I seem to remember hearing somewhere that whatever turns watch crystals yellow also causes the hands to rust. I have two watches with yellow crystals and they both have rusty hands. What's the connection? Does this rust tend to migrate into the movement itself? I see no rust on the steel parts in the back of the watches.

Thanks in advance for any information.

As I understand it the older plastic crystals were moisture permeable and the hands would rust as moisture worked it's way thru the plastic. The movement's were pretty much unaffected. Newer plastic crystals are made from a different material and don't exhibit the same problems.

I am not sure if all the blame would be moisture, most severly yellowed crystals you see cases where the hands were corroded beyond what light moisture would do. Many times with early plastics there were photo-chemical reactions with UV radiation that cause outgassing and probably whatever was being out-gassed was a bit corrosive and being trapped between the dial and the crystal it started acting like "pac-man" on the hands. Early nitro-cellulose lacquer was also affected the same way by UV radiation from sunlight, that case the lacquer resin structure to break down, yellow and shrink. The plastic would have to be very permeable to allow the water molecule to pass through, you could easily test that concept by putting some water in the dial and seeing if water passes through, which I doubt you would see.

The "yellow" crystals are made of a cheap highly hygroscopic cellulose material that was used during WWII and sold as "war plastic" to conserve our supplies of the better Acrylics used for Military windshields, etc.

These crystals were popular during the war only because they were all you could get. Putting one on a watch had the same effect as putting your watch hands in a humidifier.

As a rule, when you get a watch with a yellow crystal, you are far better off disposing of that crystal even if it means that there is no crystal at all.

Thanks Dave, Roger & Claude, very good answers, all....

Christopher, if you would like additional information, at the header at the top of this or any page you can click on the "find or search" block and type in 'yellow crystals' and there is a whole host of topics that have been written about the subject....very interesting reading....

Regards,
Jerry

Claude & Dave:

Thank you! Best answers I have heard so far.

One of the reasons that I don't think moisture is the main culprit is because if for example you have permeable membrane of some type dividing a tank, on one side you pour in a salty solution and on the other you pour in fresh water. After a period of time the permeable membrane will allow the two sides to come to equilibrium and you will end up with two sections of the tank being slightly salty. In the case of the yellowed crystals if the ambient humidity between the crystal is lower than the outside air humidity the moisture would equalize over time so that the air between the dial and the air outsie would be at the same humidity level, if the outside air was "dryer" the more humid air between the dial would equalize with the outside over time. If the plastic was hydroscopic normally you would see a milkiness appear in the crystal itself. With old nitro-cellulose lacquer you would see this milkiness and even newer synthetic lacquer if the humidity is high when you brush or spray the lacquer you in high himidity conditions get a "blush" or a whitish look caused by trapped water vapor in the film. Many old tube radios have round flower pot rings from the lacquer being permeable and allowing water to get to the wood underneath and staining the wood long after the water has moved back though the lacquer layer into the less humid outside air. If the moisture moved only one way from the outside to the area between the dial and crystal you would at some point see droplets of water on the crystal or dial and that is not something I have seen unless someone forgot these old watches are not waterproof. All plastics "out-gas" something which in the 60-70's cars was referred to as "new car" smell but in the case of these crystal whatever was being out-gassed when mixed with the normal moisture in the air was a bit corrosive and being trapped in a somewhat confined space tended to pac-man the hands.

Youi make a very good point, and still the fact remains, with "war plastic crystals" which are hygroscopic and retain water as they are not acting as a permeable memberane but rather more like a sponge, all hands in war plastic crystalled watches are destroyed by rust.

Pre and post-war acrylic plastic crystals which also stay "water clear" have no consistent record of messing up hands. Many of those acrylic were "steam tempered" in their production to properly combine water molecules in their structure and improve shock resistance.

There may be another factor in the chemistry of the war crystal vis-a-vis the metal hands, but the fact is the "war plastic" destroys hands.

Dave, even a sponge will dry out if the outside air is less moist that it is. If the lens was a hygroscopic as you mention if you put water in the lens and kept it there at some point the lens material would become so saturated with water and then would finally drip water out the other side (if you put the curve side up and supported it and kept it full with water)I have a few yellowed lens, and for the ones that don't have any cracks I will do just that and see if within a day or two if water drips out the lower surface which would have to be the case because the lens would absord so much water that just as a sponge, when it reaches more than it capacity the excess water will be pulled though via gravity. Regular lacquer was made out of a combination of gun cotton (cellulose) and nitric acid and the end result was a nitration of the cellulose compound and they yellow because old nitro-cellulose laquer had zippo resistance to UV radiation which lead to yellowing (breakdown on the side chains on the benzene ring structure) and in extreme exposure to sunlight, gatoring (cracking), and finally just falling off. Many things besides water will cause oxidation (rust) but the safe thing to say is if it has yellow lens replace it asap. By the way I am more than willing to "bet a beer or two" that if I did the experiment mentioned above that I would not see a drip. I did check and the first acrylic plastics were developed by DuPont ~1941 and it was commented that acrylics were highly resistant to sunlight.

I would be interested in reading your results if for nothing but the sheer scholarship. This reason has been repeated on every watch forum and discussion group I've read to date. It would be interesting to read what actually happened in an actual test.

Regards! Mark

Well after nearly 24 hours there is no evidence that the water that I put in the concave part of the dial is being absorbed and no evidence that the water is seeping though by pure saturation. If this plastic is supposed to be such a sponge that it absorbs enough water to turn hands into mush then surely some evidence of moisture would be easily seen. I will keep it up until after the holidays but I think what has been stated in the past is "urban legend". The crystal that I used is not "baby duck" yellow, more like "pee" yellow but it is noticeably yellow when you compare it to a good crystal. I used one that is free of cracks or chips but it has been exposed to UV enough to yellow it but not enough for the plastic to totally break down like some you see that fall to pieces.

This is just my personal opinion, but I don't think droplets of water is a valid test. It would be simply too large to pass into or thru. Surface tension of the droplet of water would prevent the passage. What I think might be at play is the possiblility of the chemical reaction of the outgassing AND the possibility that normal moisture/humidity in the air could over a long period of time pass thru microscopic pores in the plastic material. I doubt if droplets of water could ever do that.

Roger