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Sensitizing and the Brashear Process

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Hi, it is me Eric Keaveny from Harrison High School. I have been doing research on electroplating polyethelyne. One of the steps is called "sensitizing" and involves the use of SnCl2. I am not sure why this process is used or what type of reaction takes place when the plastic is immersed in this solution. Then I am going to use the Brashear process to deposit silver on the surface and then plate it from CuSO4. I was just wondering what the stannous chloride was for.

Eric Keaveny

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Eric,

The present school of thought is that the SnCl2 separates in water and leaves Sn++ in solution. This then wets to the surface of the substrate and presents a "sensitized" surface for deposition of, in this case, silver.

SnCl2 sensitization is commonly used as a precursor for a PdCl2 "activation" before electroless deposition of Nickel as well.

Maybe you can help me. I am looking for information on the Brashear process. Do you have documentation on the subject?

Thanks,

Milton Ives



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In recent days, I have been collecting information on and chemicals for the Brashear Silvering process. I have run a few test runs in glass and have not tried plastic of any kind yet. Some references mention the use of the stannous chloride and some do not. In my experiment to date, I have not used it, but will be ordering some to try later.

The material I have states that the sensitizer solution has the available tin ions (Sn2+) that are free to react with and bond to the negatively charged anions on the glass silicate surface. (I do not know if this applies to other surfaces such as polyethylene). To continue, this produces a monolayer of tin on the glass that is ready or sensitized, to reduce silver ions (Ag+) in the silvering solution. In this process the tin is oxidized to Sn4+ and metallic silver (Ag0) begins to plate out on the tin layer.

 

The information, as applied for producing the silvered inside surface of a glass dewar, states that the freshly silvered surface is rinsed with the sensitizer solution followed by a distilled water rinse. This leaves a surface layer of tin that does not allow oxygen in the air to form silver oxide (Ag2O). The dewar is allowed to dry, is evacuated and heated etc.

If the silvered surface is to be copper plated or simply painted, then the sensitizer rinse may not be needed.

It is interesting that the recipes I found for telescope making for producing the silvered first surface do not mention the sensitizer rinse. They are concerned about tarnishing, maybe this will help?

The recipe I have for the sensitizer is as follows:

Concentrate: 1 (one) gram stannous chloride in

10 (ten) ml distilled water.

Dilute concentrate for use as follows:

Dilute sol'n: 0.2 (one-fifth) ml concentrate in 1 (one) liter distilled water.

The dilute solution has a shelf life or not more than three hours.

The concentrate will last much longer but does slowly degrade forming "an unappetizing mess of yellowish solids"

The stannous chloride does not deteriorate in the dry state in the reagent bottle.

Lester Scovitch



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DEAR SIR ..

THANKS ALOT FOR YOUR ADVICES. I WOULD LIKE TO SEND ME COMPLETE INFORMATIONS ABOUT SILVERING A GLASS WITH EASY OBTAINED MATERIALS AND CHEMICALS WITH A CLEAR CONCENTRATION ,GM PER L. I HAVE PRINTED MY PHOTOGRAPH ON A GLASSBY USE SILK SCREEN AND I NEED TO COVER IT WITH SILVER TO BE A MIRROR CARRY MY PHOTOGRAPH WITHOUT THE SILVERING PROCESS EFFECT OR DETERIORATE MY PHOTOGRAPH.

YOURS TRULY,

Maruky Mohammed
-BENGHAZI, LIBYA


 

I am doing research on sensitizing glass for silvering, and then protecting the film. I am looking for documentation on the Brashear Silvering process. Can anyone tell me where it is published?

Thanks,

Dan K deleted
- Greensboro, North Carolina

Ed. note: letter 916 provides a little information about the Brashear process, Dan.



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A general comment: the Brashear process produces some [explosive] silver fulminate as an intermediate compound, so add & mix chemicals SLOWLY, and be sure to wear eye protection.

Also, I can respond to 2 of Lester Scovitch's comments:

(1) The sensitizer is not used for telescope mirrors because it micro-roughens the glass surface, preventing ideal optical resolving power ("diffraction-limited" performance) from being attained.

(2) The degradation of the "concentrate" probably is preventable by adding HCl (but I can't specify, how much): a commercial chemicals catalog states, Tin (II) chloride [is] soluble in HCl (dilute or concentrated), Decomposes in excess water.

William D. Fountain
- Carlsbad, California, USA



October 10, 2012

Q. How to dilute Stannous Chloride and use as a sensitizer for silver spray?

Hi,
I am currently using a silver spray method to apply silver to glass or high gloss substrates.

I am now buying stannous chloride in solid form (for shelf life) to use as the sensitizer.

Can someone please explain how I would break this down so that it is ready to apply?

For e.g. 4 grams of stannous chloride to 1 ltr of deionised water?

Also to preserve the life of the stannous chloride would it be advisable to keep it in a dark airtight container?

Thank you in advance .... a total beginner :-/

Ian Morris
- Belgrade, Serbia
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