From: Guy Brandenburg 
To:  atm{at}shore.net
Reply-To: Guy Brandenburg 


A cautionary tale about using ammonia to strip mirrors.

A person had a nice old smooth, spherical mirror that needed recoating. He
brought it to the NCA MMW to be recoated. Before doing that, he put it in
ammonia overnight, and it stripped it off nicely. Our aluminizer was not
working properly that week, and it only gave a partial coating. (naturally,
he didn't have to pay for a bum job.) The owner was unable to come back any
time over the next few months, when we had it working properly, so he sent
it off to a commercial place after once again soaking the mirror in ammonia
overnight to remove the partial coating. When he got it back, it had
hundreds of visible pinholes, which he didn't think were acceptable. So he
sent it back to the commercial coater. The coater put on a second coat, and
sent it to the owner. The owner was, if anything, less pleased than the
first time, because (he said) it looked like somebody had rubbed phlegm and
mucus all over it. He stripped it again (I think), same method, and sent it
back to the coater. The coater looked at the mirror and decided that he
would be unable to complete the job, saying that there was still a partial
layer of silver on the mirror, so he refunded the money and sent the mirror
back. So the owner of the mirrorr once again soaked it overnight (either
the 3rd or 4th time) to strip off what was left, and brought it to us last
night to finish the job.

When we saw the mirror last night at our mirror-making workshop, it was
pretty obviously hazy, and had a few places that looked like they still had
some aluminium coating on them. We tried to clean it off with soap, water,
and cotton balls, but it didn't remove the haze at all. We put it on the
test stand, and found that it reflected EXTREMELY little of the light, even
laser light, a phenomemon that I had never seen before. It seemed like it
had almost completely lost its polish. What little reflection I could see
was extremely uneven.

My conclusion is that the process of soaking the mirror in ammonia,
repeatedly, had actually attacked the glass in a microscopic and
macroscopic manner, and that the haze was not something that could be
removed by cleaning, but, only by polishing with a pitch lap and either
rouge or cerium oxide. So we cast a lap and the owner began polishing using
cerium oxide. After about 50 minutes, a shine had returned to most of the
mirror, though there are still some spots that are obviusly not polished,
and we were able to see that it still had its nice spherical figure (more
or less). He will have to keep polishing next week.

So, I guess the moral of this story is that it is unwise to use ammonia to
strip mirrors unless you do it very, very briefly. On the other hand,
soaking overnight in muriatic acid (or HCl) dooes not seem to cause any
problems whatever.

What caused the pinholes seen by the commercial coater, or the 'snot'
effect, I am not sure - those effects may or may not have been related to
the ammonia.

Guy Brandenburg

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