Telescope Making

Telescope Mirror Silvering

In order to aluminize a telescope mirror one needs vacuum equipment which is not easily to come by for the average amateur but silvering of the mirror can be done by himself. It's almost a miracle to observe the silver form on a mirror!

Silvering can be done via the following processes:

  • Formaldehyde process, which is easy to use but gives less satisfactory results.
  • Rochele salt process, also easy to use but the deposition of the silver occurs very slowly and thus makes it more usefull to make semi opaque coatings.
  • Brashear process, which gives excellent results.
  • Martin process, a modified Brashear process which require less precise handling and which is the one described here.

The silvering method given below is largely based on the Martin method as described by Texereau [1] and my own experience. In some regards the information given by Texereau is laking eg. the cleaning of the glass and the utensils used. He says that it must be thoroughly cleaned but doesn't tell you how to do it. The necessary information was obtained from ATM 3.[2]

Risks inherent in the silvering process

Before we get into the details of the silvering process lets discuss the inherent risks in the silvering process. It is not to scare and discourage you from silvering but to make you aware of them.

Please pay attention to the precautions in what follows for your own safety! I don't take any responsibility for any injury or damage that might be caused by incorrect or lack of information in this document, although it was gathered from reliable sources as far as possible. In other words you use the information at your own risk.

People

The first risk is to the people in your house especially children since dangerous chemicals are used. The bottles that the various solutions are kept in should be clearly marked and should be safely locked away out of the reach of children.

Chemicals

The second risk lies in the chemicals themselves during their use. Nitric acid vapours is dangerous when inhaled and could cause death. The vapours are formed when the nitric acid reacts with substances like mild steel, limestone and mortar. It also reacts with organic compounds like wood and paper. The vapours are invisible and only cause serious problems 8 to 24 hours after exposure so initially you might feel fine. Exposure to high concentrations can cause life threatening pulmonary edema. At least a respirator should be worn during it's use and the place should be well ventilated. If after working with nitric acid you have any symptoms of cough and breathlessness, seek immediate medical attention.

Explosion

The third risk is due to the mixing of the chemicals which could lead to an explosion if not handled properly. When the silvering solution becomes too concentrated via evaporation or mixing the solution has a tendency to decompose into ammonia, water and silver amide. A part of this silver amide decompose into ammonia and silver nitride. Both silver amide and silver nitride is very explosive and can explode at the slightest touch. Silver amide appears black and has a luster of something like graphite when floating on top of a silvering solution.

However the solutions used in the silvering of telescope mirrors is very dilute and far below the concentrations needed for silver amide and nitride to form. High concentration can only form through carelessness.

One incident is described where the workers left a beaker with spent silvering solution in it on a shelf over the weekend. When they returned on the Monday morning they found glass shards scattered all over the place. The beaker had exploded and had blown the shelf on which it was placed off the wall.

The spend silvering solutions should not be allowed to stand since silver fulminate could form with resulting explosion. Treat and discard as discussed later in this document.

Bottles used to prepare the silvering solution in should not be left to dry but thoroughly cleaned after use. If left to dry they could explode due to the thin silvering residue film left in them.

Accident

Lastly the chemicals could be splashed into your face and eyes by accident. One time during silvering I lifted the mirror from the silvering solution to see what was going on. The pitch that held the wooden sticks to the back of the mirror gave way and the mirror fell back in the solution splashing it over me. Some of it landed in my face and luckily not in my eyes. It browned my skin for a few days at the places of contact. So it is recommended to wear eye protection while working with these chemicals.

