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by george
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Author Topic: Tempered safety Glass  (Read 3275 times)
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withoutbubbles
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« on: January 05, 2013, 04:32:40 AM »

Hello All,

I am new to kiln forming Glass, just starting out and need some advice regarding melting down Tempered Glass.

I have been given a quantity of patio table Glass tops, which I am 99% certain it's safety glass, Can these be melted down in a mold?, I have read that you can detemper this Glass  http://www.warmtips.com/20050921.htm , or is it just a matter of breaking them up into manageable pieces to fit in to the mold and away you go with the right firing schedule?

Any help advice will be most appreciated

Many Thanks
lee
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Pauline
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« Reply #1 on: January 05, 2013, 05:43:27 AM »

I'm not sure you will be able to cut it,  I'm sure my daughter Ann said it just shatters if you try..  hopefully someone who knows rather than guesses will come along soon
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qwirkyglass
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« Reply #2 on: January 05, 2013, 07:00:40 AM »

Mums right, as far as I know and what I have been taught is that you cannot cut safety / tempered glass. If it has been made properly it will shatter in ranges of explosiveness. My tutor tests to see if glass is tempered by hitting it with a hammer. You probably could de stress it by reversing the process ie putting whole sheet in the kiln and taking it above annealing, to slumping temp and bring it back down properly, but this is by no means garenteed.
Good luck
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withoutbubbles
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« Reply #3 on: January 05, 2013, 07:15:37 AM »

Thank you, to you and your Mum,

Well, detempering is what I was thinking of, but not the whole all sheet, I was thinking Breaking it up first, then cramming into a mold, melting it Down into Flat discs.

Many Thanks
lee
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Glyn Burton
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« Reply #4 on: January 05, 2013, 08:41:18 AM »

I suppose you have to ask yourself is it cost effective in terms of time and energy. You may put a lot of effort in to something which is not that great in the end. A few other things to consider.

Differing COE's if you did decide to go ahead you would need to separate out the glass from each sheet.

Float glass is designed to be 'stiff' and needs higher temperatures to work.

Float glass is inclined to devitrify at higher temperature.

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withoutbubbles
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« Reply #5 on: January 05, 2013, 10:08:16 AM »

Glyn, thank you for your Thoughts & advice

I do see what your saying, but I must sort of own up to the project I am trying to undertake.

I have an Idea to make me a 18-20'' telescope mirror Blank, roughly around about 1 3/4 - 2'' thickness, if I were to buy standard none tempered glass from retailer to melt down  it would cost me Quite a lot of money, which would kill my project off, but with me having 5 sheets at a meter square of this tempered Glass for free!, maybe it could be fused /capped of  some how with a nice clear piece of Glass

If you think this can't be done please tell me, I am listening for advice, or maybe you have an idea how it maybe done.

Many Thanks
lee 
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ARBeads
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« Reply #6 on: January 05, 2013, 10:16:53 AM »

Hubbys now raving about sand,chemicals, turntables, refectory bricks,torches, loads of gas and lots of days......seems to think you used to be able to buy kits from America for doing it. Oh and about 4" thickness not 2".

Think I'm going to be listening to this for a while, he's got excited about something from his dim and distance past!
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Ruth & Andy


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♥♥Tan♥♥
cuntbuckets
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« Reply #7 on: January 05, 2013, 11:17:06 AM »

Ok, I have done quite a bit of work with tempered glass so can tell you this much.

If you want to break it down lay it on a sheet or towel, anything that is bigger than the area of the glass, fold the edges over and whack it as hard as you can with a hammer. It's tougher than you think so don't go tap tapping. If it will fit into a couple of bin bags then pop it in bin bags because those bits fly far and wide.

I have never tried detempering it but you would need a big kiln to fit a whole sheet in. If you are just going to break it down then there is no point detempering it because you have already removed the stress. If you put it in the kiln now it will behave like normal glass.

This is where it gets interesting/tricky. Tempered float glass that I have fused in my ceramic kiln full fuses at 860c. My notes are out in the shed but I have done several very nice rounded tack fuses at a lower temp and these have lived in the garden for a few years now with no problems. They hang on the wall and bang all over the place with the winds off the sea and I have never had cracks or breakages.

However I have noticed that at full fuse it is never a clean fuse. All of those little cracks and edges stay for the final fuse, think cobwebs in water effect. At the tack fuse it is a mix of devit and clear because you can't control which side up you fuse. This is a gorgeous effect but not what you are going for I think?

