ID help again

[paul monfils]

Thought I would address your other question as a separate post.  Rust stains on shells are almost impossible to remove.  Actually they are not just "stains" lying on the surface of the shell.  Rather, they are in effect "part of" the shell, the iron having actually replaced the calcium in the shell structure.

A brief chemistry lesson ... metals are arranged in what is called the electromotive series, or the replacement series.  Without going into the actual molecular changes that occur, suffice it to say that a metal higher in the series will replace a metal lower in the series in a compound containing the lower metal.  The shell of course is composed largely of calcium compounds.  Calcium is very low in the replacement series, and iron is quite high in the series, so when iron comes in close contact with a calcium compound in the presence of water (especially salt water), iron atoms actually kick calcium atoms out of the compound and take their place.  What was calcium carbonate becomes iron carbonate.  The only way to effectively remove the iron would be to expose the shell to a solution of a metal higher in the series than iron.  Two problems here ... first, most of the metals higher in the series than iron are very expensive and/or very toxic metals (cobalt, mercury, lead, silver, platinum, gold).  And secondly, any metal that would replace iron would very rapidly replace calcium.  So there you have it - such stains are essentially permanent - with one exception:

There is one approach I have used occasionally to remove a light rust stain from a non-glossy shell like a Buccinum.  Application of a weak acid solution with a cotton swab, just on the stained area, will remove such a stain if it isn't too deep.  Vinegar will work but it's a very weak acid, so it will be slow going.  However, if you use this method, you should be aware that what you are actually doing is removing the outer layer of shell in which the stain resides.  If the iron replacement is fairly deep, you'll have to remove quite a bit of shell to get rid of the stained part.