[ATM] Silvering Woes

["Donald R. Bates" <dbates3@houston.rr.com>]

After finishing the figuring of a 6-inch  mirror recently, I decided to try
my luck with coating it with silver myself. I researched the process, and
found a good recipe from Michael J. Coslo that was posted to ATM Archive
site back in the late 1990's. After some searching, I found a local chemical
supply shop that had everything I needed to get started. I was amazed how
easy it was to obtain things like Nitric Acid, Silver Nitrate, etc. With my
lab assistant, (my father, a retired chemical engineer) I carefully followed
the directions. When I pulled the mirror from the silvering bath, it was
still completely transparent, no silver had attached itself to the face of
the mirror. 

Undaunted, I cleaned the mirror with Nitric Acid, and tried again the next
day. This time, all the predicated color changes in the solution occured
exactly as described. Once again, I agitated the mirror face down in the
silvering solution, and pulled it from the bath. Again, no silver clung to
the mirror! After wasting two batches of Silver Nitrate, I gave up and sent
the mirror off for aluminizing. Does anyone out there know what I might have
done wrong? I have eliminated what I think re the most likely culprits:

Temperature - The "Lab" temperature was perfect, around 65 degrees; same
with the chemical solutions.

Surface of mirror not clean enough - I scrubbed the surface with so much
Nitric, that it was super-clean.

Too much Ammonium Hydroxide - I was very careful not to add to much AH, and
even added a few more drops of Silver Nitrate for good measure.

 

A few other considerations: 

- No silver attached to the tray container either. Just a grey sludge at the
bottom.

- The only thing I did different from the instructions was to put a suction
cup on the back of the mirror, and put it face DOWN in the solution. The
mirror had a few chips in the outer edges that prevented a good seal for a
face-up dam. I have also read that face down produces a better quality
coating.

 

I have included the exact directions that I followed below. I would still
like to give this a try, but I need some help on where I may have gone
wrong. It would be really great to be able to coat my mirrors without send
them off to the aluminizer. Plus, I love playing the part of the mad
scientist! Thanks!

Donald R. Bates

Chief Observation Officer

Cypress Creek Station (My backyard)

 

 

 

Telescope Mirror Silvering Method 

 

Chemistry needed:

Silver nitrate, .75 ounce

Ammonium Hydroxide .90 specific gravity aqueous solution 250 ml

Potassium Hydroxide pellets .5 ounce

Distilled water - 1 gallon

2 ounces sucrose

Citric acid

 

Cleaning mirror:

Dishwashing detergent

Acetone

Concentrated Nitric Acid

 

Labware:

Stirring rods

Rubber gloves

Tray for mirror

Absorbent cotton. Get the good stuff, it has longer

fibers of cotton that won't be coming off on the mirror.

Beakers and bottles for various solutions. The largest you need

 is 1 liter.

2 Eyedroppers

Popsicle sticks (unused of course)

Eye protection

Cellophane type tape, two or three inches wide, for tape dam

2.5 or 5 gallon plastic bucket

Mixing Silvering chemistry:

 

Silver Nitrate solution:

21 grams Silver Nitrate dissolved in 250 ml distilled water

Ammonium hydroxide solution -- .90 specific gravity (other s.g's will work)

around 100 ml

Potassium Hydroxide solution:

15 grams Potassium Hydroxide pellets dissolved in 250 ml distilled

water

Invert sugar solution:

Dissolve 60 grams sucrose in 500 ml distilled water

add 10 grams citric acid.

Bring to slow boil for 30 minutes (remember a slow boil, we don't

want to make candy here). The volume will decrease by approximately half.

Remove from heat and let cool to room temperature.

 

After the chemistry is made and is stabilizing to room temperature,

you can now start cleaning the mirror. All the steps should be sequential,

and the mirror should not dry after the nitric acid steps.

 

Cleaning Mirror:

 

1. Wash the mirror with detergent solution at room temperature.

Rinse completely.

2. You now will swab the concentrated Nitric acid onto the mirror. Keep the
acid on the top of

the mirror only, you want to keep the sides dry so you can put the dam

around the mirror. You don't need to use great amounts of acid, just wet

the swab and swirl around, getting all the surface of the mirror.

3. Use another swab to remove the Nitric acid from the mirror.

Leave the surface of the mirror wet.

4. Repeat steps 6 and 7 at least three times.

5. Place a tape dam around the circumfrence of your mirror. Get

this part done without letting the surface of the mirror dry. Plan on

around an inch over.hang above the mirror surface. If at this time you see

any beading of water on the mirror, hang your head in shame. It means you

didn't clean the mirror properly, and you have to start over again. If you

followed the instructions carefully, your mirror should be quite clean.

6. After the dam is in place, pour distilled water into the dam to

cover and protect the clean surface. The mirror is now chemically clean and

grease free

 

Preparation of the silvering solution:

1. In a 1 liter beaker, pour 225 ml of the Silver Nitrate solution.

Reserve the other 25 ml of solution for later addition.

2. With the eyedropper, add Ammonium hydroxide to the main

solution. The Silver Nitrate solution will turn a brown color. Stir

constantly, adding the Ammonium hydroxide a few drops at a time. The

solution will start to clear after a bit. You will see the solution

clearing in the region of AH addition as you drop it in. This will give you

some idea of what the clearing looks like. Don't rush this part. You don't

want too much AH in the solution.

3. After the solution has cleared, add a few drops of silver

nitrate solution, using a different dropper. Adjust the solution until it

is just a little bit cloudy.

4. Add all the Potassium Hydroxide very slowly, and with stirring

to the main solution. If a giant dollop of Potassium hydroxide solution is

added very quickly, other chemicals can form, which are not safe, and will

destroy the operation. This won't likely happen if added slowly. The

solution will turn brown again.

5. Add AH solution again, using the eyedropper, to clear the

solution. Work very carefully, as we don't want too much AH in the

solution. When the solution clears, it will likely be a light brownish

color, and may have little black specks in it.

6. Add more of the reserve Silver nitrate solution to the main

solution, a few drops to be sure that there isn't an excess of AH.

The next parts happen pretty fast, so you may want to rehearse a

few times before mixing any chemicals before trying it with the chemistry.

7. Pour the invert sugar into the main solution with stirring. The

solution will change to a different color of brown.

8. Pour the distilled water out of the mirror dam. The plastic

bucket will suffice for this.

9. Pour the main solution onto the mirror surface. The solution

will continue to change in appearance. All the appearances will be pretty

ugly. You'll wonder how anything shiny can come out of something that looks

so much like sewage.

10. Agitate the solution on the mirror fairly often. Constantly is

okay, but don't overdo it and send the solution over the sides of the dam.

11. This process will continue for about 8 to 10 minutes.

12. The solution will now be a really ugly brown-black with a lot

of sludge at the bottom. Pick up the mirror and pour the solution out and

rinse with plenty of room temp. water. Be sure to get the main portion of

the gunk off quickly. Then work your away around the mirror more leisurely.

Beware, as the wet silver coating is fragile. I would not even wipe it off

at all. Cut the tape dam off, being careful to not let the tape come into

contact with the mirror surface.

13. Before the mirror drys, place it at an angle against a support

in a tray or in the sink, and rinse it off with distilled water so that no

water spots are left on the mirror surface after drying.

14. Stand back and admire.

 

 

 

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