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Ceramic Mirrors

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#1 opticsguy

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Posted 23 October 2020 - 06:09 PM

Ceramic mirrors.

 

In the December Astronomers Workbench of Sky & Telescope, a presentation was made about Ceramic Mirrors.  Wanting to see this topic expanded and explored.  Are there any ATM's working on this methodology for mirror making?   Looks like an interesting, challenging and educational subject to be explored.     I now need to wander down to my basement and find those S&T mags for OCT. 1975 and  March 1976. 

 

Posting your explorations, experiments and results from hands-on users would be greatly appreciated here.


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#2 opticsguy

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Posted 23 October 2020 - 06:11 PM

Ceramic Mirrors Part 2

So, just to get started, a bazillion questions about Ceramic Mirrors . . . . . .

 

Porcelain Slip (among others) is recommended for this project.
Where to purchase?
How to mix for a “best” mirror blank?
How long to dry before the blank can be handled and hexagonal blocks of plaster can be removed?
Is there a coating to be placed on the hex tiles so removal can be accomplished without damage?

 

Curve generation.
How much time is needed for Porcelain to dry prior to generating the curve before firing?
Curve generation can be accomplished without an abrasive. Is this the best method?

 

Glaze:  What glaze materials can be used for the surface coating?

OK, just a few questions to get going.  Would appreciate responses from those who have hands-on experience.



#3 kb58

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Posted 23 October 2020 - 07:15 PM

I remember at an old RTMC they were saying that the Corning (I think) high temperature glass (the reddish brown cookware) was being touted as the next big thing for mirror glass. Never saw a single mirror made of it. Just saying that everyone seems to be looking for the next miracle glass with zero Tc and low cost, and it never seems to pan out for a number of reasons.



#4 luxo II

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Posted 24 October 2020 - 06:19 AM

Three come to mind - Sitall (Russian), zerodur and quartz. All three have been used for mirrors but few amateurs would have one due to the cost - firstly of the blanks, and the increased work to grind polish and figure as they’re harder than glass.

Zerodur blanks are available (Schott) and you’ll find zerodur on eBay easily enough.

Intes and Santel used Sitall primary mirrors in some of their premium maksutovs, also quartz was an option, but these were/are expensive. FWIW my 10” has a quartz primary made by intes, while my two smaller maks and the M1008 and MK91 owned by a friend are Sitall.

Likewise Questar offered quartz or zerodur options.

Another wonder material which was touted for ATM mirrors - 20 years ago - was “black vitrified ceramic” (BVC), but I’ve never head of it since, nor seen one.

Edited by luxo II, 24 October 2020 - 06:51 AM.


#5 davidc135

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Posted 24 October 2020 - 07:46 AM

Interesting topic.

The two questions that come to mind regarding porcelain as a mirror material are: How stable would this substrate be compared to glass, quartz etc and how nearly would the glaze surface behave like glass when it came to optical polishing and figuring?

I'd have thought that they'd be in with a good chance of success but perhaps they've been found wanting.  David


Edited by davidc135, 24 October 2020 - 07:46 AM.


#6 chipe450

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Posted 24 October 2020 - 07:52 AM

BVC glass in sandwich format, very lightweight.
Look at the vendor pages.

Clear skies
Andre
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#7 Alan French

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Posted 24 October 2020 - 08:09 AM

Seems like a perfect ATM project, figuring out if ceramic mirrors are viable. From the article, it doesn't sound like an expensive undertaking either. 

 

Clear skies, Alan



#8 MellonLake

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Posted 24 October 2020 - 09:18 AM

I am a ceramic engineer by training so am familiar with ceramic materials and glasses.  Most ceramics and glasses can be slip cast to near net shape.  This includes fused silica (quartz), glasses, silicon carbide and porcelain.  This process is very very easy to do regardless of the material.  Porcelain is nice because additives do not need to be added to the slip to give the green body (unfired shape) stability but you can buy slip solutions that will work to give all ceramics some green body strength. 

 

While porcelain is a readily available material to produce a near net shape mirror, it does not offer any major advantages to silica (which has a much lower thermal expansion coefficient).  Silica does take a higher sintering temperature but it would likely be worth the effort to find someone with a kiln that can sinter silica rather than porcelain.   

 

I am personally interested in seeing if an ATM mirror can be made in Silicon Carbide.  Silicon Carbide Newtonian mirrors have been made and specialized firms offer them.  The 3.5m Herschel Space telescope mirror is/was a silicon carbide. 