Accessories needed

  • An enamelled pan slightly larger than the mirror
  • Large vessel in which to rince the mirror
  • 1-liter flask for preparing solutions
  • 250-milliliter graduated flask
  • glass stirring rod
  • absorbent cotton
  • rubber gloves
  • 5 liters of distilled water for preparing solutions and rinsing
  • Eye shield to protect the eyes in the event of an explosion or an accident

Chemicals needed

The following chemicals are needed:

Chemical Remarks
Nitric acid (HNO3) Use specific gravity of 1.42. More dilute nitric of 1.22 may not effectively clean the mirror and hence may result in silvering failure.
Silver nitrate (AgNO3)  
Ammonium nitrate (NH4NO3) Use pure as possible grade.
Potassium hydroxide (KOH) or Sodium Hydroxide (NaOH) Sodium Hydroxide is commercially available as caustic soda. Whether the commercial version is suitable for silvering I do not know.
Tartaric acid (C4H6O4)  
Ethyl alcohol (C2H5OH) 90% Possibly any other alcohol can be used as it only acts as a preservative if solution D is going to be kept. If not then the alcohol is not needed.
White table sugar  
Distilled water  
Acetone Only needed for cleaning if the alcohol isn't going to be used.

Cleaning of the utensils

Inhalation of nitric acid fumes can be dangerous and can lead to death if high concentrations are inhaled. Cleaning with nitric acid should be done in a well ventilated place and a respirator should be worn. Put on the rubber gloves to protect the hands from the nitric acid that will be used in the cleaning process. Wash the gloves with soap and water and then rinse them off with a little alcohol or acetone. After scrubbing with soap and water, followed by alcohol or acetone, dilute a little nitric acid with a equal amount of distilled water and go all over the utensils again. Rinse with distilled water. Failure to clean the utensils thoroughly could lead to a failure in the silvering process.

Preparation of the solutions

The weighing of the various chemicals needn't be very precise as the process depends on the molecular ratio of the various chemicals.The correct ratio is obtained by a rough titration process.

Quantity needed: About 50 ml of solution A for every 15 square inches of mirror area, thus about 150 ml for a 8-inch mirror.

The following solutions must be prepared. The quantities here is of course more than your need for silvering for example the 8-inch mirror. One could prepare only the amount necessary for silvering the mirror but keep in mind that the ratio of solutions B and C is a titration process. If one only prepares the amounts needed one might end up with the situation where one has added all of solution C where it has need reached the titration point yet. The silvering attempt might also fail. So it would be better to prepare more than you need.

When preparing the solutions allways add the chemicals to the water and never the other way around. Sodium hydroxide can for example react violently when water is added to it causing it to be ejected from the container. Wear eye protection since potassium and sodium hydroxide can cause eye damage or blindness if it gets into ones eyes.

Number Ingredients Amount Method
A Silver nitrate crystals
Distilled water to make		 60 g
1000 ml		 Store in glass-stopperd bottle. Avoid getting it on your hands as it blackens skin.
B Ammonium nitrate crystals
Distilled water to make		 90 g
1000 ml		 Store in glass-stopperd bottle.
C Pure potassium hydroxide
or
Pure sodium hydroxide
Distilled water to make		 150 g
 
105 g
1000 ml

Store in a plastic bottle suitable for storing of strong bases. These strong bases dissolve glass given enough time.

Wear eye protection if you bought it in stick form and need to crush it.
D White table sugar (sucrose)
Tartaric acid
Ethyl alchohol (90 %)
Distilled water to make		 100 g
5 g
150 ml
1000 ml		 Dissolve sugar and tartaric acid in a small amount of water in a small, clean beaker. Bring solution to a boil and let simmer for 10 to 15 minutes to effect inversion of the sugar. Cool by adding a little distilled water. Add alcohol as preservative. Transfer solution to measuring flask and add distilled water to make 1 litre. The solution should be allowed to age for one week which enhances it's action.

Cleaning of the mirror

The successfullness of the silvering depends largely on the cleanliness of the glass that needs to be silvered. Again work in a well ventilated place and wear a respirator. Put on rubber gloves to protect the hands from the acid. The optical face is rubbed with nitric acid using a large cotton wad. If the mirror has an old silver coat the acid will destroy it. If the mirror has an old aluminium coat it can be removed with a small quantity of solution C followed with the nitric acid. The rubbing with nitric should be continued until one hears the characteristic squeak of cotton against clean glass. Pay particular attention to the edge of the mirror as it's the most difficult to clean. When the cleaning's done rinse the mirror with distilled water and place it in a tray so that the whole mirror is immersed in distilled water. The optical face must not be allowed to dry after cleaning as the silvering will fail at that spot.

