This answer nicely summarizes the general picture of historical use of silver for mid to large size telescope mirrors, and the shift to aluminum once good quality vacuum pumps and chambers suitable for this process were available. A freshly evaporated, atomically clean aluminum surface will can sometimes violently react with oxygen, so this has to be done under high vacuum, and vented carefully with inert gas first.

When did the term "resilvering" generally start to actually refer to aluminum coating, and not silver? Was that actually the case by the 1960's or 1970's?

From what I remember, evaporated aluminum will form a protective oxide layer ("native oxide") of the order of 10A thick within seconds or minutes that is self limiting - no further oxygen can penetrate this layer.

If this is so, why was re-silvering actually necessary? The cost, risk, and downtime associated with removing the primary and other mirrors every few years sounds like something you wouldn't do unless absolutely necessary.

  • $\begingroup$ @JamesKilfiger can you help me out and find some kind of reference that says silver was rarely used after 1930? I'm pretty sure - along with everyone else - that by the middle of the century that aluminum is the wise choice, but I really need to find a cite-able source. And if it is aluminum, why (actually) would it need to be redeposited regularly? Why would regular re-aluminizations (resilvering) be required? If you can help with a reference to a paper, or give a quote from a book (if it's hard to find) that's very helpful, thanks! $\endgroup$
    – uhoh
    Commented Jul 15, 2016 at 14:01
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    $\begingroup$ The problem is making the aluminum coating stick to the glass of the mirror and keeping it stuck, particularly if you are using techniques like CO2 snow cleaning to remove dirt and other "junk" that accumulate on the mirror. The coating does degrade over time, even with cleaning and can flake off. Our 2-m telescopes were re-aluminized after ~2.5yrs (we don't have a local tank) after reflectivity approx. doubled. Silver has better reflectivity in the IR (hence use for Spitzer) but is much worse in the blue and UV; see plot in astronomy.stackexchange.com/a/29372/23748 $\endgroup$ Commented Jan 31, 2019 at 16:50
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    $\begingroup$ In early 70's you either had your mirror silvered or aluminumized. The distinction was important. You could silver at home, but aluminizing required sending the mirror out. You specified silver or aluminum on the order. There were also all sorts of fancy coatings you could have applied over your aluminum/silver. You could also get a vacuum gold coating for IR work. $\endgroup$ Commented Apr 18, 2019 at 17:48
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    $\begingroup$ The Gemini telescopes have silvered mirrors. $\endgroup$
    – ProfRob
    Commented Mar 10, 2021 at 8:50
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    $\begingroup$ @ProfRob The main advantage of silver is that it reduces the total thermal emission of the telescope they go hand in hand of course, but it's not the higher reflectivity they are after, it's the lower emissivity, for the same reasons discussed in my link about radio telescopes. The mirror is hot, but with the lower emissivity $\varepsilon$ of silver it radiates somewhat less thermal power in IR via $\varepsilon \sigma T^4$ than an aluminum coating would. $\endgroup$
    – uhoh
    Commented Mar 10, 2021 at 13:10

3 Answers 3


Amateur telescope and mirror maker here. Not sure if I qualify as a "citable source" but anyway, here it is:

All metals will eventually tarnish. It may take a long time, but it will happen. The process is not entirely chemical always. Sometimes it's purely mechanical (abrasion). Other times it's in between. Surface phenomena are complex.

Even gold-coated mirrors will tarnish. You could use gold for infrared observations. But no matter how much care you take of it, eventually you will have to re-coat it. Sure, gold lasts a pretty long time, but not even gold is forever.

Aluminum is covered with a natural layer of colorless sapphire (aluminum oxide), which protects the metal from further destruction - sapphire is very hard and very inert chemically. But oxygen still diffuses through sapphire (albeit extremely slowly), and so do other corrosion agents like sulphur oxides, marine salt, etc.

So, yes, replacing silver with aluminum was an improvement. But not even aluminum is forever. In practice, you will maybe double the lifetime of the reflective layer when you migrate from silver to aluminum.

It should be noted that silver is still used. Aluminum is most popular nowadays, but you'll still find silver-coated mirrors (its reflectivity is higher than aluminum), gold-coated (for infrared), etc.

There are further coating techniques, like layering silicon oxide on top of metal, which further improve the lifetime of any kind of metal.

