hugotommy12
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Ground Glass Joints Not Fitting?
Hi everyone,
Long time observer, first time poster. I searched the forums but couldn't find any answer to my question, but I apologize if this has been answered in
the past.
I've been sourcing the glassware for a reflux setup, and I'm having some issues with joint fit. All glass is new or gently used Pyrex with ST 24/40
joints, yet there is about a 2ml bead of glass showing on the male joint of my column when fully inserted into a female joint. It's like the male
joint it a little too long, despite being labelled identically and coming from the same manufacturer.
Is this something that can be solved through joint greasing? I was under the impression that standard taper joints should just fall together.
Thanks so much!
[Edited on 22-11-2011 by hugotommy12]
[Edited on 22-11-2011 by hugotommy12]
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fledarmus
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I have noticed that occasionally in ground glass joints. As long as the taper matches, the actual length of the taper isn't that critical. Try
dampening one surface with water - if the tapers match, the joint will turn clear along its entire length and the joint will seal the way it is
supposed to. If not, you will show some white either at the top or the bottom of the joint and it won't seal properly.
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Dr.Bob
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Could be one of several things:
1) dirty joint, piece of junk is in joint, I am guessing you would have seen that, but I see this often
2) someone heated the glassware and warped it, quite likely if they flame dried it or left it in a drying oven for a long time at a high temp.
3) really bad glassware, Pyrex is normally not that.
Grease will help, but only so much.
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hugotommy12
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Thanks for the help. I'm really looking to establish if this glass is safe to use. The column was purchased new (deadstock) from a respected supplier,
so it must be a Corning quality control thing which is really odd. It seems to seal properly, there is no shifting of the joint (I cant move them
around when put together) and it wont be put under vacuum anyway.
Is there a consensus on the regularity of this type of thing happening, and if the glassware is still safe to use? Also, if not usable can this be
fixed by a glassblower without replace the whole tube?
[Edited on 22-11-2011 by hugotommy12]
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watson.fawkes
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Quote: Originally posted by hugotommy12  | Thanks for the help. I'm really looking to establish if this glass is safe to use.[...]
Is there a consensus on the regularity of this type of thing happening, and if the glassware is still safe to use? Also, if not usable can this be
fixed by a glassblower without replace the whole tube? |
In short, it seems just fine for use. You're seeing a
tolerance issue. In order to assure adequate joint sealing area, it's OK if a male taper is a bit longer past the small diameter and if a female taper
is a bit longer past the large one. I think you've just got something that's a little longer than normal.
As for fixing it, you could jig the whole thing up and cut off the end with a diamond saw, or similarly grind off the end. Both require the right
machine and leave you with a sharp edge. If you flame polish the end you'd have to regrind the taper. An easier "repair" would be to cut off the taper
and seal a new one on. None of that seems worth it.
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hissingnoise
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| Quote: | | I think you've just got something that's a little longer than normal. |
Yeah, yeah, what about the glass?
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hissingnoise
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It doesn't seem a big deal, anyway, and a few turns of teflon tape, carefully applied to avoid creases might reduce the protrusion . . .
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Vogelzang
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Try wrapping the male part with teflon tape.
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Magpie
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I think I've seen that before. If it seals well along the whole taper I wouldn't give it a second thought.
[Edited on 23-11-2011 by Magpie]
The single most important condition for a successful synthesis is good mixing - Nicodem
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bbartlog
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| Quote: | | As for fixing it, you could jig the whole thing up and cut off the end with a diamond saw |
This won't help, as it is the top that is too wide. Anyway, the piece looks perfectly functional to me. It would be annoying if an issue like this
prevented the use of a Keck clip that would otherwise fit, but it doesn't look like your male piece has a lip for that anyway.
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edgecase
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Has anyone had trouble with cheap no-name glassware? I have a condenser, stillhead, and a few other pieces where the taper angle seems way out of
tolerance, compared to name-brand stuff like Pyrex and Ace-glass (which fit together real nice).
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hugotommy12
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Cheers everyone, you've all been extremely helpful. Thanks again!
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lxvnrsw
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hugotommy12: Take it from a glassblower, here's the trick with getting ground glass joints to fit perfectly.
Getting your glass joints to fit is an easy problem to fix. Get yourself some very fine carborundum powder and make a paste out of it with water. Wet
down both the male and female joints you want to regrind. Apply a small amount of paste carborundum around the male joint and then stick the two
joints together lightly, twisting the joint a few times and then pulling the two joints apart (if you push too hard while grinding or keep the two
joints stuck together for too long you're liable to get the two stuck, and then you've got a real problem). Repeat this process two or three times,
rinsing off the old carborundum and applying fresh carborundum paste to the joint before rinsing everything with water and drying and checking the new
fit, again wetting down the male joint as fledarmus suggested.
This technique also works for leaky glass stopcocks (this technique is what is used when glassblowers want to "match" male and female ground glass
joints to assure a secure fit, hence why you shouldn't go swapping ground glass stopcock internals in lab). It won't fix extremely mismatched glass
joints, but it will even give rather ill fitting glass joints a new lease on life.
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