Silvering arrangements

The mirror might be silvered in two positions, face up or face down.

Face up

In the face up method a strip of rubber or paraffined paper is placed around the mirror's edge so as to form a dam in which the silvering solution is contained. The advantage is that less of the solution is needed. Disadvantages is that tiny pinholes may form as mud settles on the mirror which might be difficult to remove without damaging the fragile coat. Another problem I found was to attach the dam after the mirror is cleaned. It's very difficult to attach the dam while wearing rubber gloves. Drying of the mirror face is another potential problem. One solution is to attach the rubber strip before cleaning the mirror and just fold it down during the cleaning process.

Face down

In the face down method, the mirror is placed in a tray with distilled water about half the thickness of the mirror. Four wood bars are pitched in the corners so that the mirror's edge will rest on them. Two of the bars are thicker than the other two so that the mirror can be rocked in order to bring more solution in contact with it. When the mirror is placed on the bars is likely that a bubble of air will be trapped under the mirror's face. Tilt the mirror to one side so that the bubble moves to the edge of the mirror and can escape. It's important that there remains no bubbles trapped under the mirror.

Silvering process

Now we are ready for the silvering process. The proper temperature is important. A good range is 18 to 24 degrees Celsius ( 65 to 75 F). It's not advisable to do silvering if it's too hot as the risk of explosion increases with temperature. The temperature refers to the solution temperature and not that of the air. Measure the correct volume of solution A with the measuring flask and pour it into a flask. Rinse the measuring flask and measure out an equal volume of solution B and add slowly to solution A while stirring vigorously with the glass rod. Rinse the measuring flask and measure out an equal volume of solution C and add slowly while stirring to the other two. It is here that the titration process comes in. Ideally the solution should have a light tea colour. Do not add all of solution C at once. Even a few drops can turn the solution too dark. When the mixture has the correct colour use the mixture immediately. Pour the mixture into the container containing the mirror. Measure a volume of solution D equal to one-third of solution A and add it to the mixture. The reaction starts immediately. Rock the mirror gently. Do not remove the mirror from the solution to view it. It should remain in the solution for 10 to 15 minutes. The solution is spend when the solution clears. Remove the mirror from the solution and rinse it with distilled water. The thickness of the deposit can be checked against the bright filament of a lamp which should be hardly visible. Hold the mirror in a vertical position to dry it and wash it down with a small quantity of alcohol. Drops gathering at the bottom can be picked off with blotting paper.

The film can be burnished to improve it and it's adherence to the glass. Burnishing is done by making a soft pad by placing a cotton ball inside a piece of chamois leather. Rub some optical polishing compound like rouge or cerium oxide on it. The pad should be tested first on a scrap piece of glass to ensure that it is not going to scratch the mirror. Burnish the entire mirror with circular strokes of the pad. In my experience it tends to damage the mirror so I generally do not do it.

Disposal of the spent silver solution

As already said, the spent solution should not be allowed to stand since it might explode on the slightest disturbance.

The best way to dispose of the spent solution is to first treat it with a solution of common salt and water. When the salt solution is added to it a dense white cloud will form. Let the cloud settle and then again add a little salt solution. Repeat until the white cloud fails to form. Now the solution is safe and may be kept indefinitely. You could either keep the solution in order to recover the silver in it, see Recovery of the silver from the silvering residue, or else dispose of it. If you are going to get rid of it do it in an environment friendly way and within the guidelines of your local municipality.

Lacquering of the mirror

The main disadvantage of silver is that it oxidizes with time and therefore it's reflectivity slowly degrades with time. One way to avoid that is to apply a very thin layer of lacquerer to the mirror. It may cause a slight deteriotation of the image but for an amateur mirror it is probably inconsequential. Bell used it successfully on the Harvard 24-inch reflector and Porter also used it on many mirrors. Lastina lacquer thinned 10 to 1 with amyl acetate may be used. I have also read that clear nail lacquer, that women use on their finger nails, thinned with acetone should work.

