Some questions about glass working.

[jrich7720]

I'm doing some research into certain aspects of glass working, as I am formulating a plan to cast my own telescope mirrors. My tentative plan at the moment begins with constructing a bell kiln in which I can place molds for my mirrors. My reasons for wanting to build the kiln myself are twofold; I can build the kiln to the specifications that I need for my work, and it will save me a considerable amount of money.

I plan on using fused silica as the substrate. I'm having difficulty locating a supplier for this, all I've managed to find online is a European-based technical glass retailer that carries UV-grade fused silica plates. The problem is that these plates are undersized for my purposes and shipping to the States would be outrageous given the weight. I'm not sure where else to check, as it's not like I can buy directly from Corning. I guess I have it in my mind that I should be able to buy the stuff in amorphous, granulated form in 5 gallon buckets. If fused silica turns out to not be a viable option, a borosilicate such as Pyrex or an equivalent would be my second choice.

As far as the forming, I am under the impression that I will need to heat the silica to its softening point of 1665C(3029F) in order to melt and consequently cast my parts. After allowing the mirror to cool, I would then need to reheat to its annealing point. In my research so far, I can't seem to come up with "melting points" for glass substrates, only "softening points". What would be the difference? I have also been unable to determine whether the silica would remain amorphous throughout the process, or if an additional step in the process would be necessary.

Sorry if that seems long-winded. This has been a frustrating subject to research. If anyone can shed some light on any of these questions I have, I would be very appreciative.

Thank you!

James

[Matt P]

I don't have much info, but glass rarely has a melting point listed because it arguably doesn't have one. It has a softening point/range where it begins to move.

[Jimi The Don]

try fdglass.com they may be able to provide what you're looking for.

[Shattered Dreams]

and as you let your mirror cool, do it slowly and stop at its annealing point, then continue the slow cooling, instead of having to reheat.
if you cool it without annealing it may crack.

[somewhere]

I want to hear about this kiln your going to build. I'm guessing this will be a gas fired kiln?

Sio2 is easily obtained in its raw form (sand) from many suppliers such as standard sand. It is commonly used in all glass making. That said to melt it without any flux requires some extreme temperatures which I doubt you can do in a homemade kiln. Just the refractories involved are not common to say the least.

I have quartz ingots and large quartz disks up to 27" if that does anything for you.

In this path lies the mind of a mad man. LoL!

[jrich7720]

I want to hear about this kiln your going to build. I'm guessing this will be a gas fired kiln?

Sio2 is easily obtained in its raw form (sand) from many suppliers such as standard sand. It is commonly used in all glass making. That said to melt it without any flux requires some extreme temperatures which I doubt you can do in a homemade kiln. Just the refractories involved are not common to say the least.

I have quartz ingots and large quartz disks up to 27" if that does anything for you.

In this path lies the mind of a mad man. LoL!

At the moment, I am basing my design off of this guy here:

http://www.enfusiasm.fusedglassartists.com/kilnbuilding.pdf

This is the only sort of information I've been able to turn up on a DIY kiln, and like I said my plans are still quite tentative. It's an electric kiln, but I imagine the design could be converted to gas. I don't think the design as it is presented would be able to suitably melt silica. What sort of heating elements are used in a gas fired kiln? I imagine if I were to use propane I should be able to source the proper components such as lines, manifolds, and regulators, using 40lb tanks for RVs. I'll check with my local propane supply to see if they could outfit me with 100lb vertical cylinders, as I'm sure I would burn quite a bit of gas to get the kiln hot enough and keep it running through the annealing process. Or would you recommend a different fuel?

I'm considering picking this book up to help me along:

http://www.amazon.com/Kiln-Book-Materials-Specifications-Construction/dp/0801970717/ref=pd_cp_b_0

I'm not sure how much information it provides on bell kilns specifically, but I imagine a great deal of the information is still relevant.

As for the mirror I will be making, it will be 1 meter in diameter and will need to be cast to a thickness of 200mm. I will have to cast two, one will serve as the primary objective and the other will be used as the grinding tool. Pretty big! :devilish:

[jrich7720]

As far as refractories, I've noticed that K23 bricks have a maximum limit of 1648C, which is a little below the softening point of fused silica. So, would anyone have any recommendations?

