I'm trying to construct a hollow glass sphere, about 0.5-1 cm in diameter, with thin walls, like 250 microns.
I'm using a borosilicate tube 4 mm in diameter, and a bunsen burner. After sealing one edge, I'm gently applying some pressure and a hollow sphere is
formed, but it collapses when the wall thickness gets thinner about 800 microns in thickness.
Does anybody know if this is possible to achieve at all, do I need to use other equipment? Should I use another technique? Any references would be
greatly appreciated.
In reference 5 they don't say how, but 100 microns seems to be routine, and that's ~100 years ago...Morgan - 3-1-2021 at 18:57
Maybe the bubble collapses if the glass is too hot to support a bubble shape for that thinness. Or perhaps there's an elasticity that needs to harden
or cool before releasing pressure. I wonder too if the bubble that forms has a relatively uniform thickness or that if variation might somehow affect
the tension or stability of the bubble.
[Edited on 4-1-2021 by Morgan]densest - 3-1-2021 at 21:37
Please forgive me if I'm writing things you already know. I hope this helps.
There are lots of flameworking videos on YT. Borosilicate glass is NOT what most people work with. Make sure to learn boro not "soft glass".
From my experience & what I've read:
PROTECT YOUR EYES! White hot borosilicate glass emits damaging amounts of IR and even some UV!
You are wearing eye protection, right? Sunglasses aren't anywhere near good enough. You want a sodium glare filter anyway in order to see what you're
doing.
Unfortunately good lampworking eyeglasses aren't cheap but you really want something good.
Working boro on a bunsen burner is marginal. Propane/air is barely hot enough.
Flameworking is a craft and there is some technique to learn.
Learn where the hottest part of the flame is and the gradient working out from there.
If you want something symmetrical you >must< continually work to keep the thickness AND the temperature even over the work piece.
This means constantly rotating the tube in the flame.
The hottest glass will move quickest so as soon as you >think< you see a bulge move that out of the flame and the thicker part into the flame.
If you stop turning or stop distributing the heat evenly the hottest part will burst.
Once the bubble is >almost< as big as you want, start moving away from the flame (still turning) still blowing gently so that as the bubble
hardens it's the right size.
If you can, turn off the air so the flame is very smoky and cold. Cover the bubble with soot. This is a crude way to anneal the glass. If you have a
little ceramic wool, vermiculite, or other high temp insulator let the bubble & tube cool there.
[Edited on 4-1-2021 by densest]Chemetix - 4-1-2021 at 02:13
Whats the application? Because making something this small isn't going to be easy. Does it need to be boro? Temperature and chemical resistant?
If I was to make a perfect sphere this small with walls this thin then a very complex torch is going to be needed.
I'd pinch off a small piece of glass tube at either end then using a cold seal ( it's a semi permanent way of joining two bits of glass) to hold it on
a rod, I'd then introduce it to a air /oxy flame and slowly raise the temp of the piece so it begins to inflate. If you can get it to a decent
spherical shape then dislodge the cold seal and introduce it into an air /oxy flame. Now this is the tricky bit. The torch is going to need to be set
up to suspend the small spheroid in a Bernoulli flow of air while and an exterior jet of fuel and oxy is introduced with precise control. The sphere
is going to be inflated slowly with increase in temperature, then the fuel shut off abruptly to cool the sphere and keep it suspended in the air flow.
I know! This sounds like an industrial process and will take ages to set up with testing and calibrations. So I'm guessing you need thousands of these
right? Because that's the only way this will make sense.
If it's just a sphere on a stick like a pH probe then it's a little easier. An electric drill and a small torch and some practice. Getting the pinch
to inflate to the right amount with heat, you just have to choose the right starting diameter. Otherwise you will need an open ended lathe chuck of
sorts to connect a rotating blow hose and then inflate to the size you need.
If it's as thin as you say they will be then annealing and temperature control on cool down isn't needed; and the soot technique doesn't anneal by the
way, it's something that just makes a mess. Junk_Enginerd - 24-1-2021 at 02:34
What do you mean "collapses"? That's not how glass behaves in my experience. It should be literslly impossible for ot to collapse while applying
pressure, just as a rubber balloon isn't going to collapse while there's air in it.
I'm a novice lampworker and a 1 cm sphere with 250 micron walls is dead simple. I've made them with probably 10 micron thickness, mostly by accident.
Just heat a sealed end, remove from flame, and blow to desired diameter. Wall thickness of the sphere will be determined by wall thickness and
diameter of the tube.
Are you maybe applying pressure while the glass is in the actual flame? Because that's never going to work well. You want to do that while it's out of
the flame, so that thinner portions are cooling down faster than thicker portions making the thickness even out automatically.Fyndium - 24-1-2021 at 10:52
Working boro on a bunsen burner is marginal. Propane/air is barely hot enough.
Flameworking is a craft and there is some technique to learn.
I don't know if the temp resistant flasks I annealed and formed into ampoules were made of, but using a 20kW high pressure propane burner was very
barely able to make the glass to form, and it was only very slightly glowing when the room was dark. The whole flask was immersed within the flame.
So, actual working in boro without supplemental oxygen is hardly doable.Junk_Enginerd - 1-2-2021 at 04:32
They were most likely borosilicate then. Not a lot to choose from. Soda lime will easily soften in flame, so it isn't that. The other end of the
spectrum is quartz glass, which won't budge the slightest from an air/propane flame. I tried working quartz with my oxy/propane torch and even that
was only barely enough to move it even while in the flame.
I'd say you can just barely do some minor things with borosilicate with propane. I use oxy/propane, but before I had my oxygen generator I had to
contend with air/propane. I could do bending and very minor blowing with propane/air but it's very tedious and it rarely turns out exactly the way you
intended. It is doable, but not really reasonable.
