a 24-incher.....IF I only can find the time.

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#13
In reply to #12

Re: Hissing Glass

03/08/2012 10:53 AM

I take it you have a good dark site. We do not and I am just too tired of dragging a 10" Mead around.

However, I will pull it out on June 5th for the transit of Venus. I have a nice camera waiting to be mounted to it for that one.

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#16
In reply to #13

Re: Hissing Glass

03/08/2012 12:34 PM

Anon Hero complained: "I am just too tired of dragging a 10" Mead around."

Really?

Stop complaining!

That's 75 pounds of ballast you see, just to balance it! The four saddle bearings weigh 18 pounds a piece!

And this doesn't show the ladder needed to get to the eyepiece which is ten feet off the ground at Zenith!

And your tired?

Laughing Jaguar (and with good reason!)

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#26
In reply to #16

Re: Hissing Glass

03/08/2012 4:15 PM

Ha! Got you beat. :)

Mine is 100+ pounds. It's the Schmidt-Cassegrain LX-200.

Nice Newt. I envy your weight. :)

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#28
In reply to #26

Re: Hissing Glass

03/09/2012 12:25 AM

"However, I will pull it out on June 5th for the transit of Venus. I have a nice camera waiting to be mounted to it for that one."

Let me figure that out:

1. one month to put down the credit card

2. one month to get off the stool

3. One month to dust off the scope, read the manual and set up the scope.

What passion!

It's a good thing it's got computerized tracking. It will find the sun for you!

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#34
In reply to #28

Re: Hissing Glass

03/09/2012 6:46 AM

1. Ah, well, it is long paid for.

2. Stool or pot?

3. Already dusted. We had company last week.

4. This is Florida, everyone knows where the Sun is. :)

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#23
In reply to #13

Re: Hissing Glass

03/08/2012 3:14 PM

Same here. I just finished rebuilding my old 10" Meade Newtonian with a new tube, lightweight mirror cell and Crayford focusser. I've got decent skies where I live, so I have it permanently mounted on a pier in a roll-off roof shed.

It sure is nice to just go out, unlock the roof and roll it open, set the scope to North, flip on the switch and get started.

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#30
In reply to #13

Re: Hissing Glass

03/09/2012 4:01 AM

I don't have my own telescope but I can always pop down the road to Herstmonceux to use theirs. We've done a lot of work with them in the past & know the scientists well.

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#19
In reply to #12

Re: Hissing Glass

03/08/2012 1:17 PM

Moose wrote: "I now wish I had never sold the 16-incher."

I don't blame you! As a boy I used to spend summers on a dairy farm on CR 82 near Pine Plains. Great skies back then!

Look for me at Cole Palin's old Rhinebeck Aerodrome when they host the quarter-scale RC event!

I'll be riding in on a noisy, 1800 cc, two wheeler, not flying in. Sold the taildragger years ago. I'll be North of you at a Lake Taghkanic Star Party (RAC) with the van and a 20" F-3 next summer

L.J.

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#21
In reply to #19

Re: Hissing Glass

03/08/2012 1:41 PM

My my, a Pine Plains guy, eh? That's only a 20 minutes drive east of here on Rte 199! My Doctor's office is located there, so I know the area well! I take it you're not originally from Pine Plains? Where from? I think you told me once IF I'm not mistaken.

When is that RC airplane show at the Rhinebeck Aerodrome? I'll be sure to be there if you're attending! 1/4 scale!? Wowsers, now that's some big stuff! I never had anything that large as far as RC goes. The last plane I built and flew was a Topflite F8F Grumman Bearcat with a 60-inch wingspan and 0.60 engine. I ended up flying it "through" a chain-link-fence type baseball backstop at the high school.......literally like pressing soft cheese through a coarse cheese grater! LOL I haven't built anything since and have sold off the Futuba radio and servos. It's too costly a hobby for this old fart.

So, are you bringing any airplanes to the show and flying it?

Just curious LJ, did you ever attend any of the RC 1/4 scale shows/meets at the Schenectady County Airport? I used to go to those show a lot back in the 90's!

A 20-inch scope? Now I gotta see that! Staying at the State Park? That's a great place to set-up a scope....no lights for miles! If the weather is good it'll be stupendous viewing up there!!!!

Ya bring the scope and I'll bring the icey cold brews!

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#22
In reply to #21

Re: Hissing Glass

03/08/2012 2:38 PM

I take it you're not originally from Pine Plains? Where from? I think you told me once IF I'm not mistaken.

Grew up in Astoria, Queens about two hours south in a borough of NYC. Dad eventually bought a house up there, near the parkway exit for West Taghkanic & Rt 82. The house was small. A week-ender east of the parkway that overlooked the North entrance to the park off 82.

When is that RC airplane show at the Rhinebeck Aerodrome? I'll be sure to be there if you're attending! 1/4 scale!?

http://www.mhrcs.com/pages/2010RhinebeckV4.pdf

It has been the first weekend in September but when I went to the current website, it wasn't seen. It's too early, we aren't ven out of Winter yet!

http://www.oldrhinebeck.org/

So, are you bringing any airplanes to the show and flying it?

No. I'll be riding one of my motorcyles from western NJ. I live on the Delaware River across from Bucks County, PA. Probably the big V-twin 1800cc VTX Honda Cruiser. It's good for long trips

Just curious LJ, did you ever attend any of the RC 1/4 scale shows/meets at the Schenectady County Airport? I used to go to those show a lot back in the 90's!

Nope!

A 20-inch scope? Now I gotta see that! Staying at the State Park? That's a great place to set-up a scope....no lights for miles! If the weather is good it'll be stupendous viewing up there!!!!

