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I re-silvered the mirror on my favourite Mk. IX-BM when I found it had 
deteriorated to the point of being unuseable.  
In fact it was this exercise which revived my interest in navigation again.  I 
had been promising myself to do the job for a long time.

The process is straightforward and has been known and used for instruments, 
telescopes, and ordinary flat mirros since Mr Brashear developed it in the 
1880s up to about the 1930s. (Mr. Brashear actually modified an existing 
process).  Tollen's reagent for chemical analysis indicating reducing agents 
is the same process.  It was used for the very large telescope mirrors in 
observatories until large object vacuum coating became possible when the 
mercury entrapment vacuum pumps needed were invented and good vacuums were 
then possible in a large container. 

The Brashear Process is very simple, but there are difficulties, principly the 
obtaining and use of the chemicals. 
Also the glass surface has to be really scrupulously clean for good 
deposition, the glass repeatedly cleaned with alcolhol and water; then 
acetone, tri-chloroethylene, ether or other powerful degreasant, and 
preferably in concentrated nitric acid for final cleaning,  and care must be 
used in preparation as highly explosive silver fulminate can be precipitated 
if not careful with procedure.  Most of these chemicals are not available 
easily in the EU now as they are banned except for specialist use.
 
The main trouble for anyone wanting to do this is the high cost of silver 
nitrate and other chemicals for what is a very small amount needed for a 
small mirror like this (less than one gramme of silver nitrate); and the 
chemicals which are highly corrosive or dangerous when not handled correctly 
or knowledageably. 
Trichloroethylene for example, which used to be used everywhere in industry in 
the 1960s for de-greasing everything and anything and was used as an early 
cloting 'dry-cleaner',  produces phosgene gas when heated to high temperature 
- for example in your cigarette when you handle the chemical in the cleaning 
baths and then have a drag on your cigarette.  Phosgene was a poison gas used 
in the first world war!  
I use acetone which is a good explosive mixture in air (highly dangerous) - 
besides rotting your brain if you are stupid enough to like sniffing it.

The concentrations of solutions needed has to be adjusted for very small scale 
use too, which needs laboratory equipment, as the amounts used in the 
reference articles about Brashear process are for large mirrors some feet 
across.
I suggest you only attempt this if you have experience of chemicals or general laboratory work.
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Included are pictures of the silvered mirror.

First picture is the mirror held in a dish as silvered just after the process 
- you are looking at the top surface of the silvering on the top surface of 
the glass: it is a nice golden colour and is still a fair mirror on this 
'outside' surface of silver, which is not the used mirror itself i.e from the 
glass side. This was removed by wiping with conc. nitric acid.
The final (used) mirror silvered surface is the lower one as held in the dish 
- as the mirror is superior because precipitate does not affect the 
deposition process when held upside down. Brashear noted this especially for 
large optical telescope mirrors.
It takes about three to five minutes thats all.

(In case anyone wished to ask: I am very sorry but I cannot take on any 
mirrors for silvering I simply do not have the time - and besides, small 
laboratory vacuum coatings are readily available in the spectacle optics 
trade for a reasonable price I believe, and are probably superior).

Douglas Denny.
Chichester. England.
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