SILVERING A TELESCOPE MIRROR AT HOME

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Notes on the Silvering of Mirrors

by Michael J. Coslo

This is a slight modification of other processes that have been around since Methuselah. I make no claims of originality.

Materials

For mirror chemistry

Silver nitrate, 21 grams
Ammonium Hydroxide .90 specific gravity aqueous solution 250 ml
Potassium Hydroxide pellets 14 grams
Distilled water - 4 liters
60 grams dextrose sugar
Citric acid

For cleaning mirror

Dishwashing detergent
Acetone
Concentrated Nitric Acid

General 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

Wash the mirror with detergent solution at room temperature. Rinse completely.
Wash the mirror with detergent solution at room temperature. Rinse completely.
Wash the mirror with detergent solution at room temperature. Rinse completely.
What I'm trying to say here is to be very thorough. You just don't want to ruin your silvering job for lack of cleanliness. I personally washed and rinsed my mirror 6 times.
Take a popsicle stick and wrap some of the cotton around it to make a swab.
Put some acetone on the swab and swab the surface of the mirror. Make sure you get all of the mirror surface. Repeat several times. The idea is to get any leftover grease or other gunk off the mirror and onto the swab. I took the used swabs and let them evaporate outdoors. Remember acetone is quite a flammable substance, and has a low flash point. Wear goggles, especially if you wear contact lenses. You DON'T want to get acetone on your eyes. Otherwise it is one of the safer strong solvents. Allow to dry thoroughly.
Make another swab with a fresh popsicle stick. 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.
Use another swab, wet with distilled water to remove the Nitric acid from the mirror. Leave the surface of the mirror wet.
Repeat steps 6 and 7 at least three times.
Place a tape dam around the circumference of your mirror. Get this part done without letting the surface of the mirror dry. Plan on around an inch overhang 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.
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

In a 1 liter beaker, pour 225 ml of the Silver Nitrate solution. Reserve the other 25 ml of solution for later addition.
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.
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.
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.
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.
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.
Pour the invert sugar into the main solution with stirring. The solution will change to a different color of brown.

Silvering the Mirror

Pour the distilled water out of the mirror dam. The plastic bucket will suffice for this.
Pour the main solution onto the mirror surface. The solution will continue to change in appearance. All the appearances will be pretty ugly.
Agitate the solution on the mirror fairly often.
This process will continue for about 8 to 10 minutes.
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.
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.
Stand back and admire.

Notes

Some people have needed to polish their mirrors when a whitish precipitate called "bloom" is deposited on the mirror. I have not gotten this at all yet, so I cannot comment on the polishing process except to say the method I was told about involves rouge on a wad of cotton with a piece of chamois wrapped around the cotton.

Cleanliness is paramount. If you think you'll get a good mirror without being paranoid about cleanliness, forget doing the process at all. Distilled water is just that - distilled. Do not use deionized, or spring, or anything else but distilled water. Some of the chemical processes may work with ordinary tap water, assuming the water is pretty clean to begin with, but at 79 cents a gallon for distilled, why take the chance. And if you rinse your mirror with tap or DI water, you'll get water spots.

The silver deposit on the face of the mirror is very fragile. The less reasons you can think of to touch the thing, the better. Don't give into temptation.

The mirror surface will probably not look perfect your first few tries. Unless the results are terrible, try using the mirror anyway. I gave in to temptation, and swabbed at the surface of my mirror while the mirror was still wet, and a bit of the coating came off. The effect on performance was not visible. Otherwise the mirror looks wonderful, I have been told by someone that has seen a few of these things that this is one of the better looking silver jobs he's seen.

Warnings

Nitric acid is scary stuff, handle with great care.
Ammonium Hydroxide is smelly. It smells like ammonia window cleaner on steroids.
Potassium Hydroxide is a strong base.
Silver Nitrate is a poison (though my main problem with it is stains on my hands, don't ask how they got there)
Acetone is quite flammable.

All this is pointing out that you exercise diligent caution and care when performing this operation. Wear those gloves, wear those goggles. Use proper ventilation. If you don't, you will after getting a whiff of Nitric acid. The Ammonium Hydroxide will clear a room out if good ventilation isn't used. Rinse everything down with lots of water. Read and follow the Material Safety Data Sheets before using these chemicals and performing this process. Be sure to dispose of used solutions properly.

Official CYA Stuff on: ******

I offer this as a descriptive of the process that I used, not as an encouragement to use such processes, and the potential user must realize and determine if this process may or may not be suitable for their own purposes. The user takes sole responsibility for any and all damages that may occur through use of these materials under all circumstances, foreseen or unforseen. Use under responsible and skilled adult supervision only. If you are not skilled in the safe and proper use of potentially dangerous chemicals it is specifically, strongly, and emphatically recommended that you do *not* use this process.

*****Official CYA stuff off