The brass on the balance becomes very friable and it's action is destroyed.
It's also worth pointing out that ammoniated cleaners are popular for (especially) clock repairs, but prolonged immersion of brass in ammonia destroys its integrity. I've read that it dissolves the zinc from the brass, but I don't know if that bit is true or not.
Steve,
Two things about ammonia in cleaning solutions. It is waste. Excess ammonia means the proportions for making the soap are wrong. How do I know? I hold the tradmark to "Historic TImekeepers Ultrasonic Cleaner" as sold by Timesavers and others for two decades. It was a low ammonia alternative to L&R which forced them to reformulate.
Secondly, ammonia only degrades items like stamped brass. It was uncovered as a result of brass cartridge failures in the imperial UK army. The myth about it attacking brass clock parts was created by a guy back in the days of the Syracuse Clocks listserv (1994 to about 2002). This guy said he had thousands of photomicrographs of SCC failures but when we started calculating man hours and dollars, we pinned him down and he had to admit it was all a thought experiment a al "Sir"Cyril Burt (nature vs nurture twin studies).
But when a lie is repeated often enough....
Look up research on Stress Corrosion Cracking. Last I looked it all involved drawn or stamped brass in high pressure ammonia environments.
It is really like "personal magnetism" used to explain why a watch performs poorly.
Cavitation is the concern for timepieces. In fact,for ancient items, I will use ivory soap in very hot water lathered up on a very soft brass bristle brush (feels like a dust brush almost). With this, I can remove fingerprint stains on chronometer plates with
out harming the spotting (decoration). But if the prints are acid etched into the brass, that remains.
Gold gilding comes up like new.