Thursday, January 23, 2014

"Orange Peel" on Chinese Brass Coins and Medals in the Late 70's and Early 80's

It has been observed that on early modern Chinese brass coins and medals, the field is not totally smooth. Instead, it has an "orange peel" look:

Even gilt brass medals have this orange peel texture, too:

When this observation was brought up with the moderator of the Chinese brass medal QQ group, Li Haoyu, he stated that the bumpy surface (orange peel) resulted from the annealing process, as brass blanks were subject to extremely high temperatures. He had learned it from insiders of Shanghai Mint. This revelation is similar to the discussions in this thread:

Interestingly, the "orange peel" texture can be used as a characteristic to tell brass coins and medals from this period of time from later fakes.

1 comment:

  1. I disagree that annealing of the brass planchets ("blanks") would produce the orange peel texture of the mirror surfaces. That texture could only come from the dies, not the planchets. I have seen this texture in industry before, and it usually results from sand blasting, then polishing. Pickling, tumbling, buffing, needling, shot peening, or any other surface alteration process could have a similar result if it were polished afterward without first grinding it down smooth.

    Normally, a surface intended to have a mirror finish would be rough machined, finished machined, rough ground, finish ground, coarse polished, medium polished, satin polished, mirror polished, and it would be protected at each step to preserve its perfect flatness. Those steps are a lot of work, and the mint may have decided that the "cheap" brass coins were not worth that much effort. So, they probably used surface roughening process like shot peening followed by polishing. The orange peel surface that results still looks nice, but it would also do a great job of hiding all the imperfections of a sloppy process.

    For example, if any of the polishing grit from a previous step contaminates a later step, the process must be restarted, if you want a flawless mirror surface. That problem goes away when you decided not to make a mirror surface. You can see some examples of a "matte" finish for proof USA coinage here:

    Now that I'm thinking about it, chemical polishing methods might be yet another way to produce an "orange peel" surface. I think this mystery is far from solved, and deserves more research. The one thing I'm almost 100% sure of is that the planchets play no role in what texture is pounded into them by the dies. A few tons of force from the coining press ensures whatever personality the planchets had before has been completely replaced by what the die forces into it.

    I have a hypothesis. I think the planchets were tumbled and rolled to remove burrs, and mint employees were curious what they would look like if they were polished in that state. They got this orange peel texture, and they liked it for its ease of production, and it being a little different from the "look" of most other coins. As you mentioned, it also makes it much more difficult to counterfeit the coins, just like the variations in uneven, shimmering cameo frosting do. Imperfection is much more difficult to duplicate in any forgery process.

    So, the next step would have been to tumble the dies, which was probably not normally done because any burrs could easily be removed by the production machines. But, tumbling is a cheap process (gently rolling and sloshing around in a cylinder with water and gritty little rocks for a few hours), and it's perfect for roughening the surface in a way that doesn't make deep scratches, so it is still easy to polish. The tumbling would probably have to be done to the die blanks before the hub designs were pressed into them, but there are ways to make it work after the hub designs are pressed into them.

    More research is necessary!