Slumping 2 mil glass - WarmGlass.com

Slumping 2 mil glass

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Buttercup
Posts: 555
Joined: Mon Jan 28, 2008 5:22 pm
Location: S.E. Queensland Australia

Slumping 2 mil glass

Postby Buttercup » Tue Dec 13, 2016 7:01 pm

So, I have been asked by a friend if I could form a piece of picture glass (2 mil) to replace a broken semi-antique piece. It's quite small, approx. 6 x 8 inches. The edges where they slip under the picture frame would be, of course, flat for about 1/4". The glass would then rise in a gentle curve to about 1/4", flattening out again for most of the surface area.

I can't post a photo because the glass is too badly broken. If it can't be visualized I could do a drawing.

I'd like some comments on the best way to do this (other than take the project to a company that does this all the time). The plan is to make a plasticine or clay flattened 'hump' shape then make a plaster cast off that to slump into.

1. If this would be the best approach would I need to weight the edges to keep them flat and stop them sliding down?

2. What time/temperature would 2 mil picture glass need to slump that short distance without softening to the point of marking the surface?

Am I dreamin'? Should I just suggest she send it off to the professionals? I don't want to do that because her partner keeps my kilns functioning and refuses to be paid.

I look forward to some input, answers to my questions, better method? Thanks, Jen

Kevin Midgley
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Location: Tofino, British Columbia, Canada

Re: Slumping 2 mil glass

Postby Kevin Midgley » Wed Dec 14, 2016 10:46 am

Balance the glass on top of the hump and let it drop to the shape required.

It may take a few firings but at least the glass is free from the local picture framing shop.
I would suggest low and slow to do it without getting ripples and more importantly texture.
This is where an 8 minute firing Hoaf would be nice.
You might have a whole new market there once you have it mastered.
In any case, it will make you appreciate the skills of the person who made the piece you need to replace.

Buttercup
Posts: 555
Joined: Mon Jan 28, 2008 5:22 pm
Location: S.E. Queensland Australia

Re: Slumping 2 mil glass

Postby Buttercup » Wed Dec 14, 2016 11:47 pm

Thank you for that, Kevin.

The glass from the framer's shop was not going to be free! The supplier I have an account with doesn't carry 2 mil and I don't want to minimum-amount special order it. I did, however, buy a couple of cheap pictures from an Op shop and took the glass out. It’s not ‘old’ glass, not having the characteristic slight distortions, so I’m guessing it’s float.

Can you suggest a suitable time/temp please? I haven't dealt with anything so thin before, or slumped/draped float and I'm trying to avoid a lot of trial and error hovering around a hot kiln because the weather is so hot now as is my workshop (galvanized shed).

A. Do I slump/drape it tin side down?

B. Would this schedule (possibly) work?:

1. 200 ℉/H to 900 ℉ No hold
2. 100 ℉/H to 1030 ℉ Hold 10 (look and take higher if not draped?)
3. AFAP to 960 ℉ Hold 30
4. 200 ℉/H to 700 ℉ No hold
OFF leave closed to room temp.

I'm not looking at this as a potential market...... just want to do something for a friend, without too many trips to local op shops or too many kiln firings in this heat.

I've looked on-line and browsed through Stone, which has confused me further, no mention of 2 mil.

I look forward to clarification and further suggestions. Thanks Jen.

Kevin Midgley
Posts: 713
Joined: Mon Mar 10, 2003 11:36 am
Location: Tofino, British Columbia, Canada

Re: Slumping 2 mil glass

Postby Kevin Midgley » Thu Dec 15, 2016 10:06 am

Well glass from the framer's shop is free if you get it out of the tip or dumpster as we'd call it here. Just got to be extremely careful when sourcing and wear cut proof/barb wire proof gloves. I've never know a frame shop that doesn't have tons of scrap.
No real reason why you couldn't try this out with thicker glass scrap and then move onto the thinner. The thicker may also work in the frame. There is not going to be a lot of weight difference or visual difference for that matter with a piece that size stuck in a picture frame.
The only reason the thinner was used would be it was cheaper, faster to form and lighter to ship the finished item.
If you use thicker, it won't break as easily the next time.
Tin down as per usual
As for your kiln, too bad you weren't doing this in winter.
Get your kitchen timer out and start watching.
Wear it on your top. that way you will remember.
Count down type of timer can be good for these situations.
I've checked firings at one minute or less intervals.
Wear your eye protection.
I can't suggest a schedule, don't know your kiln.

JamesL
Posts: 29
Joined: Wed May 18, 2011 1:08 pm

Re: Slumping 2 mil glass

Postby JamesL » Thu Dec 15, 2016 12:38 pm

I don't often add my 2 cents for firing questions as there are more knowledgeable people here than me but, from an engineering perspective, I have a couple of thoughts.

First, i think draping it would give you the most consistent shape, take less steps and give you a wider range of materials to use. A flat section of an old broken mold could be shaped with the edges ground to the desired profile and if it isn't thick enough, use fiber paper under it to adjust the height. The edges are sure to be flat this way as they will end up truly flat on your shelf. At the lower temps it would take to shape this glass, I don't think any air trapped would give any headaches.

Second, I would consider using a bigger piece of glass than the finished size. A single 2 or 3 mil piece would be easy to cut afterwards and it would fit the frame better with a custom cut. The extra glass will also assist in the shaping process either way you decide to use.

