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Warning about treepaning and slumping soda lime glass:

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#1 stargazer193857

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Posted 05 December 2019 - 03:26 PM

This thread has no advice on how to treepan or slump glass. It only warns what can go wrong if you do certain things. It is not a complete list, just what I read so far, luckily before making those mistakes myself. I don't plan to post a correct way, for liability reasons, but thank those who did post useful info on what worked it did not.


The tin side of soda lime glass is harder than the air side. The tin side has diffused tin atoms that make it hard. It is harder than Pyrex/Borofloat33/Supremacy. This is confirmed in Schott PDFs, glass scoring sites, and was told to me on a glass fusing site.

Another PDF says Borofloat takes twice as long to wear down as soda lime does, and requires almost twice as much minimum force for the first scratch. It is not stiffer. The scratch resistance comes from its tensile strength, which helps if you drop the mirror but not with flex in a cell. I'm guessing the tin side of soda lime is 3x the hassle of the air side.

People scoring glass look for a non-bevelled edge and look for which side started with a score and which side a snap. They assume that the cut was done at the glass place by someone who knew not to where down their wheel on the tin side. But most glass is beveled or sawed. So the other way to tell is with a $40 short wave UV light, not a black light. Darken the room, shine the 254 nm into the glass, and see which side has glowing tin clouds. Homesciencetools has one. Do not look into this light! I would wear polycarbonate safety glasses when doing this, since it naturally blocks UV. Test your googled once by placing them between the light and the glass. If you still see tin clouds, the glasses don't block UV.

I'd you hog your curves, the difference between the sides matters a bit, but not after you break through to the soft inside. It matters far more to those who slump or treepan:

Slumping, you want the soft side to be concave, and the tin side convex. That way, you can start fine grinding and not have to work 3x as hard. It is harder than Borofloat33 remember.

Treepaning, the most difficult part is getting the hole started. If you don't want your blade dancing around a long time before digging in, then put the tin side on the other side and start on the air side.

Treepaning surfaces should be wet enough to avoid glass dust getting in your lungs, or heating up the glass and blades. I recommend at least 1/8" of sitting water if you are cutting fast. With glass, blade speed should be slow. Fast metal can fling grit, and metal flanges can flare as they go in. Always let the abrassive do the work, while pressure merely follows it.




Comments or other tid bits?



...

Also, if the glass has any scratches, those could precipitate cracks, by greatly reducing how much stretch is needed to start one in the kiln. Used table glass should be polished like brand new if you want faster heating and cooling.

However, it is the cooling that would cause a surface scratch to crack. If the glass is heated hot enough, they might disappear. But at the tack fixing temp, it can also stick to the kiln if kin wash is not used. A disposable surface might be good for initial tests.

Edited by stargazer193857, 06 December 2019 - 11:55 AM.

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#2 brebisson

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Posted 06 December 2019 - 01:07 AM

cool info! thanks...



#3 stargazer193857

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Posted 06 December 2019 - 09:53 AM

Glass thermal specs use these words, which one site said have these meanings:
Strain point: this is the lower end of the annealing range.
Softening point: this is the higher end of the annealing range.
Annealing temp or range: compare it to the above two to see the accuracy of that site.

The softening point is also the very beginning of the slumping range, though 200 degrees higher is faster. It may also be where you can finally raise the temp fast, if you did a soak at this temp already.

Tack fusing point, is where slow fixing is possible and slumping gets faster still.

Fusing point, is where fusing is fast, slumping is very fast, and print through gets fast. Around here devitrification can happen, if not a bit lower.

Get higher past melting, and devitrification is no longer an issue. People often open the door to quickly cool the the devitrification range, but at the cost of wearing down the kiln parts. Best practice is to just turn off the heat.


The thicker the glass, the more the glass temperature does not match the air temperature of the kiln even on the surface of the glass.

A hard rule is there is no reason to look inside the leap hole when the kiln is below the softening temperature or has not been soaked. If the glass blows up, shards might get you through the hole. Only above this point and after a good soak will slumping occur, so only then should you look. Never open the door below or near the softening point.

Once certain you are above the softening point, you may rapidly increase the temperature.
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#4 stargazer193857

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Posted 06 December 2019 - 10:14 AM

Probing for firing schedule best times:

You have heat some glass above the softening point and back down to room temp. What does that tell you? Something, but not as much as you'd like.

The heating and cooling below the anneal point does not affect the anneal. You won't know you are close to those limits till they crack. That crack could be an explosion that damages the kiln. How long each thickness takes is likely proportional to the square of the thickness. If you are probing for the best schedule, you have to crack some glass to know. And once you find it, don't let anyone open the door for a second, since glass on the fastest schedule is more fragile.

Do a polarized light test. Do a polarized light test by looking through polarised lenses through the glass at a laptop monitor or polarized light source. Rotate the lense till the light blacks out. Annealed glass let's light pass exactly how it is. Tempered breaks up the polarization, and strained does it with a variety of paterns, color indicating worse strain than black and white. If the light blacks out evenly with the unblocked monitor light, that means you did the anneal well.

If you heat or cool glass too fast, kaboom.
If drop it or try to grind it with a very bad anneal, kaboom. Some exploded on Augustus this way, cutting his hands and covering him in shards. He was wearing eye protection though.