One way of applying the lacquer is to wet the whole of the mirror surface and pour the excess off. Stand the mirror on edge and delicately blot off the drops collecting at the bottom. The downside of this method is that the lacquer is not applied uniformly but forms a thin wedge from top to bottom.

A better method of applying the lacquer is to use the spinning method. The mirror is spinned face up on a turn table. About 20 RPM is about right for a 6½-inch mirror. It is very important that the mirror and room is dust free before applying the lacquer. If the spinning is stopped before the lacquer has dried a ridge will form. The ridge at the very edge can be avoided by holding a swab of cotton wool or soft blotting paper lightly against the edge during the whole process. When dry a uniform inteference color should be seen when viewed obliquely, no bands should be visible. If the coat is too thin there will be a perceptible dulling of the surface. The lacquer layer can be removed with amyl acetate and a swab of absorbent cotton. Before re-applying the lacquer the silver coat should be lightly burnished otherwise the lacquer will not flow evenly.

Recovery of the silver from the silvering residue

During the silvering process a very tiny amount of the silver is actually used and the rest is wasted. However most of the silver in the residue can be recovered and re-used by the following two methods.

Warning: Never keep the residue UNTREATED since it could lead to an explosion.

Method 1

Add hydrochloric acid to the residue until the solution is slightly acid which makes it safe to keep it indefinately. New residue can be added to it. Again add hydrochloric acid until the solution is slightly acid.

The amounts in the method described here are for 450 grams of silver nitrate.

  1. Filter the solution and add a pint of water to it.
  2. Boil the solution with 200 grams of potassium hydroxide and 340 grams of sugar for about three-quarters of an hour, stirring vigorously to hasten the reaction. A grayish brown precipitate will form.
  3. Wash the precipitate by decantation about ten times, using distilled water the last two times.
  4. Dissolve the precipitate in 230 grams of nitric acid (specific gravity 1.42) diluted with an equal amount of water. Heat the solution to hasten the reaction.
  5. Filter and add enough water to make about 900 grams. Now we have a dilute solution of fairly pure silver nitrate.
  6. Heat this solution to boiling and add 110 grams to 140 grams of ammonium formate. Again a grayish brown precipitate will form.
  7. Wash the precipitate in distilled water by decantation.
  8. Dissolve the precipitate in 230 grams of nitric acid diluted with an equal amount of distilled water and filter it.
  9. Evaporate the solution to crystallization and store the pure silver nitrate crystals in an air-tight brown bottle.

Method 2

Immediately after silvering transfer the residue to a suitable glass or plastic bottle. Add to it a solution of common salt and water. A dense white cloud will form. Let it settle and again add a little salt solution. Repeat until the white cloud stops forming. The solution is now safe and may be kept indefinitely. Fresh silver residue can be added and then treated with the salt solution as described. The clear liquid at the top may be poured off to make space for new silver residue to be added.

The recovery of the silver is done as follows:

  1. Pour of the clear liquid as completely as possible and place the residue in a large open glass or plastic dish. Drain off any excess licquid and spread the residue evenly into a thin layer. Let it stand in a warm location to dry thoroughly.
  2. When dry mix the residue with a mixture three times it's bulk made of potassium carbonate and sodium carbonate in equal parts.
  3. Transfer the mixture to a crucible and heat until melted and settled in a hot coke fire or gas furnace. Remove from fire and let it cool.
  4. Break out the button of silver and wash it clean from adhering salts.
  5. Dissolve the silver button in nitric acid and evaporate the solution to crystallization over a vessel of boiling water.
  6. Store the silver nitrate crystals in a brown bottle to prevent the deleterious effects of light. The bottle should be air-tight to exclude chlorides and sulfur compounds which also has a deleterious effect on it.

Suppliers

See the Resources page for suppliers of chemicals in South Africa.

References

1. Texereau, Jean: How To Make A Telescope, Interscience Publishers, Inc., New York, 1957. The town library of Stellenbosch, South Africa has a copy of this book.

2. Ingalls, Albert G.: Amateur Telescope Making Volume 3, Willmann-Bell, Inc., 1996.