Tutorial Lesson 17, Silvering Glass (Advanced Topic)
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### Silver Plating - The Brashear Process

The Brashear process is commonly used to silver the internal surfaces of Dewar flasks, which are evacuated double-walled vessels (much like a Thermos® bottle) used for handling and storing laboratory cryogens such as liquid nitrogen and liquid helium. The combination of the evacuated double-wall and silver mirror surface greatly reduce heat transmission from the inside to the outside of these vessels.

The chemistry of this process is described in the Tollen's test which is a physical method that tests for the presence of aldehydes in an organic chemistry laboratory. A video of this process is available through the Royal Society of Chemistry.

Three Part Solution Mix for Internal Plating
Solution 1 2 liters distilled water, 50 grams silver nitrate. Store in a dark glass bottle.
Solution 2 2 liters distilled water, 90 grams potassium hydroxide.
Solution 3 800 ml distilled water, 80 grams cane sugar, 100 ml ethyl alcohol, 3.5 ml nitric acid. Age 30 days or boil 30 minutes before using.

Solutions are used at the following ratio: 1 : 2 : 3 = 64% : 32% : 4% by volume. Cut out this handy table and stick it on your shop wall as a quick reference so you don't need to calculate volumes:

Solution Required Solution 1 Solution 2 Solution 3
50 32 16 2
100 64 32 4
150 96 48 6
200 128 64 8
250 160 80 10
300 192 96 12
400 256 128 16
500 320 160 20
600 384 192 24
700 448 224 28
800 512 256 32
900 576 288 36
1000 640 320 40
1500 960 480 60
2000 1280 640 80

To use: First ensure that your glassware to be silvered is scrupulously clean. Then, add concentrated ammonium hydroxide dropwise to Solution 1 with stirring. It will form a precipitate with the first drop; continue adding until the precipitate clears. This should require no more than an eye dropper's worth of ammonium hydroxide for volumes up to ~500 mL. Add Solution 2 to Solution 1. The combined solutions will have a creamy white precipitate. Stir the solution and again add concentrated ammonium hydroxide dropwise until the solution is slightly cloudy. Pour the solution into the glass vessel to be silvered. Add solution 3 and stir/shake while the mirror develops.

Warning: Wear protective clothing and read the MSDS for all materials that you use. Promptly and safely discard unused mixed solutions.

We hope to post some pictures here that demonstrate a Dewar flask being made. Stay tuned!

Tutorial Lesson 17, Silvering Glass (Advanced Topic)
Lesson 16