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Sean OKeefe
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Votes: 0 (Vote!) | | Posted on Wednesday, March 31, 2004 - 03:00 pm: | 
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Paul Ron provided a method for silvering mirrors/prisms that he had earlier received from a Mamiya user. I adjusted the volume to ~200ml rather than 2L and measured volumes of solutions added. I am copying the procedure below.
I had a Pen F with a deteriorated silvered surface, corrosion right in the middle of the viewing frame where it bugs you most. So, I decided to open the camera and it was not the mirror (which I can easily replace) that was corroded, but it was the prism. We all know how easily you can find prisms for the Pen F, so I decided to resilver. I have a lab with fume hood, silver nitrate, sodium hydroxide, nitric acid and all the goodies so I decided to do it myself.
Let me say first that the second attempt was perfect. My first try had good silver coating but it was uneven and you would be able to see this in the viewfinder.
RESILVERING A PRISM
Silver nitrate solution is made by adding 6g silver nitrate to 100ml distilled water and mixing for a few minutes.
Ammonium hydroxide (ammonia water, NH4OH I used lab grade which is ~28% NH4 aq.) is added dropwise. The solution gets dark, muddy brown and will eventually clear. The goal is to get just to the point where it clears. I added 13mls.
1/2 of the volume (there are 113 mls now, so 56.5mls) of NaOH (4.2% in water) is added. The solution turns muddy brown. Again, NH4OH is added until it almost clears. I added 10 mls.
This solution is activated by adding a reducer, which in this case is 100mls of 10% dextrose in 83% water and 17% ethanol. So 17mls ethanol and 10g dextrose brought to 100mls total volume (dextrose = glucose = corn sugar). I used lab ethanol that was denatured with 5% methanol.
After adding the 100mls dextrose reducer directly to the ~180mls silver nitrate solution, you need to act fast and get the solution on your prism/mirror. I hung the prism surface to be silvered on a clamp and immersed it into the silvering solution. I did not stir.
After 15 minutes, I removed the prism, washed with distilled water, and then with ethanol. I used distilled water for all of the manipulations.
After printing the back of the prism, I carefully removed the bit of excess silvering using a toothpick.
Viola, a shiny, clean silvered prism.
The extra silvered surfaces oxidized very quickly, turning white.
To clean up the extra silver, I used nitric acid. Nasty stuff that, and if you wipe up concentrated acid with paper, it may catch on fire. I used gloves, a fume hood, eye protection and lab coat.
Surfaces you don't want silvered can be coated with wax before treatment. It is probably a good idea to do this to the vessel you have the prism/mirror in for silvering, but I just cleaned up by rinsing with nitric acid.
I previously tried the Rochelle salt procedure described on a Do It Yourself web site, but that did not work at all at room temp, and the silver that was deposited overnight at 50 °C was very fragile, and washed off during rinsing .
Not everybody has access to these chemicals and a fume hood (for nitric acid), but for those who do, this procedure should be useful. Do not try this at home if you do not have extensive chemistry experience and I am not liable for problems should things go horribly wrong. Tell that to your next of kin as well.
Cheers
Sean |
Sean OKeefe
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, March 31, 2004 - 03:03 pm: |
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One more thing. I cleaned the glass to be silvered carefully in nitric acid. If the glass isn't clean, the silver won't stick. |
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