I checked out Standard Sand, but their products look to be focused on abrasion applications; I don't think they would qualify as optical grade. I think on Monday I'll give Heraeus Quartz's sales office a call tomorrow and see if I can order. Hopefully, they don't limit their sales to wholesale.

Also on the silica, does anyone know if the glass will remain amorphous throughout the softening and annealing processes? I know that when it is manufactured it is fused at a temperature above 2000C, much less than what I will need to melt it. So, I'm hopefully wishing that it won't recrystallize unless it is reheated to this fusing point. Anyone know anything about this?

Thanks again for all the input!

[khan]

Electric is not going to work. Propane will do for fuel but you will need power burners. http://www.wardburner.com/burnersparts/venturiburners.html
Frank Olsons book has several home built kilns in it for reference. I used to build kilns for firing porcelain to 2500* F. if you start going much above that your asking for VERY expensive materials and building methods.
Making mirrors is an art all its own... i would advise you to write to the ppl who are making them... Find out who is making them for like the hubble and write to them about the stuff you want to know. The ppl in this forum just dont deal with mirror making so much. But i would like to see what plans you come up with.
I am pretty sure the big mirrors are made in kilns that spin so you get your parabola shape from the centrifical force. Other wise you will have to grind it by hand. Polishing will still have to be done by hand even after you get the parabola. Your in for some serious work.
I have a 13inch scope myself and after careful consideration i decided there was no way i could make that mirror ... that big... for what i paid for it. Besides if i made a scope bigger than that i couldnt move it to dark skys. You can see more with a small mirror from a dark sky location than from a big mirror under bright skys.
Have you ever seen someone grind or tryed grinding a mirror? Damn it is a long long process. I would give all this some serious thought.

Khan

[CheeseNip]

Have you looked into making a glass telescope lens' much? I have done a bit of research on it, and not to sound like an ass or anything, but I have a hard time seeing somebody without a lot of scientific knowledge (not saying you don't have scientific knowledge) or any knowledge of glass molding or grinding and polishing doing so successfully. As I'm sure you know, telescope lens require very strict tolerances/quality control to work properly. How are you planning to add the Ultra high transmission coatings to the lens? As far as i can tell, it either requires machinery to do so, or being sent off somewhere (which is easy enough and I'm assuming much cheaper).

^ I also don't really think the people who made the lens' for the hubble or anything like that are going to offer much help honestly. There are probably patents on the lens' that they use i would guess.

[menty666]

That's odd, didn't someone else ask about this within the past month? Not to say there's anything hinky going on, it's just weird that two folks showed up looking to cast telescope mirrors.

[mellofello]

I'm just wondering why it is you want to do this? Just for fun? Certainly isn't going to be any kind of money saving here and you will struggle to get a product that is of a quality you could sell especially with a home made kiln. 1 meter by 200mm is pretty damn big. You will also need a high level of control over the cooling process so you'll be looking at using high quality components that are going to be very expensive.

I don't want to put a downer on your plans jsut trying to get an idea of what your goal is. If this is jsut a fun project be prepared for it to be a very expensive one too

[CheeseNip]

http://www.npr.org/blogs/thetwo-way/2013/08/24/215156780/tons-of-molten-glass-go-into-making-mirror-for-giant-telescope

[Greymatter Glass]

jrich7720, welcome to The Melting Pot!

So... to get to "brass tacks" here, a few questions before you proceed as planned...

Are you willing to spend in excess of $5000 to build this "kiln" to melt silica?
Do you have any experience with high temperature glass forming, glass physics, chemistry, engineering, etc? (what is your background in approaching this?)
What, if any, assumptions have you made thus far?


There's a BIG difference between a glass annealing kiln and a furnace capable of fusing silica. When you get into talking about optical grade crystal you further complicate thing because now you're also dealing with atmosphere and contamination. Controlling those two factors is not easy, and it sure as hell isn't cheap.

You're dealing with a process that large multi-national corporations generally handle. That's not to say you can't do it, but there will be challenges that you will have to overcome that go far beyond the cost and build. It's more than just "throw some sand in a box and add fire lol!"

