Cleaning bronze crossbow mechanisms

The alloys used in the mechanisms is for whatever reason fairly corrosion resistant. However, they are still bronze, and are subject to what is referred to as "Bronze disease" ... the formation of cupreous salts which in turn form hydrochloric acid as a by product, and self perpetuate. One of my favorite stories on the matter is that of a US diplomat in Taiwan in the late '60s, early '70s. He mailed himself a Shang Dynasty vessel in the Diplomatic pouch, only to open the package a few weeks later to find it had essentially turned to dust.

Bronze disease usually appears as a soft, pale green powdery deposit. Depending on factors such as tin content, the color ranges from almost white to pale blue. The key is its texture ... it can be scratched easily with a fingernail or toothpick. It forms both macro and micro pores into the metal, which is the primary difficulty in treating it ... it is impossible to remove it all.

Causes are rather complex, and I can't say that I understand all of the chemistry involved. Suffice to say that, in burial situations, pieces are usually protected to one degree or another (though not always ... I've got a couple of minis that are good examples of that). However, the increase in oxygen provided when exposed, the presence of salts in the air and on the skin, acids and oils from the skin, and ambient moisture are all potential causes of bronze disease. Oh ... and metal formerly covered by protective corrosions, such as malachite, cuprite, and azurite (the greens, reds, and blues typical of archaich) are more
susceptible to it.

The old method of dealing with it was to heavily wax or oil the piece ... which from personal experience i can tell you is one of the worst things one can do. Unless one is able to completely seal the metal off from the atmosphere, it actually causes the problem to fester.

I use two primary methods of treating and preventing it, one of which I'll mention here as it is easy to do, though it requires some maintenance. The other, more permanent ... it requires specialized handling, some equipment, and the handling of a rather nasty chemical. Part of it I also semi invented, so it would be easier and cheaper to have me do it if it is really desired, or if one has a really problematic piece. (For that, or any other questions, just emial me at [email protected], my business email).

Cleaning: given the general topic of the forum, the end result desired is function, as opposed to the typical collector's ideal of complete preservation of as much patina as possible while revealing details that may be obscured.
For the functional result, most of the minutia of cleaning can be skipped. However, I would offer two bits of advice: First, for the sake of future authentication, try to leave a patch of the patina intact someplace; Second, to avoid marring the surface, use bronze tools (I use the broken shafts of period arrowheads, etc., sharpened to a chisel point). Also ... be aware that relatively frequently, makers and users left marks ... and you may also encounter silver or other decoration. One of my mechs has been tinned or silverd over much of its surface (I have yet to test the metal, but mean to soon). If you're really lucky, you'll encounter inlay work (see http://www.pricelesspast.com/gallery_Other.html for an example).

For bronze disease treatment and prevention: Remove any oil, dirt, wax, etc. (hot water and Dawn dish detergent is fine). Remove any surface deposits one can find, using wood, antler, or bronze tools. In visible pores, one, if careful, can use dental pics to get inside of them as much as possible. Bring the piece to a boil in distilled water and allow to cool .... repeat 5-10 times. The reason for this last part is that the expansion and contraction of the water molecules can flush the macro and micro pores to a decent extent. This should be follwed by one of two drying methods: bake in an oven at 150F for a couple of hours, or place in an acetone bath overnight and then allow to dry.

That was the hard part ... the easy part is: Pick up a bottle of Corrosion X (available at www.BuyCorrosionX.com). There may be other anti corrosive oils around that would be effective... however, I have found none that is as effective for cupreous metals as this stuff. And, one bottle should last quite a while. I use the trigger spray, and with dozens of bronzes inclueding a 2 meter statue, it lasted me three years.

Wipe on a decent coating of the stuff, let it seep in for a few minutes, and wipe it off. Repeat every 6 months to a year, though given the type of use they may recieve, with the mechs one may want to wipe the trigger and any other touched/used area more frequently.

Best,
Nathan