  • $\begingroup$ Thanks! We should call the amorphous $\text{Al}_2 \text{O}_3$ layer alumina. Sapphire is a crystal. As a rough analogy, we usually call amorphous $\text{SiO}_2$ silica, not quartz Why do you think aluminum actually continues to oxidize significantly after the native oxide layer forms - even years later? Did you read that somewhere - is it possible to track that down? Is there any data anywhere that actually shows that good quality aluminum coatings degrade by chemical reaction of the aluminum with something in the air? Are you sure the gold "tarnishes"? $\endgroup$
    – uhoh
    Commented Jul 15, 2016 at 21:37
  • $\begingroup$ My theory is that the actual thing that needs to be done is simply cleaning, and the easiest way to clean such a delicate coating is to strip it and redeposit it. As for the gold, I am not sure it chemically reacts with the air. However if it builds up a thin layer of organics from the atmosphere, those can be strong absorbers at some IR wavelength, and if an organic layer is able to absorb moisture from the atmosphere, that might be an even stronger absorber. But I don't think the gold is actually chemically reacting with the air an actually tarnishing. $\endgroup$
    – uhoh
    Commented Jul 15, 2016 at 21:40
  • $\begingroup$ Like I said, it's pretty complex. Your theory is wrong. There are definite cleaning procedures for telescope mirrors that don't involve re-coating. Regardless of the nature of the process, the fact remains that any reflective metallic layer working in conditions other than vacuum will, in time, become less and less reflective. There are many things in the air, not just oxygen, that will attack metals; e.g. mirrors used near oceans will tarnish more quickly compared to those used in deserts. There are also purely mechanical tarnishing processes from particulate matter hitting the mirror. $\endgroup$ Commented Jul 16, 2016 at 5:38
  • $\begingroup$ Is there any data to back this up? Maybe a plot of reflectivity vs years, for an aluminum astronomical mirror, with some discussion of why its known to be degradation of the aluminum, and not just a build-up of dirt? You seem pretty sure, so there must be some scientific evidence that convinced you. $\endgroup$
    – uhoh
    Commented Jul 16, 2016 at 7:05
  • $\begingroup$ For example if you look at this paper they seem to report that both the control mirror (Figure 1. protected from the environment) and environmentally exposed mirror that was washed once a month (Figure 4.) remained at about 90% reflectivity for 32 months. But the mirror that was exposed but not washed (Figure 2.) dropped to about 80% over 32 months. There's no theory here, it's data. $\endgroup$
    – uhoh
    Commented Jul 16, 2016 at 7:20

It's been quite a while with no activity, so I'll post an answer based on my comment.

tl;dr: It started in the 1930's but even the Kepler space telescope still used silver!

This post in cloudynights.com says:

Aluminizing mirrors was developed by John Strong, in the early 1930's. One can read about it in his book Procedures in Experimental Physics

From Archive.org's https://archive.org/details/ProceduresInExperimentalPhysics/page/n181

enter image description here

The 40-inch tank, Figure 13, shows the type of equipment used at the California Institude of Technology for larger mirrors. Still larger systems have been used26

26 http://articles.adsabs.harvard.edu/full/1936ApJ....83..401S The Evaporation Process and its Application to the Aluminizing of Large Telescope Mirrors also here

From that paper:


The aluminum film is automatically protected from tarnishing by an oxide film (presumably corundum, Al203, or bauxite, Al2O3*2H20) which starts to form as soon as the aluminum comes in contact with the air. This oxide becomes thicker with time for about sixty days, when it is very hard and tough, forming a surface not easily scratched when it is being dusted and cleaned.

The formation of the aluminum oxide film on an exposed metallic aluminum surface has been studied by Vernon,31 who weighed the sample to a precision of 1/100 mg and plotted the weight increment due to oxidation as a function of time. The period of time required for the oxide film to attain its natural thickness was seven to fourteen days. After this the thickness of the film remained nearly constant at 100 A.

This oxide layer is, of course, too thin to give interference effects. It could be much thicker, however, and still not give interference, because of its transparency and the high reflectivity of the under- lying aluminum.32

The article shows why shows an additional reason why aluminum was more attractive than silver for astronomers, even when aluminumization was a challenging and difficult process, spectral range!

enter image description here

Wikipedia's Silvering

An aluminum vacuum-deposition process invented in 1930 by Caltech physicist and astronomer John Strong, led to most reflecting telescopes shifting to aluminum.9 Nevertheless, some modern telescopes use silver, such as the Kepler space observatory. The Kepler mirror's silver was deposited using ion assisted evaporation.


From that archived source, a post by Jim Destefani: Mirror, Mirror - Keeping the Hale Telescope optically sharp:

Why Clean? You might think the Hale mirror, nestled deep inside the telescope workings in the Observatory dome, would be relatively sheltered from dirt and soils. But telescope mirrors get a lot of use, and the Hale Telescope is no exception. According to Palomar Observatory’s W. Scott Kardel, the 200-inch Hale Telescope at the Observatory is used an average of 300 nights per year. Even with weekly cleanings, mirrors periodically need to be given a new coat of reflective aluminum.

“Soils that may accumulate on the mirror include oils from the machinery,” Kardel explains. “We have motors driving the telescope workings that are more than 60 years old. Some of them are suspended above the primary mirror, because we also use smaller mirrors that have to be moved in and out of position. Occasionally, one of the motors might leak. Oil may come from elsewhere, too.”

Other possible soils include occasional drops of rain or condensation, which leave water spots. “A water spot with a bit of dust or ash from a fire, for example, can actually form an acid eventually,” Kardel says. Obviously, that’s not acceptable on a surface that needs to be as optically perfect as possible.

To combat routine soil accumulations, observatory staff perform a weekly carbon dioxide dusting of the mirror surface. But even this will not remove water spots.

