Stripping the Aluminum Coating from a Telescope Mirror
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- เผยแพร่เมื่อ 12 ก.ย. 2024
- To prepare a mirror for refiguring, Gordon Waite shows how he removes the old reflective aluminum coatings. Gordon used ferric chloride to eat away the shiny aluminum without damaging the underlying glass.
It also eats copper, etches steel, and many other metals. From experience it stains concrete quite well too!
Jon Steel It stains pretty much everything!
I know it's old, but I can't thank you enought for this video.
My pleasure to have helped you! If you ever have a question, please don't hesitate to ask, either here or on private email. Thanks! -Gordon
How do you recoat a mirror? Btw good tutorial!
wow that was one interesting aluminum mirror coating removal. 😎 thanks, now for the new coating video. 🥰
Thanks a lot for sharing this. I've been looking for a good solution to remove aluminum coating.
I use a very small amount of etch and place filmy plastic over that. Take the back of your hand and sweep it across the plastic to disperse the etch. There will be some air bubbles. You have to come back and rotate the plastic. Not as scary as your method.
Well I did it almost exactly like you said, and it worked beautifully. I only made one mistake, and that was to dump the Sodium Bicarbonate dry onto the mirror to clean up. Got a little messy... note to self, do it like the video says. Thanks for the video!
You are welcome. Glad you had good success!
Good stuff. Keep that ferric chloride away from copper, too!
Is it a good idea to neutralize the etchant with baking soda on the mirror? That would precipitate iron oxide and hydroxide particles that could etch or scratch the glass, Ive seen how a rusting nail stains and etches glass. I would say its better to rinse the mirror with slightly acidified distilled water (vinegar or citric acid) first and then after that a good rinse of pure distilled water no need to risk damaging the surface with iron oxide particles.
I use this for printed circuit boards. if you warm the solution it will act a lot faster
Mariano Gaston Yes, that's true! Thanks for a good tip.
It is called Arrhenius equation, and there are two ways to look at the results of that equation "every 10°C increase in temperature doubles the rate of reactions." (Which is fantastic for etching copper and aluminium).
Or "Every 10°C increase in temperature reduces the MTBF by half", usually applied to electronics but works in lots of areas.
Excuse me for asking a totally naive question, but would the aluminium coating not also be quickly ground away during the grinding process, thus obviating the need for removing it chemically?
Hi, Dutch! When a mirror comes in for refiguring, I usually take off the coating right away because it makes it easier to test the mirror. Often the coatings are in terrible shape, and sometimes mirrors have defective coatings that hide the true surface shape of the mirror. So by taking off the coating I get a clear look at the real condition of the glass. In most refigurings, you don't have to go all the way back to grinding. Grinding would obliterate the coating in short order, but the gentler polishing can take a long time to remove a coating. I once "polished off" and older coating, and it took two hours of hard work! In any case, I just chemically remove the old coatings. It's cheap and easy.
Well done sir.
Thanks! Glad you enjoyed the video.
How do you apply a new coat of Aluminum? Can one person do it? How much does it cost? Can you do a video on recoating? Thank you.
FlameHue Checking into doing a video on coating. The cost usually runs about $12 to $14 per inch of aperture.
Hi. The video helped me a lot.
Can a 15-year-old UK Orion Optics 12-inch mirror remove the coating in the same way?
where is the video to coat it again with aluminium?
You could warm the bottle of FeCl_3 with hot water before pouring, which would make the reaction happen much quicker. Agitating the liquid would also help speed things.
+Arnþór Gíslason You are correct on both accounts! Thanks... -Gordon
OK I'm confused. Most mirrors have a SiO2 overcoat on the aluminum, right? How does the FeCl attack the SiO2 (glass) passivation coating?
These ones definitely don't have a glass coating. You'd need to use hydrofluoric acid to etch the glass.
A very great job sir but here is request please tell us about all the meterals or ingredients used in the whole process like black liquid which you boiled and you used for grinding the mirror , ceramic tiles , dental powders , for polishing and silver coating their chemical names pleas . actually I want learn from you every thing and I may become pupil . if you don't mind please . please sir tell me also about neumarical measurement please please sir anxiously waiting for your positive response please . thank you sir . I do all these for my family own job and also to observe and see tha planets and stars etc. Once again thank you so much .. Please
If you heat up the FeCl_3 to luke warm it works faster
You are right!
Will the ferric chloride remove anti-reflective coatings from lenes?
Just noticed this video. At the end, after the aluminum is gone and the mirror is cleaned up, it looks frosted; is it? I have a small aluminized mirror that I want to strip, but I don't want to damage the underlying surface; I just want it re-aluminized.
No it's not frosted. You are seeing the back of the mirror through the front. The back side was ground flat, so the back is something like a 25-micron surface. The front is shiny and perfect. I have never seen a mirror damaged at all using Ferric Chloride.
How do i know when i have to do this.
Cool video bro...
+salvador allende Hi! Thanks for your comment. You only have to strip a mirror if you are going to refigure it yourself. If you just need a fresh coating, send the mirror as-is to the coater, and they can do the stripping for you. Much easier that way!
I haven't used FeCl to etch PC boards in some time. Instead, I've used a mixture of 2 parts Hydrogen Peroxide to one part HCl (muratic acid from pool supply store). I don't know if this will work on aluminum, but I can tell you it's faster acting on copper than FeCl, and doesn't stain. It DOES burn your fingers though, so I have to wear gloves when using it (which is a good idea with FeCl too because of the staining).
It also goes flat pretty quick and you need to keep adding the oxidizer to revive it. Also, not as safe to have around in storage. I used it for awhile, but instead started making my own FeCl3 which lasts forever and can be re-used over and over.
