Glass and Quartz

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Glass and Quartz

Postby Doug Coulter » Thu Aug 26, 2010 9:47 am

Note, as they say on wikipedia, this is a "stub" that I plan to fill in with details as I warm up the scanner and go take some pix of doing all this in my lab.

One wants to read just about any of Kohl's books on the topic of glass or ceramic -- an utter gold mine of good how-to information. John Strong's book is a great reference on quartz and working it -- but that is way not for beginners unless you have some good amount of practice stock and some time to master it. I guess the good thing is that it makes pyrex seem easy after that.

I use 3 types of "glassy" things here. One is Pyrex 7740 (same as Kimble K-33), another is lead glass as used (and bought cheaply in bulk) for neon signs, and the last is quartz from quartz.com.

These pretty much span the range of properties you'd want in a normal lab. Quartz is a fantastic insulator, makes good fibers, a great light pipe but is hard to work without some special tooling. Pyrex is easy to work, a lousy insulator (by glass standards) but that's not always a problem -- you get an intrinsic voltage divider, sort of, using it, and it seals to tungsten quite well and fairly easily. Lead glass is a good insulator (nothing beats quartz, but this is better than pyrex), and has a high tempco that matches some other metals well -- I've sealed Ti wire through it with no problems. I lack a source of DuMet or Kovar to try with, but they should do fine as well -- this is what the bases of many vacuum tubes are made of, while a less insulating glass is used for the bulbs to keep electron charge from building up on the bulb and messing up tube operation.

I found some "didymum" [sic] glasses for working with pyrex (or any soda containing glass) that work like magic and only cut the sodium optical lines out of the view so you can really see what you're doing without the yellow flare blinding you. They are becoming harder to find these days, as they don't cut UV or IR so the newer types (that cost ten times more) are what you mostly find unless you know what you are looking for. I found mine on an art-glass supply webpage I lost the link to (I will look again, but if anyone finds it first, chime in!) for about $40. I made my own propane/oxygen micro torch for pyrex and lead glass,

newtorch.jpg



and for some of the smaller quartz work, but for anything not tiny, you need oxy-acetylene for quartz and real welding goggles, not to mention a fair amount of serious practice as quartz vaporizes not far above the working temperature, and leaves this white coating of condensed quartz stuck all over the work if you push it at all -- it's a delicate temperature balance right around blinding white hot. You have to work quartz while it's still pretty viscous, you can't heat it "runny" and get away with it, so more prep is also needed, as well as jigging for any good work with it.

I cut quartz and some larger glass tubing with a tool post grinder I made for my lathe, with a diamond wheel on it. I run the lathe slow (about 60 rpm) and the wheel fast, and use water from an eyedropper for lube and get perfect cuts everytime. Further, you can also use other diamond bits to chamfer ends (so for example you can slide them over O rings easily) as well. I also use diamond hole saws (sometimes found cheap at harbor freight) to make holes even in the sides of tubing. For that case, you use some modelling clay to make a little dam around where the hole is going and fill that with water as a lube and coolant, and just go slow with constant pressure on the drill press -- and you get through pretty quick. For holing flat things, I wax or water-stick a sacrificial backing plate so the tool won't chip the work when it breaks through -- going slow and with low feed pressure is key for that.

For stress checking any work, we made a crossed polarizer light table to put the work in to see if there is a lot of stress in any joint. Most things other than quartz need to be annealed after working them (various techniques are possible) or they will suddenly explode without warning sometime later on. Kohl's new book (at least, didn't check the old one) has some time/temperature curves for best annealing, and I have a heat treat oven I use for critical pieces that can be programmed to do that. Many other glass blowers simply turn off the oxygen when done, but keep the sooty flame on the work till it gets coated thickly with soot, cooling it slowly enough -- most of the time, but this isn't always good enough or at least not for me, but then again, I'm not truly expert, just good enough to make what I want, which tends to be tempered with what I think I can pull off ;) .