Eventually, the mirror surface degrades to a point where full refurbishment, including cleaning, stripping of the old vacuum metallized aluminum coating, and recoating becomes necessary. “On a historical basis, we’ve done the full stripping, cleaning and re-metallizing about every 18 months to two years,” says Bruce Baker, skilled trades senior at the Observatory. “Of course, some of that is contingent on scheduling of the telescope and how it’s being used.”

Aluminum quickly forms a native oxide when exposed to atmosphere. Aluminum is incredibly exothermically reactive with oxygen. The resulting thin Al2O3 or alumina film is only tens of angstroms thick but the reaction is self-limiting because the resulting layer can not be penetrated by any further oxygen once it gets this thick. It's a bit similar to the formation of native oxides on silicon for similar reasons.

Here is an example of what the "resilvering" of the Hale 200 inch mirror looks like. Between 01:00 and 01:30 you can see what an involved project simply washing the mirror can be. At this point it's probably a better idea to resurface than to try to do this without damaging an existing aluminum surface.

The paper Reflectivity Degradation Rates of Aluminum Coatings at the CFHT Publications of the Astronomical Society of the Pacific, v.109, p.303-306 shows that aluminum mirrors that are washed regularly are just fine without resurfacing.

The experimental data in the paper supports the basic science; native aluminum oxide forms quickly, stays very very thin (tens of angstroms) and becomes a permanent barrier to further reaction of the aluminum. Alumina is un-reactive as well; alumina ceramics are used in a wide variety of chemically reactive and corrosive situation because of their non-reactivity.

Resurfacing of aluminized mirrors is necessary from time to time when cleaning with soap and water would cause more damage than it would benefit because dirt is stuck to the mirror's surface. It is not done because the integrity of the aluminum is compromised, but because it's a better choice than cleaning for practical reasons.

GIF ("blink comparator")

enter image description here

From http://global.kyocera.com/fcworld/charact/chemistry/chemiresist.html - alumina can potentially be remarkably resistant to chemical attack compared to other materials - note logarithmic scale.

enter image description here

The individual figures:

enter image description here

enter image description here

enter image description here

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    $\begingroup$ I silvered my first mirror at home because I could get the chemicals for cheap. (don't know if we peons are even allowed nitric acid anymore). In the early 70's sending a mirror out for Aluminizing was much more costly. $\endgroup$ Commented Dec 20, 2018 at 18:03

I apologize for the length of this answer, but this is a complicated subject to address. Thank you all.

I owned a camera store for many years, and have 60+ years in photo. I got my start in photo via Astronomy in high school. Went to the University of Iowa, started in Physics and Astronomy, but my math skills were not up to the task, so went to Radio-TV-Film. The physics was a great basis for photography as you might well imagine.

I knew just enough Physics and science to be a danger to myself and those around me.

Several years ago, a product called ROR (Residual Oil Remover) came to market, invented by a man from Illinois whom I knew casually. His premise was that in today's world, much of the 'dirt and dust' we perceive on optics of all kinds, is suspended in microscopic droplets of oil in the atmosphere, and as we wear glasses and walk in or move about, those microscopic droplets, also containing dirt/dust of all sorts, coat optical surfaces and everything else, and we breath the stuff.

I would guess that this oily deposit may also be a cause for some of the 'rapid' deterioration of optical coatings on telescope mirrors, it builds up over time, dries, which forms a glue or sticky layer which also causes dust and dirt to build up more rapidly.

If you can find ROR in a local camera store (or similar), get a bottle, I used a Kleenex tissue to clean my glasses. Spray the ROR directly on the glass, both sides, one lens only, gently rub it until it dries, then put on the glasses and compare the views, you should be amazed. The other effect of ROR was that as you cleaned the glass once, then a second time, the glass simply becomes slippery. The ROR, somehow, cleans the micro pores of the glass and removes the deep down sticky, oily material.

Of course, it can be used on any optical glass surface as far as I know

It was endorsed, as I remember, by NASA, FBI, Nat Geo, etc. so good credentials.

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    $\begingroup$ Your answer length is absolutely okay! And its content, too. :-) Welcome on the Astronomy SE! $\endgroup$
    – peterh
    Commented Apr 18, 2019 at 3:56
  • $\begingroup$ Thank you for your answer! Yes I think your experience agrees with what's written and cited in this answer in that "deterioration" of optical surfaces (in the case of the question reflective surfaces, but it doesn't matter) is due to a build-up of material, not degradation of the physical surface or what's underneath. Lenses and metallized mirrors are both covered with dielectric coatings both in manufacture, and in some cases (actual silver if exposed) from exposure to the atmosphere. $\endgroup$
    – uhoh
    Commented Apr 18, 2019 at 4:04
  • $\begingroup$ In cases where "re-silvering" refers specifically to re-aluminization, it is probably easier to remove the coating as part of the cleaning process than it is to thoroughly optically clean the mirror without optically damaging its surface. By the way, yours is a well-written, concise and relatively short answer. Here's an example of what a long answer looks like for comparison! ;-) $\endgroup$
    – uhoh
    Commented Apr 18, 2019 at 4:07

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