 

Silicon carbide has a thermal conductivity more than one order of magnitude higher than glass, quartz and porcelain.  It more than twice as stiff as glass and porcelain and has a similar expansion coefficient.  What this all means is a Silicon Carbide mirror would cool 10 times faster than a conventional mirror and would be much much less likely to distort under any conditions.   A 20" mirror of conventional thickness would likely cool to ambient in less than 20 min.!!!

 

A Silicon carbide mirror once craft would be highly resistant to scratching.  Additionally Silicon Carbide is much less likely to be damaged by impact loads than glass!!

 

Silicon carbide raw material is exceptionally inexpensive.          

 

A silicon carbide near net shape mirror blank would be very easy to cast and it would be relatively simple to find someone to fire it for you (kiln it).  

 

The big problem with silicon carbide from an ATM point of view is that the material is super hard (Mohs 9.5).  It would be very very difficult to shape an polish without expensive abrasives (diamond, tungsten carbide, cubic boron nitride).


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#9 MellonLake

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Posted 24 October 2020 - 09:57 AM

FYI - I have not read the article but this is what I would do. Or you could just make a round blank if you wanted.  Also, I have never tried to make a mirror so may know all the limitations. 

 

The process would be as follows:

 

1) Make a positive of the mirror shape out of aluminium or I would just use an existing mirror (scrap preferably) find someone who scratched their mirror and use it. 

2) get a flexible bucket large enough to cover the mirror with the plaster

3) Spray the bucket with silicon oil spray as a parting agent.

4) Pour a thin layer of plaster (anhydrite based plaster of Paris) in the bottom of container and let it harden overnight. The plaster should be thin (runny) such that any bubbles can escape (there will be directions on how much water can be added).

5) Put a layer of 2mm poly sheet on the the layer of hardened plaster.  Make sure it lays very very flat and extends fully to the sides of the container.  The better the fit the easer it will be to separate later.

6) Put the mirror on the poly centred with the parabolic surface facing up. (You may need to coat the mirror in silicon oil but I would try first without as it may make the plaster less likely to work well as the mold).

7) Pour in a thin (runny) plaster again making sure the bubble can rise and cover the mirror with at least 1" of plaster. 

8) Allow the plaster to set overnight.

10) Demold from the bucket and separate the upper (flat bottom and mirror from the mold).  I am hoping the mirror will slide out, if not the plaster will have to be broken off and a vertical mold split line added)

11) Drill a 1" hole through the flat bottom of mold such that the porcelain or ceramic slip can be poured into the mould.

12) You can sand the mold lightly to remove any unwanted features.

Mold complete

 

Making green casting.

1) Make the slip.  The porcelain slip should be made quite runny to allow the air bubbles to rise.  If you are making a slip of another ceramic or glass follow the instructions for the commercial slip solution. 

2) Put the Mirror portion of the mold (what was the upper portion before) on a supporting grid to allow airflow under the mould but still support it well.  

3) put the top (former bottom) on the mold.

4) pour in the slip through the hole, making sure the bubbles rise out the fill hole.  

5) the green mirror blank will be ready to demold when the plaster again appears completely dry (this could take a few days).

6) the blank should slide out of the mold.  If it does not allow the open mold with the green mirror blank to dry further (it will shrink more and then fall out)  If it does not come out you may have to cut the mold off or a vertical mold split may be needed (vertical poly at 180 degrees appart).

7) Follow the required sintering process in the kiln for the porcelain or ceramic.  I would place it with the parabolic side up in the kiln and the bottom on a very flat kiln tile.

 

Notes:

1) the end mirror blank will be smaller than the original mirror due to shrinkage of both the slip and further shrinkage on firing.  May want to consider using an oversized blank. 

2) distortion of the mirror blank can occur in both the green state and kiln.  Care is needed in handling and support.

3) the green mirror blank can be sanded/cut as desired taking care not to contaminate it (using files is better than sand paper).

4) Bubbles are your enemy

5)Surfactants can be added to the slip to make bubbles riser better.  Surfactants can aid in making the slip runnier.

6) Degassing with a vacuum mixer can help remove bubbles as well (use a vacuum mixer to make the slip).   

 

There would be mistakes and trial and error to get it right but this should work to make a near net shape blank.  

 

FYI - There is no glazing required if the porcelain is fully fired and sintered (same with ceramics).  They fully densify.  If you do as I have indicated hopefully any bubbles rise away from the surface which will be polished.  