As for raw feed stock... if you want to start with raw silica that's going to be an issue. The people who make a business of making fused quartz all start with raw SiO2, or silica sand, from a sand quarry. It's usually supplied as an "industrial" grade that's around 95-98% pure SiO2 straight from the ground. That's pretty good, often good enough for general glass making. You can pick this up at any ceramics supplier that sells clay and glazes and such... but you need reagent grade PURE SiO2. There's a few ways that's made, and none of them are easy or cheap.

You can start with a reagent grade silica compound like TEOS, and reduce the silica out as a nano particulate... that's common enough if you need small amounts, grams or ounces... you're going to need up to several hundred pounds when this is all said and done....

If sticking with sand, you'll need an analysis of the batch, find a good analytical lab that can give you a count of contaminants in the PPM range. Then once you know what's in the silica besides silicon and oxygen you can formulate a plan to purify it...

You can use powerful acids and solvents to wash the silica sand. Throw enough acids and bases into the mix and get something like 99.95% pure silica, that might do you right. Corning Vycor is made by boiling borosilicate tubes in various salts and acids to remove the flux and then it's heated up and re-fused... results are not pure silica, but it's a start.

Also, you could look up the vapor point of contaminants, if they're lower than the silica, just burn them out. If they're not lower, you have to find a way to get rid of them.... maybe turn it all into gas and react it with a catalyst of some sort? I did mention this isn't cheap or easy. Especially not in small batches.

You might contact someone GE or Corning and see if they can supply you with some high purity SiO2.... and chemical suppliers like Sigma-Aldrich and Strem sell it for big bucks.

Of course, there's a better way, one most glass blowers are familiar with, and that's to start with something that someone else already did all the hard work on. Just buy fused silica. You can get it in rods, blocks, tubes, ingots....break it up, lay it in your mold, and add enough heat and it will re-fuse, chances are they already got it as pure or better than you'll ever do on your own.



So now you have your quartz. As you found out, normal kiln bricks and wire elements don't like to go much above 2400ºF so you'll need special stuff.

Bricks are easy enough, but expensive. AZS (alumina zirconium silicate) blocks washed in a platinum ceramic slurry might handle the heat? Don't really know.... I would look into refractories for plasma arc furnaces.

Then you need a heat source. Propane and air probably won't cut it. You'll need some amount of oxygen enrichment, and some way to control the furnace atmosphere so you don't reduce you quartz.

Plasma Arc furnaces are very useful for stuff like this.


or.....


You could just do what 99% of the people who get started on this project do and buy a lens/mirror blank and get on with what they really want to do, which is make a telescope.


Also, all the above would be about 1,000,000x easier if you stick to glass instead of fused silica, for which there's not a significant advantage to using...

But lets say you DO go on with this project, please document it and share the pictures here. I'd love to see it.


-Doug

[Greymatter Glass]

I imagine if I were to use propane....using 40lb tanks for RVs.at deal of the information is still relevant.


...

As for the mirror I will be making, it will be 1 meter in diameter and will need to be cast to a thickness of 200mm.

I should have read this before I replied.

Propane would work, but you'd be better off with a 4" natural gas line. To use propane you'd be looking at several tons of gas, think lining up rail cars on your street.

A lens that size, you're looking at high tens if not hundreds of thousands of dollars to build a furnace capable of doing that.



Sorry man, if you do it that would be awesome, please post pictures. But even if you wanted to cast a 1000x200mm borosilicate blank you'd be looking at similar costs, not to mention the months you'd spend annealing it, and the years it would take to plan the melt.

go to youtube and look at the corning videos of them making big telescope mirrors. scale down the 100 million dollar price tags on those to a few million, assemble a team, get a few phd's under your belt and call us in the morning.

[mellofello]

Greymatter thanks for a really interesting post man, I knew there would be all kinds of consideration with the raw material for the glass but its something I know nothing about so thanks for taking the time to go through that.

Like you when I saw the size he was looking to produce I knew this project needs crazy $s

jrich I'd love to see you try this, sounds like an interesting project indeed.