In a way glass working is like welding or some other things -- it's all the prep, the jigs, and the other tools. Too late to fish for something once you get it hot, you have to be ready, and performance visualization is a good technique to practice before starting the torch. Glass (or quartz) doesn't stick to graphite (even the low grade stuff like welder's gouging rods) so you need some of that -- with insulating handles, as graphite is pretty thermally conductive, which causes other issues if you hit hot glass with it cold, and quickly burnt fingers if you don't put it into a good handle. Various shapes of graphite tools and jigs are well worth making for this work. I made some nice inserts for the tubing sizes I have to help make creating "tees" much easier -- you put one in the tubing you've put a hole in (I use the hole saw) and that keeps it from collapsing while you weld on the sidearm -- you can now use a bit of force and/or take your time at that job (maybe using some cane to fill in the weld joint and make a "bead" there for strength. On the other hand, you sometimes want something that does stick to hot glass, and for that I use a tungsten welding rod in a holder to for example, pull off excess glass for which it works better than a piece of cane.

For what it's worth, here's how I make a tungsten/pyrex seal. I use TIG welding rods, either .040 or .0625 usually, but wire works just as well -- the rods are cheaper.
First you get it clean. I've never even tried the "dip in molten nitrite" thing, just sand them off a little with fine grit seems fine. You then pass them through the torch flame for a very short time. You just want to see a blue oxide on there -- if you go too long, it turns other colors or black -- start over. No matter what size I'm going to seal it into, I begin by drawing out a section of thin pyrex from a 1/4" piece of tubing to about the same size as the tungsten, a little larger for a loose fit. Slip this over the rod, being careful not to scratch that oxide -- or start over again. Then, working from one end, put it in the torch and slowly melt the glass to the tungsten (surface tension will do this for you) starting at one end and working to the other so you dirve out any air trapped in there. A few bubbles won't hurt this, but there shouldn't be many. Now the thing is safe to leave laying around. When needed, you just melt it into the end of some pyrex tubing, using whatever heat is required to make it shrink down that much and fit. That's the easy part. You should anneal this seal before use, preferably with the "soot" method, and perhaps in the oven later If it looks good under a magnifier when you make it, it's good, but may crack later if not well annealed and leak. Tungsten rod often has longitudinal channels in it. The only way to fix that is to seal the ends. I have heard of using braze to do this but haven't used that (not sure which alloy would wet the tungsten) -- the advantage would be you could solder a wire to the air end. I simply use my tig welder to melt the ends, then spot weld to the air side. To make that work, I wind the wire around a few times for mechanical strain relief, then just spot weld the end away from the rod end (eg close to the glass part).

I have used the same basic technique to put Ti wire (.032") into lead glass. Here there's enough tempco mismatch that I use Hysol epoxy or real glyptal after to keep the seal good, and the glass is mainly the mechanical strength part. Hysol from McMaster is pretty good vacuum epoxy if you really mix the snot out of it and use it carefully, and then don't get it too hot. It takes quite a long time to cure, so you have to be patient and not even touch it for 24 hours after putting it in a warm place to cure.

As I said above, this is a big one, and I will get back here soon and fill in a lot more info from the various books and actual experiences here. I think there's so much this might deserve a fairly long set of responses just to keep it organized.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: Glass and Quartz

Postby Jerry » Fri Aug 27, 2010 2:16 am

A friend of mine recently bought a pair of glassworking glasses and they were not badly priced, i think about $100. These have full protection from IR and UV.

