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I’m interested to know if anyone in England taken the plunge and purchased the (USA import) Angel Gilding kit and how you’ve fared with it?
https://angelgilding.com/pump-spray-silver-kit.htmlI have the practical “how to do it” article written by Howard Banich & Peter Pekurer.
Thanks, Alan.I have seen videos of these kits in action and the results are impressive. The chemistry is nothing new, and I have used the materials over decades in my professional career when doing some chemical tests for reducing agents. The most important thing is to have the surface absolutely clean. If silvering a test tube or glass flask I would normally warm it with some concentrated nitric acid, but the acid can be added in the cold, as you would have to in the case of a mirror, or swabbed over the surface with cotton wool. The use of tin (II) chloride to sensitise the surface is also not a new invention, but is important in the spraying process for with that method the silver-based and reducer chemicals are in contact just briefly upon the surface of the mirror, and the pre-treatment with this powerful reducing agent will enhance the deposition rate of the silver. The price of the commercial kit is very high. Why not make your own? All the chemicals are readily obtained and silver nitrate can be bought for less than £2 per gram. You should not need more than 5 grams. There is a 12 page discussion on the Cloudy Nights Forum. Temperature is important. A warm day would work better, for in the lab one has to warm the reagents, and a large cold mirror surface will cool the spray. Again this is discussed at the Forum I mentioned. I would be inclined to have a go myself first at spraying a small test piece, rather than buying a kit. If you need any chemical advice I will be happy to help (by email).
Good morning everyone. I am a bit surprised to find this topic, I didn’t think the problem of silvering the telescope mirror still existed. And even more I didn’t know about that silvering kit. The first mirror silvering I commissioned from a well-equipped glassmaker, who did a very good job. The only major problem was the minimal durability of the Ag deposit, which began to oxidise and yellow. After a few months (max 6-8 months) it had to be removed and re-done. Then I tried to work on it myself by buying Ag nitrate, ammonia, H2O and more. Very bad result! Longer life (up to a year) but silvering with dark spots, uneven deposit, minimal adhesion so cleaning impossible, and other defects. Everything has made me prefer mirror aluminisation. However, here in Italy, aluminisation is made by very few companies and costs a lot. So I gave up the resolution power of a 200 mm and opted for a smaller (90 mm) catadioptric that is very handy and inexpensive. Nevertheless, the D200mm dream still haunts me. I would kindly ask for your opinion on what I have reported. I am a self-made astrophile, and as a basis I have the manual: Procedures in Experimental Physics by John Strong.
Thank you and greetings to you allTranslated with DeepL.com (free version)
It sounds like the original coating was too thin. When you use a silvering solution, there are two types of silver deposited. One is a brown precipitate. This is silver, but not the desirable bright adherent silver film that coats the mirror. To avoid getting it on the mirror surface, the solution can be agitated, or the mirror silvered face down. A good deal of the silver ends up being wasted as the brown precipitate, which is the result of the silver ions getting reduced too quickly to form a continuous metallic coating. It cannot be avoided, as the reduction is done in solution.
The new silver film should be burnished, and it can be made more durable by applying a lacquer. One way to do this is to put the mirror on a record player turntable, rotate it slowly and apply the solution with a brush. Perspex chips can be dissolved in a solvent like ethyl acetate, though the solvent must not be one that evaporates too quickly, before the turntable has helped in spreading the solution.
Any trace of sulfur in the atmosphere will form a yellow-brown deposit of silver sulfide upon the surface, and it sounds like this happened in your case. It may be that in your location in Italy you have some sulfur dioxide or hydrogen sulfide in the atmosphere, in which case aluminising will be a better option. (Aluminium is less suited to observatories near the sea, unless it is overcoated. Chloride ions cause pitting corrosion of the film.) As an aside, I once aluminised a mirror myself. Chemically cleaned, placed upside down in a small vacuum chamber and suspended over a heating element upon which small turnings of aluminium had been hung….. Very interesting to do. The inside of the chamber gets coated as well, of course. 45 years later the mirror is still shiny!
By mistake I have posted the reply twice, and the second version is correct. I have now forgotten how to remove the first!
By mistake I have posted the reply twice, and the second version is correct. I have now forgotten how to remove the first!
Hi Richard – I’ve deleted it for you… (on the toolbar at the right top of the post there us a “BIN” option – and after that you can then get a DELETE option to remove it completely).
Callum
Richard, thank you for the encouragement etc. I did attempt silvering 4 and 6-inch test pieces back in 2016 following the method of E. E. Hall in the 1965 June Journal. Results were patchy. Revisiting my 2016 notes today I seem to have missed out the cleaning/degreasing – oops! I do have 60% nitric acid in the cupboard so can use that as you suggest although it is horrid stuff. I will try using calcium carbonate first, as per the Youtube videos.
I had assumed the sprayers being used were ‘proper’ big-ticket paint sprayer kit but I now realise they are no more than the cheap greenhouse/garden things sold by garden centres.I am also intrigued by the idea of perspex chips dissolved in a solvent as a guard coating. I have read that alum paper can slow down the tarnishing and will try that first.
Giovanni, you are right, silvering was a thing of the past. As early as 1961 J.B. Sidgwick wrote that vacuum coated aluminium had taken over, noting also that the silvering chemicals were difficult to obtain by members of the public. I remember trying to obtain potassium nitrate for a different purpose and thought they were going to call the cops. Now almost anything is available mail order. I suggest the cost/benefit is perhaps shifting back to silvering. It is not just the cost of aluminising. There is no way I am trusting my mirror set to a courier, when a master optician said in 2016 that to make it then would cost between £6k-£10k. So I’d have drive it there myself and collect it afterwards.
If you can, read Jerry Oltion’s short article, pages 74 & 75 in Sky & Telescope January 2020,We used to silver the 14-inch mirror of the school telescope back in the 70s under the supervision of Commander Hatfield. This was followed by careful burnishing, a critical step, as Richard pointed out. It was a useful demonstration of chemistry in action. And more impressive than carrying out Tollen’s test for reducing sugars in a tiny test tube that we performed in the biology labs.
I think one of the points of the Spray Silvering technique is that the mirror does not need burnishing after.
Some of the big dob users in North West USA (Howard Banich, Mel Bartels et al.) were the trailblazers with the technique due to the cost and difficulty of getting their very large mirrors aluminised.
Callum
It must be admitted that Prof. McKim knows more than the devil. Thanks for the valuable information, Prof. I really didn’t know about the inverted mirror and other procedures you mention. Nor did I know about the possible effect of an atmosphere polluted by sulphur dioxide and hydrogen sulphide. And concerning this, the various attempts at silvering I made in the chemistry lab of the high school in the city where I teach (physics), a very industrialised city. I will not attempt to silver again, but I will at least theoretically explore the topics presented by McKim, to whom I extend my thanks. Thanks also to those who have considered my intervention.
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