[khan]

[QUOTE=CheeseNip;753744]Have you looked into making a glass telescope lens' much? I have done a bit of research on it, and not to sound like an ass or anything, but I have a hard time seeing somebody without a lot of scientific knowledge (not saying you don't have scientific knowledge) or any knowledge of glass molding or grinding and polishing doing so successfully. As I'm sure you know, telescope lens require very strict tolerances/quality control to work properly. How are you planning to add the Ultra high transmission coatings to the lens? As far as i can tell, it either requires machinery to do so, or being sent off somewhere (which is easy enough and I'm assuming much cheaper).

He is talking about making a Mirror as in for a "reflecting telescope".... not a Lens as in for a "refracting telescope" ... totally different animals
Seems ppl post about things they know absolutely nothing about.

Khan

[mojigglins]

It can be done! You are making a mirror not a lens so no need for pure silica. Lots of other cheaper options to use, And maybe 1000x200 is a bit ambitious for a first attempt maybe start small then take what you learn and step up the size. when I get to work tomorrow I will pm you suppliers of batch material and oven materials, look me up on the web www.sandfire.com I think I can be of assistance.

Just found this on YouTube.

Cheers.
https://www.youtube.com/watch?v=XBlu6vXF10M&feature=youtube_gdata_player

[jrich7720]

Thanks for all the replies, guys. I really appreciate it. And yes, I am trying to make a mirror for constructing a Newtonian reflector, not a lens for a refractor.

Greymatter - I'm only trying to achieve the softening point of the quartz, not the fusing point. So, I don't really think that this kiln should run me $50,000. I just need it to get up to 3100F or so, taking the softening point of 3029F that I am seeing for the silica. I do not need the quartz to be ultra high purity. A few impurities will not be any big detriment. I have chosen fused quartz over borosilicate as my starting point given it has an even lower coefficient of thermal expansion than borosilicate. Given that the mirror will be 1 meter in diameter, it will collect huge amounts of light and in turn become very, very hot. The lower coefficient of thermal expansion will reduce image distortion. After the mirror is ground and polished, the surface will be aluminized or otherwise coated to make it reflective.

I appreciate the suggestion for using a 4" natural gas line, it looks to be common size line that I could easily source.

The guy who posted the article on the Magellan optics - I realize the amount of work involved in figuring a scope that large, but you can't really compare a 72-foot telescope to a roughly 3-foot telescope. It's just not fair. That telescope could revolutionize astronomy. The 1/20th wavefront they are figuring those mirrors to is about as good as it gets. It's beyond the threshold of human perception. I probably won't achieve that with this mirror, but don't think that DIYers haven't achieved that level of precision with hand figured optics. There are plenty of examples. I also plan for this mirror to have a slow focal ratio which will make figuring it much easier.

And no, I have not yet ground my own telescope mirror. This mirror is not something I plan to start tomorrow, but rather two to three years down the road. It's going to require a lot of research, planning, and practice before I will even consider beginning. In the meantime, I plan to order kits for some smaller mirrors. I'll probably start with a 6", then a 12", and then a 20", perhaps grinding two of each before moving up a size. I'll also work on getting this kiln built in that span. This mirror is something I am planning to do because no one to this point has made an effort to make a mirror this size. I believe Intermountain Optics may provide blanks in the 1-meter class, but the mirrors are thin and they are fast(meaning they are figured for a short focal length). I want to build a slow scope with a full thickness mirror. At 200mm, my mirror would actually exceed full thickness. Intermountain would probably cast a mirror this size to somewhere around 60mm thick, less than half thickness. They would probably also charge somewhere around $25,000 for a mirror this size, maybe more. Obsession Telescopes would charges around $50,000 for a 36" scope. I also intend for this mirror to be a stepping stone to making even larger mirrors, perhaps 2 meters. It may even evolve into a business to support myself during or after college.