I took some scientific glassworking classes a long time ago. They taught me how to make tungsten glass seals. You need uranium glass boro tubing to do it right. Heat the tungsten rod in the flame until it gets to a "Kentucky Fried Chicken" brown inthe joint area. Slip a piece of uranium glass tubing over the joint area and heat with a broad flame. The tubing will bond to the rod and seal around it. At that point you can attach it into a piece of regular pyrex with no issues. You see these types of joints often in large power vacuum tubes and a lot of lab equipment. The tubing can be found on ebay.
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Re: Glass and Quartz

Postby Doug Coulter » Fri Aug 27, 2010 7:25 am

If you find a link to them at that price -- let us know, that's not bad compared to what I was finding about a year ago (I am NOT the master scrounger here). I don't think it's that important for pyrex, but for quartz, there is enough of both IR and UV to be a real issue...facial sunburn! I use an arc welding helmet for that one as there isn't the blinding sodium flare (D lines) involved with pure quartz.

I've never found a source for, and thus never tried the U glass, though I do see it used on definitely-not-tungsten seals in big power tubes of WWII vintage -- usually Kovar or DuMet (you can tell by the color of the metal where it goes through the glass among other things). Those are much more conductive for the high currents involved -- which can be a serious issue if it heats up right at the seal. I found that direct sealing of W to plain 7740 works fine, but if you oxidize the tungsten to the brown state, the oxide is far too thick for a good seal to be made (could be with some other glass that dissolves some more of it, it would be fine -- as Kohl points out when looking at the chemistry of it all). See Kohl on that one -- everything from his books I've tried has worked fine for me. And what he says doesn't work, doesn't work. Reliable source.

According to Kohl, many glasses (he must mention at least 100 glass alloys) are used to make "graded seals" between things of non-matching tempcos to gradually adapt one to the other. I'd bet that was the case there. For example (in non-Houskeeper designs) one uses a graded seal to get anything sealed into quartz, which has a lower tempco than just about anything conductive. I note that all the quartz-halogen bulbs I get use the Housekeeper plan as it's cheaper to make a pinch seal that way. In a Housekeeper design, the metal is set up to have enough "give" to accommodate, so thin, ductile metal is used. That would be the thin metal tape you see in the base of a bi-pin lamp, which is actually the vacuum seal part - the pins are in there mechanically, sometimes a ceramic cement is used to hold them better, but the real seal is to the thin tape. In the case of quartz the other issue is can the metal stand the heat needed to melt quartz -- most won't, so for awhile, the graded seal was widely used, but being much more complex (expensive) it was mostly ditched in cases where it's not needed.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: Glass and Quartz

Postby Jerry » Fri Aug 27, 2010 11:19 pm

Found it, took some searching. All they do is glasses.

http://www.auralens.net/e_gwrecommend.cfm
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Glass and Quartz -- data

Postby Doug Coulter » Mon Nov 08, 2010 5:30 pm

Here's what Kohl has to say about annealing glasswork for a few of the popular alloys and tempcos.

KohlAnneal.gif
Kohl's tables on annealing


I've used these (and much other info from this source) and yes, this is good data.

Here's some data on volume resistivity of glass vs temperature. Sadly, there's no one nice table that matches the names to the numbers, but things with a lot of 9's in the numbers tend to be either quartz or vicor, or quartz/alumina.

KohlResistance.gif
Bulk resistance of glass alloys


Glass breakdown voltages vs this and that:
GlassBreakdown1.gif

GlassBreakdown2.gif


Here's some info on RF losses. It will be obvious why I had to go from 7740 to quartz for the microwave ion source. Losses!

GlassLosses.gif


Anyone who needs more, ask. If you're doing serious work in vacuum, note that this is a tiny part of one chapter of a very good book on many materials for vacuum use, not just glass, and you should bite the bullet and buy it if you can find it -- well worth it -- likely to actually save you more than it costs.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: Glass and Quartz

Postby William A Washburn » Thu Dec 16, 2010 8:12 pm

Anneal, anneal, anneal...

Learned this the hard way when some of the previous day's work went "plink", flew across
the room and smashed itself on the opposite wall. Good thing nobody was in the way.

Best is to use annealing tables and an annealing oven following the curves in the tables.

