Nitrox2
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Rubber Septum
Wondering what size would be needed for 20x150mm test tube? I'm hoping I can insert a glass straw into one, that way I have a test tube with a
delivery tube of about 24" long.
This might make up a small scale distillation device capable of doing a few mL's of liquid -- at a time.
[Edited on 6-23-25 by Nitrox2]
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RU_KLO
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Maybe You could use a cork ( or the new "plastic cork" that comes with cheap wine) mold it with an exacto knife - in Argentina we know rh m as cutter)
and bore a hole in the middle for your glass tube.
I hace done it when I was starting. It did not work well because it was bumping all the time. It is really difficult to keep a constant no
bumping/splashing temp in a test tube. If the experiment allowed it, I put a Stainless steel sponge inside the test tube to tame the bumping/splashing
[Edited on 24-6-2025 by RU_KLO]
Go SAFE, because stupidity and bad Luck exist.
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Cathoderay
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A #3 rubber stopper should work with a 20mm opening.
Careful about inserting the glass tube, it can break easily.
https://www.homesciencetools.com/product/rubber-stopper-3-1-...
By the way that website has lots of stuff for starting out.
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Nitrox2
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I am talking about these.
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Cathoderay
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I know, but stoppers should work also.
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Nitrox2
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That is true. I cannot quite remember why I was thinking of using rubber septum, other than that I haven't had much luck with the black rubber
stoppers forming a seal tight enough for the test tube. (For gas/vapor generation/ usage as a glass "delivery tube")
Would this and a hammer work for boring a hole? would I want to use 3/16" hole punch for a 6mm O.D. glass tube?
I know other options exist for borers, but I'm going to have to use something local.
 
[Edited on 6-25-25 by Nitrox2]
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Cathoderay
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I think those punches you show there are for something like leather. They wouldn't work for something as thick as a cork.
Because the cost of a set of cork borers is not small, I think it would be more economical just to buy the corks already with the holes unless you
need a lot of stoppers with holes.
From what I recall putting a hole in a stopper is not easy.
I'd be surprised that you could find septa locally any more so than stoppers.
Keep in mind you do not want to pressurize glassware very much. Things tend to burst.
You definitely don't want to have pressure build when distilling in glassware.
Rubber often becomes harder with age so it may not seal as well as newer.
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RU_KLO
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I used normal drill bits (for wood/ metal). For cork, plastic and rubber.
Start with the smallest you can get, (ex 1 or 1.5mm) then increase the size by 1mm, till you get the desired diameter.
Once you have done this, put the drill in a stationary position, so it does not move, and move/slide the stopper up/down the drill bit, as to use the
side of the drill bit as a file, to smooth out the inside hole.
Go SAFE, because stupidity and bad Luck exist.
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bnull
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I second that. Drill bits come in many sizes and are cheaper (where I am) and more useful than cork borers.
Instead of putting the stopper in the test tube and pushing the glass tube through the hole, insert the glass tube in the stopper and see if it still
fits the test tube. Sometimes the hole you made is a little too tight and the stopper expands, and you'll only notice that when the test tube breaks.
If it still fits, you can do it the right way (stopper into test tube, glass tube into hole); if not, enlarge the hole with a drill bit or a rolled
bit of sandpaper (grain 80 or 100 for wood).
Electrolytic capacitors have two-hole rubber seals (basically stoppers). You can find them in junkyards and TV shops. Use sandpaper to shape the
rubber seal into a truncated cone. Enlarge the holes as needed.
Reusable straws are made (at least here) of borosilicate, as are level indicators for espresso machines.
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angrygiraffe
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Personally, I find corks work better than rubber ones. Just my opinion though.
Let nature take its course,without comparing anything!
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Sulaiman
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an option is to mold your own silicone rubber bung using silicone sealant.
the rubber cures quite quickly if not too thick,
if thick then at least a week is required.
You can get two-part platinum cured silicone rubber but it is more expensive.
______________________________________________________
I think that you will be frustrated if you use small-bore glassware for distillations.
little meniscuses and bubbles easily form in the narrow tubing and produce a lot of spitting.
also typical glass tubing (5 to 8 mm id) is very inefficient when used as a condenser
(due to low internal surface area vs thickness of glass)
_______________________________________________________
regarding glass tubing in rubber bungs:
1 the hole should be small enough so that the bung DOES expand a little when the (well greased) glass tubing is inserted.
this ensures a gastight fit.
2 this method would crack a test tube if the bung is inserted in the test tube before the tubing is inserted in the bung
3 once the tubing-through-bung has been used it is often difficult to remove the tubing from the bung without breakages (ouch!)
so if possible keep the tubing in the bung.
use a different bung with different tubing if there are different requirements.
here are a few 'stuck' glass tubes in red tubber bungs.
the silicone stopper in the filter flask shows the unstretched silicone,
the two fritted filters were pushed through similar holes.
silicone is really stretchy 
click to enlarge
PS cling film is good for keeping cleaned stuff clean.
and
I prefer silicone to red rubber due to its softness,
neither red rubber nor silicone rubber are resistant to many of the chemicals that I use.
ground glass joints are so much better.
(and no more "mark of the chemist" https://youtu.be/Wn9qxPKkRDw?si=YJn5Q5VzP9GwrdtG )
[Edited on 10-9-2025 by Sulaiman]
CAUTION : Hobby Chemist, not Professional or even Amateur
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BromicAcid
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Rubber septum are nice, you roll the sides down over your tube to create a seal instead of friction fitting the thing, and the thickness of the rubber
allows you to easily put in needles for addition of chemicals or to vent or for gas generation.
Something for a 19/22 joint might be close since it's meant to fit a 19 mm hole. Depends on the tolerances of the septum I suppose if it's a good fit
or not but even if it's not a tight fit you can roll it down over the top. Looks like Amazon sells a bunch of them:
https://www.amazon.com/Labasics-Stopper-10-Pack-Natural-Glas...
[Edited on 9/9/2025 by BromicAcid]
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MrDoctor
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at the brew shop they have silicone stoppers that dont seem appreciably different from what one would expect from a "flexible" curing silicone.
something like that you could probably DIY even
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