[jrich7720]

Electric is not going to work. Propane will do for fuel but you will need power burners. http://www.wardburner.com/burnersparts/venturiburners.html
Frank Olsons book has several home built kilns in it for reference. I used to build kilns for firing porcelain to 2500* F. if you start going much above that your asking for VERY expensive materials and building methods.
Making mirrors is an art all its own... i would advise you to write to the ppl who are making them... Find out who is making them for like the hubble and write to them about the stuff you want to know. The ppl in this forum just dont deal with mirror making so much. But i would like to see what plans you come up with.
I am pretty sure the big mirrors are made in kilns that spin so you get your parabola shape from the centrifical force. Other wise you will have to grind it by hand. Polishing will still have to be done by hand even after you get the parabola. Your in for some serious work.
I have a 13inch scope myself and after careful consideration i decided there was no way i could make that mirror ... that big... for what i paid for it. Besides if i made a scope bigger than that i couldnt move it to dark skys. You can see more with a small mirror from a dark sky location than from a big mirror under bright skys.
Have you ever seen someone grind or tryed grinding a mirror? Damn it is a long long process. I would give all this some serious thought.

Khan

I plan to make the scope portable by permanently mounting it on a trailer. That is one of many aspects of this tentative plan that may change in the future, I may decide I would be better off buying an observatory kit and mounting it permanently. Like I said above, I plan to grind a number of smaller mirrors before I take this one on.

I linked to Fred Olson's book above, I'm going to go ahead and order it today. I need a better understanding of kilns in general. I'm also afraid to bother you guys with every detail to the point of tedium. :D

Thank you for the Ward Burner link, very helpful.

Another point I forgot to make above is that I do plan on buying silica that is already fused, not fuse it myself. Just thought I should state that specifically.

Feel free if anyone has anything else they would like to throw in the pot.

[Greymatter Glass]

Greymatter - I'm only trying to achieve the softening point of the quartz, not the fusing point. So, I don't really think that this kiln should run me $50,000. I just need it to get up to 3100F or so, taking the softening point of 3029F that I am seeing for the silica. I do not need the quartz to be ultra high purity.

This completely contradicts what you said at the start of this, that you will be using fused quartz.

You need to learn a bit about what glass is and how it forms. Getting a pile of silica sand to the "softening point" would result in, at best, a sintered block. You absolutely need to fuse silica in order to make a solid mirror blank. The fusing point of quartz is around 2000ºC. Check this out, it's not all inclusive of what you'll need to know, but it's a start: http://en.wikipedia.org/wiki/Fused_quartz

Consider also that your block of sand will have micro bubbles in it, and when you grind and polish it, those bubbles will come out as small pock mark on the surface of the lens - who do you plan on dealing with that?



I appreciate the suggestion for using a 4" natural gas line, it looks to be common size line that I could easily source.


What I am suggesting is that you will need a building that the local gas company will install a 4" high pressure gas service into to get enough gas for the amount of heat you will need. You can buy all the 4" gas line you want, you can't dig up your yard and install it yourself.



This mirror is something I am planning to do because no one to this point has made an effort to make a mirror this size.


Just because you don't know about it doesn't mean no one else has tried it, or at least thought about it. The reason you don't see anything about it is because it's not as easy as you're assuming it should be - not by a long shot. And I know you already assume it's going to be difficult, so starting from that point, it's harder than you think.



I believe Intermountain Optics may provide blanks in the 1-meter class...around $25,000 for a mirror this size, maybe more.
Obsession Telescopes would charges around $50,000 for a 36" scope.


Most business is based on a cost + margin model, you take the cost it requires to make something and figure what kind of profit you need to make it worth the effort, and charge that. Obviously the supply and demand curve for 1m telescope mirrors is different than that of say, ipads, but it's still the same general principle. The cost of materials, facility, labor, R&D, failure/shrinkage all play into those prices.

Chances are neither of those companies would have any involvement in the actual manufacture of the glass, and probably not even do the all the grinding/polishing/coating in house, they would contract it to someone like Corning or Schott for that.



It may even evolve into a business to support myself during or after college.

I would contact both of those companies you've mentioned and talk to them, see if they're hiring, and/or see if they can give you some information about how to cast and produce a 1m lens. The astronomy field is full of DIY enthusiasts and I'm sure that if they won't help you directly, you can find someone through their forums to help you.

I'm sorry to discourage you, but it sounds like you maybe haven't even started college, and what you're talking about is not a small or cheap project. I'm not kidding when I say you need to be comfortable spending more to make it on your own as a one off than you would buying it pre-made from someone who has a business doing this already. I don't think you can tool up to make a 1m lens blank out of fused quartz for under $25,000 - just the parts for the furnace will cost over $10,000, then you have the sand, the energy, the time, the grinding, the coating, the shipping.... seriously.

