What do you suppose is causing corrosion on a few pocket watch hands? I noticed when looking at a couple of pocket watches that were kept in a safe. They had a dark brownish residue on the tips? It brushed off but left a dark spot. Also noticed it on the regulator. Is it moisture? If so, what can be done to control it? Thank you.

Moisture, high humidity. Also check - older plastic crystals retain moisture and cause hands to rust... can any one else add to this?
You mentioned it was in a safe - is it in your house? Where is it kept?

I've also read that when stored in a closed space with silver, corrosion can occur due to the outgassing of the silver.

Regards! Mark

Mark, interesting!! These are stored in a safe, in the house, dry but!! with a host of silver coins. Most are in plastic containers with the exception of these large (1 troy pound) coins. Can this be the smoking gun??

Moisture and possibly bad plastic crystals. Some people say that the old plastic crystals would absorb moisture but that is the real problem, most of these older crystals degrade and outgas as the result of the degradation. On the outside the outgas just goes into the air, on the other side of the crystal that gas is trapped between the crystal and the movement. With normal humidity and some outgas the mixture is a bit corrosive so on these watches where the crystal or bezel has not been removed for a long period of time the hands will be eaten away and even some corrosion happens on the pinions. Later acrylic plastics did not have this problem but the early type plastics were problematic. If they are in a safe I would think any good desiccant should prevent this, any crystals that look even slightly yellow, replace.

bit of trivia
1922 - first spectacles molded in cellulose acetate (in France)

Some plastics were either Cellulose acetate or Cellulose nitrate, and some of the bad things that happen when they break down. When exposed to heat, moisture or acids in the film base begin to deteriorate to an unusable state, releasing acetic acid with a characteristic vinegary smell

Try storing watches with some silica gel packets when moisture damage due to humidity is suspect. Also consider moisture barrier bags to isolate watches from the effects of other items. Some people store automobiles in barrier bags with desiccant (silica gel).

William

Carlos,
Often you will find grease on the hands form people touching them or handling while fitting, the grease and salt form sweat will not help and sit there eroding away over time.

You might find this article of interest:

http://rg.ancients.info/guide/toning.html

Especially this paragragh from the text:

"If sealed in an airtight environment, the surfaces of a coin will deplete sulfur and other chemicals around it and stop toning after that. Intercept Shield coin holders are designed to intercept and neutralize sulfur and other contaminants and thus prevent toning."


The term 'toning' used in this article means 'rust' or tarnish'.

Regards! Mark