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Author: Subject: Repairing star cracks
Biochemscientist
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[*] posted on 22-1-2012 at 09:44
Repairing star cracks


Would applying a flame with blowtorch be a good method of repairing star cracks and other small cracks on glassware?
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GreenD
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[*] posted on 22-1-2012 at 09:51


I don't believe so. I've heard once you get one it's time to buy a new one.

The problem is that if you do repair it yourself you are making an area of weakness in the glass. If you ever make an explosion, do refluxes, or reduced pressure you're putting yourself at risk.

But. Give it a try. :D
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stygian
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[*] posted on 22-1-2012 at 11:39


you would likely have to keep a high temperature for a long time, then a slow annealing in a kiln to avoid stressing it. proably better off buying a new one.
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AirCowPeaCock
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[*] posted on 22-1-2012 at 12:07


You might be able to put it on your hotplate at max and slowly approach it with the torch. It wont be as good as new, but it might be better than it was before.



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entropy51
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[*] posted on 22-1-2012 at 16:50


Quote: Originally posted by AirCowPeaCock  
You might be able to put it on your hotplate at max and slowly approach it with the torch. It wont be as good as new, but it might be better than it was before.
Silliest thing I ever heard. Stop the BS man. If you don't know just keep quiet.

I have had a couple of star cracks repaired by a glassblower, but I would never, ever pull a vacuum on those flasks.
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Arthur Dent
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[*] posted on 23-1-2012 at 04:54


I have had star cracks occur when glassware is in direct contact with a heating element, like a stove element, or more recently, a beaker that was put on a hotplate with aluminum paper on its surface. After heating the solution, I removed the beaker, but the aluminum foil was "stuck" to the bottom, and after it cooled-down, several spots had the aluminum paper literally fused to the bottom, and there were star cracks on these spots.

As the posters mentioned above, star cracks are the end of the road. Throw away the vessel, even if it's an expensive one. Period. Because all of know very well that this damaged piece of glassware will decide to fail at the most inconvenient of moments, like when you're heating a strong mineral acid or a flammable solvent :(

Robert




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AirCowPeaCock
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[*] posted on 23-1-2012 at 06:37


Not bull shit! Ive done this many times before and it sometimes works. The hotplate is not to melt the glass, or even heat up the glass so it melts easier. But gently preheating the glass like a hotplate does helps prevent the aggressive heat from the touch expand that crack instead of fixing it. Tiny amounts of whatever like to get in those cracks when they form, if you heat them too quickly the gunk will expand in the crack and totally ruin any chance of fixing it. Why don't you stay out of it if you don't know what your talking about.?



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[*] posted on 23-1-2012 at 06:40


What entropy was saying is that even if you do this, you will not reduce the danger of using that beaker/flask in the future. IMO, it is completely irresponsible to keep, or repair a beaker that is that bad off, with intent of reusing it. No competent chemist would dream of this. If you do any kind of chemistry wet work, the previously damaged beaker/flask will pose a serious, and unnecessary threat to your lab, you, and those around you. Throw it out and replace it.



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watson.fawkes
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[*] posted on 23-1-2012 at 08:25


Repair of start cracks is possible, but you need an annealing oven to remove stress afterwards and a polariscope to ensure that the stress is actually gone. Very experienced glassblowers used to be able to flame-anneal, but even they weren't as good as a soak at a controlled temperature. Even then, it takes a good deal of manual dexterity and skill to ensure that the wall thickness remains consistent.
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ziqquratu
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[*] posted on 23-1-2012 at 20:16


Like watson says - find a glassblower. Anyone with any type of skill can fix them, it's just the annealing needs a proper oven or you end up with a flask that's weaker than with the crack! Our glassblower charges just a couple of dollars to fix them, and I've given him flasks with up to five star cracks, which came back good as new.

As an alternative, even an art glassblower would probably have the equipment needed for the job, if you can't find a scientific glassblower. In a serious pinch, you could have a go at fixing it yourself if you could access a kiln (perhaps you know someone who works with ceramics?) in which you could anneal it. There were some good glassblowing texts in the forum library, as I recall - have a read, see if it's something you believe could try.

Final point - for many purposes, at least for semi-thick walled glassware like jointed RBFs and the like (perhaps not thin glass like beakers), you can still use a star-cracked piece for many purposes. Just be aware that catastrophic failure CAN occur, and do your best to avoid that being a problem (so, stirring a reaction at room temp is absolutely fine, as would be an ice bath, but , say, heating it to reflux DMSO would be asking for pain!). Oh, and avoid vacuum, obviously!

Be smart and careful and you can still get some use out of it, even if you can't find anyone to fix it immediately.
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[*] posted on 24-1-2012 at 03:50


Yeah, it can be done even at home if the vessel is not complicated, but I wouldn't use it for sensitive stuff anymore.
Star cracks can be fused together with a very hot flame, and after the damage is gone, the surrounding area should be bathed in yellow safety flame so it gets covered in soot. After that, keep spinning the vessel slowly bringing it out of the flame. That's the cheap alternative for not having an annealing oven.




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Zan Divine
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[*] posted on 12-2-2012 at 07:42


I have to admit being surprised at the answers given.

The fact is that star cracks on most non-flat surfaces are by far the easiest glassware repair generally encountered.