Even if my numbers are off, if there are companies that sell a 1m mirror for $25k it suggests that they are probably spending at least $10,000 in production.

good luck.

-Doug

[mojigglins]

have a look at this link below, there are some glasses you can use that have low softening points which you would not need a gas fired kiln, even if you used already fused quartz you will need to get well above it working temp 2600*c+++++ to get a consistant melt, unless you are pointing it at the sun it will not get hot, the distortion will come from the mass of the mirror flexing and bending under its own weight, you can avoid this by drilling out the back side, and also quartz has a Mohs hardness of around 6 and other types of boro etc are around 4-5 making grinding and polishing, polishing quartz is a huge PITA especialy large surfaces like a 1m mirror( I have 15 years exp in cutting grinding and polishing glasses, quartz and optical materials as well as my dads 40+ years exp)
http://www.oldham-optical.co.uk/Glass.htm

I have accounts with Corning, Schott, Momentive, etc so if you find a material that you like I can order it for you, fell free to contact me, I have built many kilns and muffle funaces in the past, I can help you if you want>

[kbinkster]

That's odd, didn't someone else ask about this within the past month? Not to say there's anything hinky going on, it's just weird that two folks showed up looking to cast telescope mirrors.
http://www.talkglass.com/forum/showthread.php?50863-Help-with-casting-telescope-mirrors&highlight=telescope

James, maybe you can contact Ron and you guys could collaborate/share info.

[khan]

a huge.....AMEN.!!!! to what Graymatter said.

He is absolutely right.

Khan

[mellofello]

Also agree with that greymatter has said. There is some great advice here for you though and I hope it helps you.

So $25 - 50k to buy one, just from that its clear that this is going to be an expensive project.

You say that it hasn't been done before. I don't know anything about telescopes so I'm just wondering about the reasons no one has before? Besides possibly cost are you aware of any other reasons? Is it that the benefits of having such a large mirror not great enough? Are there alternative methods for producing the same result which are currently preferred? It seems that even at $50 - 100k it wouldn't be enough to put everyone off, sure there are some pretty wealthy telescope users/ lovers/ collectors or what ever out there. Guess I'm going to have to go away and look into these telescopes as I really know nothing about how they work and what role the mirror plays in it all.

[Greymatter Glass]

You say that it hasn't been done before.

I know for a fact it's been done before, thousands of times. There are entire forums dedicated to people with 1m+ telescopes. Many of them have made their own blanks, mostly float glass, some boro. Thing is, they know what it's going to cost, they know they could save money just buying one off the shelf, they're not making it because they want to save money, they're making it for bragging rights and to learn the process and just *waves hands in the air frantically* because.

Thing is, for most people astronomy is an amateur hobby at best - there's not any income from owning a $100,000 telescope system (I say system because the telescope is just the crazy expensive lens of a very expensive camera that's hooked up to a computer, most people don't do direct observation with these telescopes)... so the people who own them generally have huge disposable incomes and think little to nothing of the cost.

As is often the case in "Big Boy Toys" there are inherent costs associated with these things. There are the guys who just buy something ready to go out of the box, and the guys who COULD, but chose not to, because it's more fun to make it yourself. In either case, the costs end up being similar. The DIY crowd might spend a bit less cash, but spend way more time to get the same results as an "off the shelf" product offers. In the case of 1m telescope mirror blanks... just a back of an envelope calculation you're looking at about $20-30k and 6-8 months to make ONE mirror. And if it cracks, you're out the time and $5k or so in energy costs and materials that will need to be replaced.

And there's always the guys who don't have anything CLOSE to a reasonable idea of the cost and difficulty involved and will spend more money than they can afford to lose getting to the conclusion that they'll never have enough money, skill, knowledge or access to do what they want, and fail and give up. That always makes me sad, but I would NEVER stop someone from following that dream because...

From among those people, maybe just once in a generation, someone discovers something amazing, sparks some innovation that changes the world. Invents something that makes them powerful and famous, and rich beyond any reason. Evolves society and makes things exciting again.

That's usually worth it.