If you want a two inch thick piece of crystal clear fused glass you definitely won't get it with shattered tempered glass.
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withoutbubbles
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« Reply #8 on: January 05, 2013, 02:25:09 PM »

Thank you for your input ARBeads , Tan,  most appreciated!

Being melted down, say casting sort of Heat/cycle, All of those little cracks and edges, wont they just all melt in together? , the glass I have are all identical sheets from the same company, To be honest I should research about the diversification with regards telescope mirror, I don't (think) it really matters, I know air bubbles are the things needing to be avoided more than anything, but saying that, air bubbles to a certain extent could be ok as long as they are deep enough into the Glass   
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Glyn Burton
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« Reply #9 on: January 05, 2013, 02:37:14 PM »

Hello again I fully agree with Tan you will never get a clear cast from shattered tempered glass, it is a basic rule of kiln casting the bigger the piece of glass you start off with the clearer the finished cast, so if you are starting of with what is effectivly a hand full of gravel you will get a lot of bubbles.
I presume you want to grind the lens after casting it would seem to be a pity to spend hours grinding something which will be like an Aero bar.
If you buy a bullseye crystal clear casting billet, about 26 for 1.6kg it will do the job and save you a lot of grief.
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withoutbubbles
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« Reply #10 on: January 05, 2013, 03:09:14 PM »

Hello Glyne

it would seem to be a pity to spend hours grinding something which will be like an Aero bar.


Ha Ha, yes it certainly would Grin I love the analogy.

Yes , thank you for your input, So this idea of mine is gradually sinking! I don't mind, this is what I asked for, advice.

I am sure I would need more than one Billet though for what I want , I am talking 18-20'' at near on two inch thick
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qwirkyglass
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« Reply #11 on: January 05, 2013, 03:33:04 PM »

hi,

the only way i can think of you possibly manage it is to cast the piece from a flower pot resivoir, taking the temp up to 1100degreesc at least so that it becomes like furnace glass again, but this is still very likely to devit, and with an open cast the size you are talking about would be about a week and a half annealing at least, plus a week to get the glass up to temp, and allow it a long enough soak to cast prroperly.
there would still be veiling within the glass from the broken edges, this can look pretty but not good far a telescope mirror, and would still not garentee no bubbles, so you still might have aero bar syndrome.
Where theres a will theres a way, but you might be better investing your time in trying it another way.
You could always make a mini version first and see if that works at all Smiley
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withoutbubbles
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« Reply #12 on: January 06, 2013, 04:10:46 AM »

OK, thank you everyone for all the ideas and advice,

Recon that I may be biting off more than I can chew with this idea of mine, , I will give this one a miss, maybe it would be best to have a go making a small test piece like what has been said, to see what happens.

Many thanks

lee
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♥♥Tan♥♥
cuntbuckets
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« Reply #13 on: January 06, 2013, 08:02:33 AM »

Lee, those edges would all melt together but they leave their mark, you can't thoroughly clean every single edge nor can you grind them all so where it meets and melts you will always have those cobwebs running through it. A gorgeous effect if you want it, a pain in the arse if you don't.

I'm so sorry I couldn't give you the answers you wanted but hopefully managed to save you from hours of melting and a final disappointment. Glyn has it spot on with the billet, its a much purer glass to work with.
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Moreton
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« Reply #14 on: January 06, 2013, 11:35:53 AM »

Hi Lee welcome to frit happens. What an interesting idea. However as has been explained it won't work that way.
If you were to "Un harden" the glass you would need kiln large enough to lay your glass flat , How large are the sheets that you have?
Another consideration, and this applies whatever glass you, use is the weight. By a quick calculation I reckon that you are looking at more than 25Kilos of glass. Can you handle that safely when you come to grind the radius and polish the lens? Have you looked into the complexities of the polishing? A proffessional grinder/polisher would by remarkably expensive (made by the likes of Olympus & Lietz). Home made is the way to go....how are you on acurate fabrication methods for both the support and the grinding boss? Enough rambling from me but you can see that a lens of that size is going to present some very interesting challenges. A search on YouTube for "Telescope Glass" will bring up some fascinating ideas particularly for DIY gear. How about a small version say 3" as a trial if that works then you are on the road to glory!!! Casting glass can be obtained from Cristalica in Germany and is a good (cheap) price.
If you want good advice about suitable glass and its characteristics a visit to Plowden & Thompson in Stourbridge would pay dividends. They can formulate glass to any specification anyone could ever desire.......at a price.
On here GLyn has good experience of large castings & his advice is always accurate.
A chat with the staff at Wolverhampton or Sunderland Unis would set you right also.
Let us know how you get on . Regards Pete
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Pete
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