#10 PrestonE

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Posted 24 October 2020 - 11:28 AM

Here is an interesting article on Lightweight 2 Meter Reaction Bonded SiC Mirror...

 

https://www.scienced...211379718315183

 

Final figuring is with 1 micron diamond powder...

 

Looks doable, until you want to make the Silicon Carbide part...which requires a 

Vacuum Furnace...

 

Coors Ceramic could make the blanks, but likely costly...

 

Best Regards,

 

Preston


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#11 MellonLake

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Posted 24 October 2020 - 11:45 AM

There are lots of vacuum furance operators around North America.  I am sure you could talk someone into doing it for you (I am sure it would cost a bit).  

 

Rob  



#12 Ed Jones

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Posted 24 October 2020 - 11:52 AM

I did an unsuccessful trial last year and this is what I learned.  I got a couple of gallons of Porcelain slip C/6 from Standard Ceramic Supply Co. at a local ceramics shop.  I found that it shrinks a lot on drying on plaster molds so I used weights on top of the plaster with an aluminum strip on the side to limit the diameter shrink.  When dry I had the plano-concave disk (7 X 3/4 in.) bisque fired .  When I got it back I found that it was no longer flat on the back and both sided had moved.  OK so I ground the back side flat again and generated the front curve again closest to where it was now. Then I applied a clear glaze HF-9 zinc free lead free glaze and had it fired again.  When I got it back I found that the back was no longer flat and so both sides moved.  Worse however the glaze had crazed badly which is due to different thermal expansion of the glaze and porcelain seen in the shot below. 

 

  I'm pretty much in the dark when it comes to ceramics, but then I look at the 12 inch porcelain tiles I used in my bathroom which have a polished glaze.  They evidently have figured it out.  Then there are companies who make cordierite mirrors who have figured it out as well.  I've emailed a few of the cordierite companies about glazes but never heard back.  Anyone know a ceramics expert?

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#13 MellonLake

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Posted 24 October 2020 - 11:59 AM

Ed.. my suggestions are below.  

 

1) When objects are put in kilns they place them on tiles which may not be flat.  The sintering mirror blank will deform to the shape of the tile it is sitting on.  You might have to buy your own alumina/mullite tile (they are pretty cheap) and make sure it is very flat to start with and then have the kiln operator put the mirror green body and the alumina/mullite tile with the green ware on the tile in the kiln.

2) The porcelain needs to be left in the kiln a very long time to fully densify (you would have to know the type of material and have the sintering information for the porcelain and then determine the length of time at temperature).

3) The kiln should have a controlled very very slow cool to minimize stresses which develop during cooling.

 

I would skip the glaze and try and polish the porcelain directly.  The glaze is always going to have a different coefficient of thermal expansion to the porcelain and is going to cause the mirror to distort.  

 

Rob


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#14 davidc135

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Posted 24 October 2020 - 12:48 PM

It looks like the coefficient of thermal expansion of the ceramic needs to closely match that of the glaze; in fact, exactly match it to avoid figure destroying stresses being caused by temperature changes.  David


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#15 MellonLake

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Posted 24 October 2020 - 01:14 PM

In reality coefficients of thermal expansion may be equal at one temperature but change as temperature increases or decreases.  If the ceramic and glaze have the same thermal expansion coefficient at room temperature, the difference in the coefficients may be very different at the firing temperatures (and vice versa).  Thus, on cooling we end up with differential contraction and residual thermal stresses in the object.  This makes it very difficult, if not impossible to have an object made of two materials that will have zero stress from differential thermal contraction on cooling.   In reality, we always have a compromise where the coating layer (glaze) and substrate (ceramic) are stressed (coating in tension and substrate surface in compression or coating in compression and substrate in compression), what we hope for is that the stress is low enough that we don't exceed the strength of either the glaze layer or substrate.  If the strength is exceed, crack invariably occur.  However, even if cracks don't occur there are residual stresses in the glaze layer and substrate that are going to lead to distortions in a mirror.  Hence, my recommendation for no glaze layer.   


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#16 mark cowan

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Posted 24 October 2020 - 02:50 PM

I am personally interested in seeing if an ATM mirror can be made in Silicon Carbide.  Silicon Carbide Newtonian mirrors have been made and specialized firms offer them.  The 3.5m Herschel Space telescope mirror is/was a silicon carbide.

You might find this helpful:  https://wp.optics.ar...ly-Kashmira.pdf

 

It is relatively low cost (about 30$/kg to 90$/kg), which is preferred for large size parts or
high-volume production.