Ya bring the scope and I'll bring the icey cold brews!

I'd love a cold brew Moosie but it's a State Park. Better we drive to a bar in West Tachkanic for cold ones! I used to know a neat place in Ancram Lake!

Later

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#24
In reply to #22

Re: Hissing Glass

03/08/2012 3:37 PM

Okay Astoria Queens. Been through it on the LIRR many times! LOL

Mrs. Moosie hails originally from the Village of Baldwin, Nassau County, LI. Yeah, even though she's been up this way since 1981 she still retains her mixed Irish-Long Island accent! Sometimes I cannot understand her!

Okay, I've only been to the State Park at Taghkanic once and that was 10 years ago, although I've driven pass it a Godzillion times since then while on the Taconic State Parkway.

I'll keep my eye peeled for the RC show on the website....the Aerodrome is located about a mile southeast of us as the crow flies! If the weather holds I definitely will be going! I'll look for your bike and then you! 1600cc = BIG BIKE! Wowsers!!!! Possibly swap photos before the meet, so we can find one another?

Along the Delaware River eh? Ya near Sue then?

Ahhhh I forgot that NYS doesn't allow brewski's in their parks......MY BAD. Yeah, a trip to one of the local gin mills will be in order! LOL Drinks on me!!!!! (I do mean that).

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#29
In reply to #10

Re: Hissing Glass

03/09/2012 1:02 AM

how about a 600m^2 mirror Check the new HESS telescope in Namibia

my previous job

http://www.mpi-hd.mpg.de/hfm/HESS/pages/about/telescopes/

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#14
In reply to #9

Re: Hissing Glass

03/08/2012 12:21 PM

Of Course! Cold air rushes in; gets trapped under the closed lid; gets hot and then squeezes past the lid seat. Which is a good thing. Can you imagine the consequence of a tightly sealed lid and the air expanding. Entropy where are you when I need you!

Actually the hissing sound is a 20 foot Python I have as a pet. "Honey? Where are the cats?"

No Moose. It won't be a Cass. Life is difficult enough! These are test runs in preparation for two of four new scopes I've designed, all but one with thin meniscus mirrors: 8", 14" and 20"; all are Newtonians. The remaining one is a 14 Inch honeycombed mirror. THAT one has me really excited!

The prior one I opened with, the 8", a thin meniscus mirror, will be slumped later on a mold at 900 degrees; ground and figured to f-3 and then fitted to an all-in-house-made scope. The composite tube was made on my vacuum bench too. This scope is sized to fit into an existing Meade quartz drive fork mount and is airline carry-on legal.

The 14 Inch Meniscus mirrored scope is also a fast f-3 and a modular, also airline legal carry-on.

The 14 inch, honeycombed mirror, also an F-3, will be more of a challenge in that it will take forever to cool after firing; many days me thinks, not counting the annealing phase. The only way to get any volume production is to buy a super large Kiln and stock in the local electric company!

*

When installed in a scope, this mirror will reach thermal equilibrium five times faster by virtue of the reduced mass and increased surface area. That's the whole idea for honeycombing an otherwise solid blank. However, thin slumped meniscus mirrors are showing lots of promise and may obviate the complex production process needed to produce honeycombs. The purists want honeycomb however so.......................!

By contrast, these thin meniscus mirrors are a piece of cake to make. Given the temps, maybe I should say a piece of pizza!

After all that, it's nothing but hot air (sic!)

. . . . . . the hissing sound of course!

Thanks all, for the responses.

L.J.

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#11

Re: Hissing Glass

03/08/2012 9:42 AM

It's the dragon sighing with relief now that the draft has gone.

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#27
In reply to #11

Re: Hissing Glass

03/08/2012 11:10 PM

I understand self marking your tongue in cheek answer as OT. But you conjure such a beautiful metaphor I had to give you a GA.

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#33
In reply to #27

Re: Hissing Glass

03/09/2012 6:43 AM

+1

My only regret is that I have but one vote to give.

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#15

Re: Hissing Glass

03/08/2012 12:26 PM

allowing air to enter the kiln by opening the door will introduce contaminants that will activate the flux and cause the hizzing sound. i'm not familar with the process of making optical glass, but i'm thinking that by injecting an inert gas into the kiln under very low pressure would make a dramatic difference in clarity.

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#17
In reply to #15

Re: Hissing Glass

03/08/2012 12:44 PM

DurtieDuck wrote: ". . . .contaminants that will activate the flux. . . . "

I am making mirror blanks. There is no flux. What for?

DurtieDuck wrote: ". . . . . inert gas into the kiln under very low pressure would make a dramatic difference in clarity."

Huh? No it won't. They will get ground, polished and figured as mirrors. These are borosilicate and float glass blanks for first surface mirrors, not lenses.

L.J.

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#18
In reply to #17

Re: Hissing Glass

03/08/2012 1:00 PM

ahh, now you went and hurt my feelings. i guess i'll just have to kick the dog to make me feel better.

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#20
In reply to #18

Re: Hissing Glass

03/08/2012 1:26 PM

Durtieduck wrote: "ahh, now you went and hurt my feelings"

That was not my intention. . . . . You were simply trying to solve a problem that you thought existed but didn't.

"I guess i'll just have to kick the dog to make me feel better."

Don't you dare!

When you stop trying to solve problems, that pooch will still love you, with or without a can opener!

L.J.

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#31

Re: Hissing Glass

03/09/2012 5:01 AM

Great blog, telescopes and RC aircraft, couldn't be better!!

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#32

Re: Hissing Glass

03/09/2012 6:06 AM

It's caused by the sand which the glass is made from:

http://en.wikipedia.org/wiki/Singing_sand

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