Sciencemadness Discussion Board

Ground Glass Joints Not Fitting?

hugotommy12 - 22-11-2011 at 10:49

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]

fledarmus - 22-11-2011 at 11:39

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.

Dr.Bob - 22-11-2011 at 11:40

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.

hugotommy12 - 22-11-2011 at 12:11

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]

watson.fawkes - 22-11-2011 at 13:04

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.

hissingnoise - 22-11-2011 at 13:35

Quote:
I think you've just got something that's a little longer than normal.

Yeah, yeah, what about the glass?


hissingnoise - 22-11-2011 at 13:45

It doesn't seem a big deal, anyway, and a few turns of teflon tape, carefully applied to avoid creases might reduce the protrusion . . .


Vogelzang - 22-11-2011 at 14:03

Try wrapping the male part with teflon tape.

Magpie - 22-11-2011 at 16:05

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]

bbartlog - 22-11-2011 at 16:37

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.

edgecase - 22-11-2011 at 18:47

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).

hugotommy12 - 22-11-2011 at 19:33

Cheers everyone, you've all been extremely helpful. Thanks again!

lxvnrsw - 24-11-2011 at 22:16

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.