#5 stargazer193857

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Posted 06 December 2019 - 10:28 AM

If your kiln has typical insulation it thickness, expect the outside surface to be 400 degrees and the air around it 300 degrees F. I calculated that from the wall resistance and thickness and inside temperature and such, for during soda lime slumping.

A layer of rock wool outside the bricks is a cheep way to reduce the watts needed and maybe increase safety. Soft fire bricks insulate better and heat and cool faster but also are more fragile.

Art glass kins use crystal glass that is worked at a much lower temp than soda lime. Borofloat33 needs 200 deg higher temps than plate, and fused silica much much higher, needing a kiln with more expensive bricks, or a pottery kiln.

These second two glasses can be heated and cooled much faster without breaking, due to their higher strength and lower thermal expansion coefficients. That might even pay for the glass. But when they do break, they break at least twice as violently.

#6 stargazer193857

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Posted 06 December 2019 - 10:41 AM

What temperature differential will crack glass? That depends on the thickness. 12" sculptures need to be cooled for months. 1" thick mirrors need time too.

As for 1/4" thick cookware, which is much more than 4x as fast as the mirror, here is some data:

Case 1:
Put water in a glass pan, fixing the inside temp to 212F, and place it on a hot grill directly, and it will crack. The temp differential is too much in that 1/4", and it likely does not matter how fast you heat it, since both sides are forced.

Glass classes often don't open the door to the kiln until the glass is below 100 degrees. Even then, they wrap it I a towel to let it cool slower. Some keep it in the kiln till room temperature. It is not uncommon to take an expensive piece of art out of the kiln, let cool, and the hear "tink, tink, ... tink, as it spits out pieces."

I even read of a blank that was made, came out ok, and cracked days later. So slow heating and cooling and precision testing pays off. Micro-cracks can take time to grow, like on a windshield. Therefore, the fastest schedule that looks ok at the time might not actually be ok.

#7 stargazer193857

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Posted 06 December 2019 - 11:43 AM

After that technical info, here is info on whether an amatuer even want a slump mirror mirror blanks or will find lots of others who do. Professionals likely will be bored by this:

I've seen people try to hog and grind a 24"x0.75" blank, only to get astigmatism during fine grinding. Thin mirrors that are successful can potato chip in a cell if the cell is not made right, turning off customers and prompting mirror makers to insist on their own cell being bought and used. I believe a thin center is at fault there, and a meniscus would not have flopped in a simpler cell. A center fan cools the center faster, where glass is thinner, and a meniscus would be thicker to cool evenly.

With thin mirrors cooling faster but unevenly, and big fast mirrors needing more hogging, and thick mirrors costing more, and thin centered mirrors flopping, slumping sounds desireable.

I asked many fast mirror lovers, thin mirror lovers, and big mirror lovers what they thought of slumped meniscus mirrors. I found that at least half liked only 2 of those three qualities and thus were not excited over meniscus mirrors. Though there are people who want a slumped 22+" blank, many amateurs take a year or two to parabolized large mirrors, likely not needing another till then. Even professionals might not have a line of people asking for them, but rather charge high prices for supply side reasons.

Some people think hogging is a huge pain and would love to not have to, while others prefer to save money and do it themselves, especially if they've never hogged at 16" before. There is the question of whether the kiln or float glass has the better anneal. Someone here paid $200 to have a 28"x0.75" Borofloat33 blank slumped, and won't start working on it till a year from now. I don't know if the $200 included the 2 16" blanks, but I do know they too were slumped. Fullum has his own kiln, and did these. Slumping is not a magic fix for hogging, as the mold must already be made for that focal length, else some hogging remains.

Some professional mirror makers told over-excited newbies that not even they, a professional, can figure a 25"x1" mirror. Also, such a mirror would weigh 44 pounds.

Even though Mel Bartel and an associate verified that his mirror and cell performs well, several experts doubt it or think it is due to phenomena they don't understand and might not duplicate with their own attempt. There are fence sitters watching.
Some don't want to change their mirror cell design to work out of plane, though that is actually simple.


Danny and Mark have said many times to make a bowl mold of the back, and put padding or pitch between them. Interest is definitely there, at least among some, but one person's gold is another person's trash, until more definite success stories proliferate. I'm firmly in the meniscus camp, but finally waking up to realize not everyone else is, and that more experiments are needed.

Edited by stargazer193857, 06 December 2019 - 02:30 PM.

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#8 GTom

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Posted 19 January 2020 - 05:10 PM

I don't think slumping has to be overmystified. Hard task indeed but you can buy off-the-shelf conical mirrors, guaranteed Strehl above 0.9 can't you?  This means, processing of thin glass (says 16mm, about 5/8" edge thickness) is possible.

 

Hopefully I'll be able to guinea pig the concept, although not slumping myself. I'd happily try borosilicate, but will keep low budget in case a few blanks fail.

 

OTOH, the goal of these scopes are often not 1/8 wave perfection. How often do you get a seeing that allows you to push near diffraction limit of a 20" scope? (0.2 arc seconds) If I get the resolution of a 6inch perfect APO and the light collecting capacity of a 24-28" bucket, then I'd deem the project successful.


Edited by GTom, 19 January 2020 - 05:18 PM.



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