 

Though that is assuming you're going to manufacture the blank yourself, and once made (they go into detail) it will require something other than SiC grit to grind, and special polishing techniques due to the hardness. ;)


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#17 MellonLake

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Posted 24 October 2020 - 03:48 PM

You might find this helpful:  https://wp.optics.ar...ly-Kashmira.pdf

Though that is assuming you're going to manufacture the blank yourself, and once made (they go into detail) it will require something other than SiC grit to grind, and special polishing techniques due to the hardness. wink.gif

Yep saw that one.  I don't think I am personally ever going to make a mirror, just not my thing. I am more hopeful that someone else will latch on and give it a try.  I really think that SiC would be a superb material for mirrors for a lot of reasons and a much better material than just about anything out there.  SiC mirrors can be half as thick and deflect less than glass/quartz.   They cool very quickly and are more durable. 

 

Also as noted above Coors are the guys who know this stuff inside and out.  If their engineers are anything like me (I am an engineer), they are more than willing to help for an interesting project whether the material is SiC, Porcelain, or Silica.    



#18 Ed Jones

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Posted 24 October 2020 - 06:41 PM

 

I would skip the glaze and try and polish the porcelain directly.

Does porcelain take a glass-like polish?  That would be interesting to find out.



#19 MellonLake

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Posted 24 October 2020 - 06:58 PM

Ed

  Can you polish porcelain, yes and its hardness is similar to glass.  As a materials engineer, I have polished to many materials to count to a mirror finish (lead, plastic, concrete, rock, titanium, aluminium, steel, nickel, glass, cobalt, zirconia, alumina, silica, and even tungsten carbide).  I grind and polish these materials to view their microstructures and polishing is usually to a 1 micron or better finish.  Soft materials are actually more difficult to get a good finish on than harder materials.  Harder materials take longer but the finish is easier to achieve a better quality polished surface.  Porcelain is similar hardness to glass and there should be no issues polishing or grinding it.    

 

My biggest concern is porosity not only due to surface finish considerations but also pores trap polishing media making it more difficult to get the finish needed.  Sintering to full density is the biggest issue.  Sintering, the process of densification of ceramics at high temperatures during heat treatment, is a process where it is difficult to remove all the porosity.  Almost invariably there are pores even in well sintered porcelains.  The porosity is generally in the 1 micron range in diameter but better can be achieved more time at the sintering temperatures (24 hours to days). 

 

Rob  


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#20 Ed Jones

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Posted 24 October 2020 - 07:26 PM

So then my bathroom tiles may not be glazed at all, just polished.   thinking1.gif



#21 MellonLake

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Posted 24 October 2020 - 07:40 PM

Unfortunately, tiles are usually not well sintered, they just sinter them enough to make them suitably durable but not to really densify them that much.  So they apply a glass glaze to the tile to give them colour and make them smooth but inside they are are full of pores.  If you broke open you bathroom tile you could see its porosity with a magnifying glass.  The examples of fully dense porcelain are:

 

1)Good quality plates and dishes, mugs... while still glazed the porcelain is well sintered to ensure these objects have good strength.  Bone china is particularly dense and fracture resistant (it would be really good for a mirror).  

2) Electrical insulators.  High voltage insulators were historically made of porcelain.  These were fully denified.

3) False teeth and dentures (dental porcelain) is fully densified to give it strength (Now we often use zirconia because it is even more fracture resistant).



#22 hakann

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Posted 24 October 2020 - 11:50 PM

I has look into SiC before for a fast 25”, and as a cellular and the stiffness it could be a Very light mirror and still Very stiff.
This material handle heat Way better than any glass.
There are several company’s doing this works but first design and FEM works into a cell construction will be very expensive, and the less weight the more important the cell will be.
I was not into a plano.
Most company’s doing this is for space applications so they could be very light.
Then the polishing require MRF or ION and from the ATM perspective this shops do the normal std 20 nm RMS in red so for us it means 1/3 wave in green.
This glass also need to has a thin layer of coating ( cladd ) for the final parabola as to gain the smoothness needed.
Then the protecting coating/reflecting, all all this will be very expensive.
This material is very hard so it would not just crack.
It shore would be nice, especially for bigger optics but be ’portable’ with this money spend is not wise the way I see it.
So from the AMT perspective it’s a dead horse for many aspects.
Even ’if’ money is not the object..
We fight weather, sky conditions etc so use something like this will not match a reality, and I had ruff price at 400K Euros.
Get bigger is always nice and to use good glass ex Zerodur ( but it get real expensive fast ) but I has a classic plano Pyrex at 30” f/3.6 now and it’s a good mirror, and I don’t really demand better from what I observe and this is at a marginal price.
What I learned is nothing beat the sky ( own time at the EP ) and diameter to collect the light.
If a decent cell and a thinner plano, they do its job.
We can’t fight the atmosphere.
Get darkness is also hard and if darkness we has the seeing.
But this subject is cool.

