P3D Re: Brass, again, for antique viewers

[roberts@xxxxxxxxxxxxxxxxx (John W Roberts)]

>Date: Mon, 2 Feb 1998 10:09:19 -0700
>From: Duncan Waldron <J.Waldron@xxxxxxxxx>
>Subject: P3D Re: Brass, again, for antique viewers

>On Mon, 2 Feb 1998, John W Roberts wrote:

>> Could it be that the ammonia just facilitates the reaction, without being 
>> consumed in the process? Anyone know the chemical formula for brass tarnish? 
>> (If ammonia vapors can cause it, it wouldn't be a sulfur compound like silver
>> tarnish.)

>Why not? It would depend upon what was attacking the brass; use nitric acid, 

What, you guys don't call it "azotic acid" any more? (reference to British/
American terminology :-)

>and you would surely get a nitric compound; sulphuric acid (sorry,
>but I'm British) would give sulphuric, and so on.

But ammonia solution is NH4OH (the vapors would be NH3?), so for the
ammonia vapors to facilitate overnight tarnishing (Bob W's Dec 31 post),
you couldn't count on there being enough sulfur in the air to accomplish
a tarnish based on sulfur compounds.

A quick check revealed that silver tarnish is silver sulfide, but gave no
indication what brass tarnish is made of (there could be different kinds).
It was noted that metal tarnishes are typically based on oxygen or sulfur,
and that Gustav Tammann (1861-1938) was famous for his analysis of the
chemical composition of tarnishes.

It would be interesting to know whether immersion in ammonia softens the
layer of tarnish, or whether it softens the layer of metal just under the
tarnish. In either case, since rubbing is required, it would appear that
some metal and/or metallic compounds are removed.

John R


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