I fixed glassware for decades for other chemists at my former lab. We then used that glassware exactly as we would any other, under vacuum, hot, cold, even in LN2...never a single failure.

Method:
Approach the star crack with a soft, medium warm oxy-fuel gas flame so as not to cause it to propagate. Move in circles around the star slowly closing in on it.

Increase the flame temperature until you have a well-defined inner cone about 1/2 to 3/4" long.

Focus on the star now. Heat it until the glass begins to sag inward about a mm or so. Don't let the glass become too fluid to control. Easy does it. By gently puffing on a rubber tube connected to the flask joint, push the glass outward by a mm or two. The idea is to make the star look like a slowly pulsing lump. This works the glass and causes the crack to fuse. This cycle is done 10 or 12 times and then the shape is adjusted by final puffs as you cool the glass just enough so it stay put.

Slightly, and in gradual degrees, cut down on the oxygen while playing the flame in circles around the repair. Over 3-5 min gradually cut off the oxygen. Eventually, play the yellow gas flame over the area laying down a thick sooty layer. This is called flame annealing and, for small repairs not near strained areas, it is quite good.

Let it cool, wipe it off, good to go. You will almost always see a very very slight mark where the glass fused. Microbubbles. Not a real concern.

Areas with flat surfaces are highly strained. This fix doesn't work on flat bottomed vessels. Due to the great thickness of glass, large filtering flasks (2 L and over) are also not easily repaired (on the sides only) by amateurs. Also, beakers and most erlenmeyers are so inexpensive (no ground glass) that it's hardly worth the effort to fix even if the crack isn't on the bottom.

I have fixed countless sep funnels, rb flasks, condensers, adapters, solvent traps, chromatography columns and even a spinning-band still over the years and saved many $$$.

Also, in case you weren't aware, vacuum lines (not little manifolds, but 8 foot long, 4 foot high full-scale lab versions) are constructed in place. You clamp the tubing in place, attach another piece, flame-anneal and move on to your next hundred joints. These lines are frequently heated with hot air guns, swabbed with LN2 and generally worked hard for years and they aren't annealed in a furnace. Some of the connections are somewhat large, also, like to the diffusion pump.



[Edited on 12-2-2012 by Zan Divine]
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Endimion17
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[*] posted on 12-2-2012 at 10:04


Zan Divine, that's awesome, thanks for the more detailed procedure.

The only reason why I wrote "but I wouldn't use it for sensitive stuff anymore" is because the repair wasn't done by a professional. I, too, repaired some simple flasks, but the confidence is greater when it's done by someone professional.

Have you ever experienced a sudden burst of a non-properly annealed borosilicate glass and do you use photoelasticity to check for strains?




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Poppy
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[*] posted on 15-8-2012 at 15:55


Cmon people, why so much discouraging so?

Not all classes of glasses are intended too be directly subemerged in lava or hold 167% glass-corroding solution of NaOH. By example, my graham condenser is having problems with one of its cooling water plugs. I performed a test by cracking a test tube and then blow torching it with a common propane torch to see if heat could act as a solder... IT DID, I had, say, 30% succes. The fail was due to inhomogenous heating: the surfaces around the crack slips at different speed distoring the glass, filling the crack with air, which will not solder anymore, as well as having a distorted part.

Heating must be extremely patiently slow. I willtry this, as all I have is a propane torch working in air alone:
Start in the oven having the piece suspended in a brick if needed, heat to maximum temperature with the oven cool at first instance. Remove from the oven with a wooden clamp, hold it so that you can manage it at a 360° freedom. Use the blowtorch at distance waving it around the glass: as at distance I refer the flame should not yet touch the glass. Aproach the flame 15 minutes later. Yes, it will take time. 10 more minutes and you can have full power of the blue flame touchingthe glass. Repeat the procedure backwards to prevent stress to the piece!!
That should consume you 1 1/2 hour or so, guess it compensates for the money you would expend in turn.

As off bubbles, I bought a "promotion" beaker with a bubble up to a milimiter in its bottom, lol, I tought it would crack but its still alive...and has always been working upon direct flame lol




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[*] posted on 15-8-2012 at 19:17


I've cracked a few of my smaller glass beakers- Mostly stress cracking, a drop here or there ;) I heated one in my 'lab toaster oven' set for 400*F for 3 hours, and let it cool inside and insulated to relieve some stress, then wicked it with low-viscosity superglue (Normal stuff, I just happen to stock high-viscosity stuff for modeling)... I continue to use 2 of these for warming water-saturated salts and for long-term evaporative crystal growth. No reason to throw them out! Just be careful and restrict their use.
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[*] posted on 16-8-2012 at 15:13


I'm with Zan on this. It's a question of finding a glassblower with the skill and paying for the time in the annealing oven. 12L on up its probably worthwhile to repair them. I have a star-cracked evaporator flask. I don't think I'll try to repair that. I think it would wobble like hell in the Buchi.



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[*] posted on 16-8-2012 at 17:38
Cracks


Specialty Glass Inc 15525 County road 48 Rosharon, Texas 77583
Repairs all types of cracks in all types glassware. I have had many things repaired cheaply and I just sent a 3 neck 2000 ml flat bottom flask out to them today for crack repair on the center neck. The last repair was a condenser and the repair cost was 25.00. Since, I paid over a 100 for it, that was a good deal rather than pay another 100 for a new one.:cool:
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