-But out of topics is there are cast Pyrex cellular optics aviable ( not that high in price ) but I has never seen or heard any of this into the visual ATM, but I shore would like look into the EP of a such optic !

Edited by hakann, 25 October 2020 - 04:43 PM.


#23 hakann

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Posted 25 October 2020 - 04:39 AM

Let’s look at gains and negative aspects of a SiC mirror.
It might be out of topics, but I went into this so I can cheer my thoughts.
The basic ideas I had was as big as possible and foot on ground and be portable for trips like ex Chile.
But here, let’s focus on a plano version here maybe.
But first, what do we fight here.
What are we observing.
Are we portable or stationary.
In my case I like see faint objects so the diameter plays a role and be ’smooth’ and has a decent good parabola.
As I’m most at smaller pupil I has no real gain of a fast focal but if get bigger it means sub 4.
The SiC require clad and polishing will be extreme hard for AMT and MRF is expensive and they need it at near 1 wave for a start.
If we look at a smaller optics I can’t see the idea as ex my Quartz 18” is at 9 kg.
PPC for Quartz is at 0.5 and material and labour to get a super polishing is not expensive ( from the ATM world suppliers ) and still a f/4 means no ladder.
It also just need a decent Plop cell, and if a big box or fans it get Ok very fast.
Matter of fact even in window glass, it would not be a big issue at that size.
So stiffness and glass quality will play a role, but what is reasonable if one are into visual observing.
Yes, even thinner big one planos get real heavy, but in a decent cell and wait some for the glass to come to ambient I has not seen the big issue here.
It’s more about the sky quality and going big ( see more ) it soon get rather expensive fast, especially if one like a quality glass but compare to a SiC it’s cheap.
Then how do we see the aerospace tolerances ? ( this is a big subject on its own )
So if going for a plano in SiC whit all the expense I can’t see the gain here, no matter of size.
Lets say it’s 4X stiffness, 100X faster in heat, but the parabola might not be over diffraction aswell.
And to get a IF tested 25” at 1/8 PV wf will be almost impossible from aerospace.
So yes a designed SiC plano will be lighter but going big is never a easy operation, so it’s better to be stationary and then the weight factor is less.

 

-I think this will never be done for ATM application ( my thoughts )

I still wait on a cast cellular in Pyrex that is big to read about then out on field.
Idea is the factor of 6;1 and less weight ( more easy cell and less sag so faster get heat out )
The shops I talked to swear diffration is all Ok ( and all what we need ) and no print troughs so this design will wipe a plano out easy.
I must say, I’m sceptical for field even if facts or math say another.


Edited by hakann, 25 October 2020 - 10:20 AM.

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#24 Ed Jones

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Posted 25 October 2020 - 11:42 AM

 

Bone china is particularly dense and fracture resistant (it would be really good for a mirror).

Does this mean it will polish like glass?  The slip is more expensive and is fired at 1250 C to be translucent.



#25 MellonLake

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Posted 25 October 2020 - 12:03 PM

Ed... Technically bone china is a glass ceramic like Zerodur. The composition allows for the formation of a glass phase during sintering which in turn results in faster densification (diffusion through the glass phase is faster resulting in quicker densification, at its heart sintering is a diffusion process). The faster densification results in lower porosity, higher strength and higher fracture toughness. However, the bone china porcelain will retain porosity just like other sintered objects but it will likely be less for a given time in the kiln. Until someone tries it I don't think it can be well determined if the material will polish well and produce a suitable surface for a mirror. The porosity should be on micron scale which is similar to the wavelength of light (as you know). I am concerned that the pores may trap polishing media leading to difficult producing a suitably smooth surface and that the pores will interfere with making a suitably polished surface (I am not and optician or mirror maker so I don't know what the requirements are for smoothness). Also if you want to use bone china porcelain, the content of bone should be maximized (like 45%) as it will be stronger and harder. Bone china is slightly softer than conventional porcelain due to the glass content but still in line with pyrex and conventional glasses.


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