On the other hand (if you have no annealing oven) turn your torch O2 down and bathe your work piece
in the orange flame moving back and forth for maybe 1/2 hour (depending on the mass of glass you're working
with) and don't forget to turn the flame down a little every few minutes.

It is amazing how much energy can be stored in a 1 in dia 2ft section of pyrex or silica.

It's also amazing how easy it is to produce a strain that you will never know is there until...

Anneal, anneal, anneal...
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Re: Glass and Quartz

Postby Doug Coulter » Thu Dec 16, 2010 11:58 pm

Indeed anneal, I even do quartz. I find that the the normally thin and simple stuff I do that the torch method works in only a minute or two of extra torch time (which still seems like a long time when you're doing it), especially if you really coat the thing up good with soot, while it's on a good insulator. If you can lay it flat on there, convection cooling is reduced. We have some special foamy firebrick (don't know what else to call it), that has almost zero thermal conduction or mass - instant white hot when hit with a torch flame, cool to the touch a few seconds later. I cut some grooves and whatnot in it to act as jigs for things like Tee's along the way.

The other useful (and just plain fun) thing to do is crossed polarizers. I made a stand with two big polarizing films, CCFL lamp under one (sharp spectral lines are good here), crossed polarization. You can then see the stress in your work placed between them as colored contour lines, and while that might not tell you just how it's going to explode, it does say whether it needs better annealing or design. Kohl also covers what happens in thick to thin welding (a problem in just about all domains) when the glass doesn't all cool the same speed and some sets before other. In that case, the oven is king, and do follow the tables. After awhile, you don't need the viewer as much, you can kind of predict what's going to be troublesome, and hopefully fix it at the design stage. If not, see oven again, and this time, preheat partway it so it's ready for the work. Hopefully none of your work is stone cold while some is hot, whip the torch on it some while working -- some shapes heat all over naturally pretty well too.

And yes, is is startling when something jumps a few feet seemingly for no reason.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: Glass and Quartz

Postby Jerry » Fri Dec 17, 2010 2:27 am

I helped a friend make an art piece that was specifically made to have strain. I used backlight assemblies from 32" LCD tvs and polarizers to make up a strain viewer and mount to hold the piece.

Image
cafe5 by macona, on Flickr
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Re: Glass and Quartz

Postby William A Washburn » Fri Dec 17, 2010 7:58 am

These are really nice. If it's OK with you I want my daughter (40 YO artist in Portland, OR) to see these. Would it be OK to give her
a link or JPG? She does everything including glass (electric kilns). Sometimes she gets commissions for leaded glass
(as in church widows) and they are beautiful. When she firt started out in Portland she went to work for a pot-glass manufacturer called
UroBoRos [http://www.uroboros.com/index.html]. My brother and I got a tour one night on the way to dinner. These folks would get
a pot and usually put in three colors, and mix carefully, (the mixers were the true artists), pour out just enough into a given roll press with
etched rolls to give the glass random texture so the kinda square 2/2 ft that came out was the right size and thickness.
I spent 1/2 hour going through the racks of cooled finished stock and almost bought a piece to put in a light box on my wall. It
was sky blue mixed with almost black blue and a nedium blue all with a beautiful texture in transparency and thickness.
With nobody having touched the blank it looked like a flowing river! Another had three reds and one orange and looked like fall leaves,
"again with no intervention". Those flat-looking leaded glass works you have seen made of transparent colored glass, when re-cut and
say a leaf replaced with this part number would simply make you breathless. Anyway, Katy does the kind of work you displayed and would
be really happy to see your work if that is OK with you.
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Re: Glass and Quartz

Postby Doug Coulter » Fri Dec 17, 2010 9:44 am

Really nice Jerry! I'd never taken it to quite that level of size, but I'm glad someone did. Very pretty! I'm using a "spare" LCD backlight here as a light table for drawing, and hadn't thought of that use for it. Cool!
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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