@@gettingpast4391 Yes, the home made HCl + h2o2 is a one use concoction, but the HCl I use is stored outside by the swimming pool (in a weather proof bin) where it is used to fix the pool ph when necessary. The H2O2 is borrowed from the bathroom medicine cabinet.
Hi.
George Ionas put me onto this video. Looks interesting. I use "green river" solution for stripping old coatings prior to cleaning and recoating. I would be interested in your comments on the advantages of one method over the other.
Peter Wilde Hi, Peter... Green River is just distilled water, hydrochloric acid and cupric sulfate. Depending on the exact formula, it is probably quite a bit stronger than the Ferric Chloride that I use, and is probably a lot faster. I like the ferric chloride because I can buy it off the shelf at the Radio Shack, and it is cheap and very safe to use. If you get it on your hands, you'll just turn a little orange! I can leave the ferric chloride on the surface overnight and it won't damage the surface of the mirror. With GR, you probably want to neutralize it as soon as the job is done.
The question is: why?
Hello... thanks
Do you know how to make silver coating on a glass to get transparent mirror?
+Ahmet Yasar Nope! What is the secret?
I pulled the primary mirror from my 12 inch do and tried to clean it. No luck. Seems mat or oxidised. Can I use a metal polish to make it shine again or do I have to get it to an expert to re-coat it ??
Time for recoating!
Doesn't most mirror have a protecting SiO2 layer on top of the aluminium? Does this procedure also work if this is the case?
This procedure strips off the SiO2 layers as well.
i want buy this lense. . pls send the link sir .
www.waiteresearch.com
Why not start the process with the mirror in the black container?
Mirror is bigger than the black container.
@@GordonWaite Ah, I didn't see that. Thanks.
How to decide which side should coat the mirror
The coating always goes on the side you worked to a curve.
Hi Mr Gordon , i hve a quiestions plese; .... do you think its possible to remove the MC coating (antireflexive) of a lens ? than a lot
I'm thinking that's not too easy. Those coatings are usually applied with heat, and they adhere very well.
Hi Gordon - I'm in the middle of doing that with my refractor lens! It takes 2-4 hours of vigorous polishing, but it is coming off!
next time put the mirror on a cookie sheet so that it will be easy to pick up and not cause a mess!
Put the mirror in the tub before you add the FeCl3! Then all you need to do is lift one side.
Why do you need to remove the coating at all?
For a refiguring job, you might not be working on the entire surface, so you want to get the coating out of the way. Polishers act a little goofy when working on hard coatings or over-coatings. It doesn't cost but a few cents to strip, and you get to start with a nice, clean surface. Also, some coatings are very hard to polish off, so you need chemistry to do the job then.
I see - anyway great job you are doing. I would buy a telescope from you if I was in US...
Maybe start it in the dish tou use toncatch the fluid. Got nervous watching you lift it lol
sodium hydroxide solution should work well too,
+user21XXL Be careful with sodium hydroxide. It does dissolve aluminium, but it also dissolves glass, especially if concentrated and/or hot. It might make the glass dull.
Actually FeCl3 is a salt not an acid 😅
At least not a bronsted acid
While a salt, ferric chloride solutions with water are strongly acidic. Glad you set the record straight on this, and got it exactly right.
GordonWaite Yeah. Being a lewis acid (not referring to the general definition of acid by means of donating a proton -> bronsted acid definition) ferric chloride has an instable electron octet and not having noble gas configuration in chemical complexes as central atom make it react acidic.
But don’t confuse: acid =!acidic
There is a difference between an acidic solution and an acid
too bad radio shack is closed now
You can get it from Fry's Electronics, or from several sources on Amazon. Ferric Chloride is cheap and easy to find online.
wasting SO much acid!!! you only need to wipe some on and wait no need to use so much. And his ferric chloride looks quite dilute but even at this dilution you don't need anywhere near this amount.
You missed my quote at 1:11: "you can use it over and over." When I am done stripping a mirror, all of the acid goes back into the container, and is used again on the next mirror!
No I saw that, but you're further diluting your acid that way so the situation will get worse. You're better off with keeping it as concentrated as possible and putting it on with a swab and removing it with one. Overall you'll end up using less acid.
I don't dilute the acid at all. I use it full strength from the bottle, and pour it right back into the bottle. There is no dilution. From a 16 oz bottle that costs about $12 total, I can strip probably 10 or 12 mirrors. So it's about a buck per stripping. If I tried to strip a 20" mirror with a tiny little cotton swab, it would take me all day. The time would be WAY more cost than that tiny amount of acid. You have to be a little practical about things!
You can apply it pretty quickly and the acid is diluted as it takes on metals, perhaps I should have said denatured.
The aluminum coating on a telescope mirror is literally a few molecules thick. If a mirror had 0.1 gram of aluminum on it, I would be shocked. Such a small metal load is, for all practical purposes, no load or dilution on the ferric chloride. I'm just not worried at all about trying to save money on $1 worth of acid for a refiguring job that is going to run $1,000 to $2,000 in labor. A 20" mirror has 314 square inches of surface area. I can't even imagine putting ferric chloride on that much surface area with a tiny cotton swab! Especially when the goal is to save 20 cents worth of acid! You are letting a nickel get in the way of a dollar.
So, Gordon is it not possible to do the record by myself also? $150-$200 is lot of money. I have a 10" spherical what are we talking to do a reconfigure to parabolic? I don't have the patience for self grinding and polishing. Norm
+Norm7634 Working a 10" mirror from spherical to parabolic is a fair amount of work. The exact amount would depend on the focal length, and the condition of the mirror at the start. In ball-park terms, if you have a 10" spherical mirror, you are maybe half way to a good parabolized mirror.