Kevin Midgley
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Joined: Mon Mar 10, 2003 11:36 am
Location: Tofino, British Columbia, Canada

Re: Slumping 2 mil glass

Postby Kevin Midgley » Thu Dec 15, 2016 12:45 pm

James you gave Buttercup a good description of the mold required as was my suggestion to drape.
I am not sure if your idea of a larger piece of glass being used will help or hinder the shaping.
With a much larger piece of glass trimmed later as you suggest, a drop out mold might be easier to get the perfect curvature without marking.
Oh but the timing will be so critical as would be crash cooling to hold the desired shape without any extra movement.

Buttercup
Posts: 555
Joined: Mon Jan 28, 2008 5:22 pm
Location: S.E. Queensland Australia

Re: Slumping 2 mil glass

Postby Buttercup » Thu Dec 15, 2016 10:01 pm

Kevin and James, thank you for the comments….but dumpster diving.…..(helpful charity workers waiting to help one out and handing over food vouchers?) nah! I’ll buy glass.

The frame is too shallow for 3 mil. The original glass and frame is about 65-70 yrs old and the frame is small and delicate so the glass was thinner.

Kevin, you mention 2 mil forming more quickly. That’s interesting as I’d thought the weight of the heavier glass would make it slump more quickly. Good advice to watch it closely….. I do have a little clip-on timer.

As you both say, draping would be more direct and take fewer steps than slumping. Since it’s such a shallow drop the idea of a piece of shaped kiln liner, height adjusted with fiber paper would be a lot less fuss. I had planned to make the glass a bit wider all around and grind it to fit as the chance of it coming out exactly right is nil and I’d be terrified to cut it.

Kevin, wouldn’t a drop out have a high area in the centre, more rounded rather than a ‘plateau’?

Can anyone suggest a schedule that won’t cause the surface of the glass to soften to the point of marking?

Thanks again for the input. Jen

Kevin Midgley
Posts: 713
Joined: Mon Mar 10, 2003 11:36 am
Location: Tofino, British Columbia, Canada

Re: Slumping 2 mil glass

Postby Kevin Midgley » Thu Dec 15, 2016 11:10 pm

practice with some glass or be prepared to do it a few times to get it right
Slower would be better at providing control over the drape.

Done as a drop out, with the correct time and temperature you will not have anything more than the gentle curve as the glass sags in the center.
If the central area is 'flatter' then the drop has been stopped in the middle but with the glass not hot enough to mark.

A drape does have the advantage of your controlling the actual shape more than just the curve resulting from heating as you are conforming to a mold.

There is another option perhaps worth considering.
That is to take the project to your local scientific glass blower and with their powerful torches they could shape a piece of pyrex sheet for you.

Thin glass takes less energy to heat it up and wants to create its 6mil layer if it can. Float of course is a little less flowing than art glasses.
Don't forget to figure out where to wear a kitchen timer so that you don't lose it or fail to reset it during the firing.

The bonus after all this is that you will have learned a huge amount about controlling glass in your kiln.

Buttercup
Posts: 555
Joined: Mon Jan 28, 2008 5:22 pm
Location: S.E. Queensland Australia

Re: Slumping 2 mil glass

Postby Buttercup » Fri Dec 16, 2016 1:32 am

Thanks again, Kevin.

I’m resigned to doing this more than once but given our current temp of 30℃ + + would like to limit the number of trials. (Think ‘Cat in a Hot Tin Shed’, and add a hot kiln.) It’s even too hot in there for Monty (the python).

As a fallback option I have found a company in Oz that does such work. However, as mentioned, I’d like to do it for my friends as a very inadequate ‘thank you’ for all the times my kilns have been rescued as well as the bonus satisfaction/frustration (?) it will bring. :? Jen

jim simmons
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Location: Hillsboro Oregon
Contact:

Re: Slumping 2 mil glass

Postby jim simmons » Sat Dec 17, 2016 2:41 pm

Hot? Hot? Hot?
We could use some of it. That way you will get cooler and we will get warmer.

Think "Win, Win"
The other Jim

Buttercup
Posts: 555
Joined: Mon Jan 28, 2008 5:22 pm
Location: S.E. Queensland Australia

Re: Slumping 2 mil glass

Postby Buttercup » Sat Dec 17, 2016 8:35 pm

Happy to share, Jim. May I send over a load of politicians full of hot air? :D Jen

linn keller
Posts: 52
Joined: Mon Mar 10, 2003 12:22 pm
Location: south tx

Re: Slumping 2 mil glass

Postby linn keller » Sun Dec 18, 2016 9:32 pm

jen,
please keep the ones you've got and be willing to accept some of ours - i think we're going to have more than enough to share.
thanks, though,
linn
linn in deep deep south texas

Buttercup
Posts: 555
Joined: Mon Jan 28, 2008 5:22 pm
Location: S.E. Queensland Australia

Re: Slumping 2 mil glass

Postby Buttercup » Tue Apr 18, 2017 3:48 am

Late update.....

When the temperature cooled down I made a mold from a piece of old kiln shelf.
Mold made to slump picture glass.png
Mold made to slump picture glass.png (139.28 KiB) Viewed 9045 times


The glass is a piece of 2 mil (+/-) picture glass from an op-shop. I don't know if it was tin up or tin down. It was old so may not be float. It was cut slightly larger so it could be trimmed to fit after firing. It needed very little trimming other than the corners rounded off a bit.

The schedule used was:

1. 1050F over 2:00, no hold
2. 1100F over 0:30 held until it started to move @ 2:43 of program, watched until it was where I wanted it
3. 1000 AFP held for :20 mins
4.Off. Left kiln closed till next morning.

Can't believe it happened in one shot!
Attachments
Glass installed in frame.png


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