The E-Sylum:  Volume 7, Number 16, April 18, 2004, Article 16

NUMISMATICS' GREATEST MISNOMER: CARBON SPOTS

  Dick Johnson writes: "Eric von Klinger?s article in April 13th
  Coin World, ?Qualifiers: A Guide to Lingo,? perpetrates one
  of the most erroneous terms in all of numismatics --
  CARBON SPOTS.

  Carbon is not involved in these surface finish anomalies and the
  process is not oxidation (so often described as the chemical
  process). The culprit is sulfur and the process is sulphatization.

  Carbon spots are found inside diamonds [inclusions from
  imperfect pressure during formation millions of years ago] ?
  not on the surface of coins and medals. The proper term in
  numismatics should be ? SULFUR SPOTS.

  These dark brown to black spots appear on both copper
  (including bronze) and silver coins (including silver clad).
  These are formed, not with contact with carbon, but contact
  with sulfur from the environment. The sulfur comes from any
  variety of sources. The curing of rubber, for example, includes
  sulfur by vulcanization (thank you, Charles Goodyear!). Thus
  rubber should never come in continuous contact with coins and
  medals.

  Sulfur is also used in some manufacturing processes of paper.
  This is why coins tone in certain paper envelopes. Anti-tarnish
  tissue is made without any sulfur at all.

  Sulphatization is a greater problem for the field of frescos than
  coins and medals. Here a sulfur atom replaces a carbon atom,
  physically changing the plaster UNDER the pigments of the
  paint. In numismatics at least our sulfur problem is on the
  surface of the metal, where it can be treated.

  More evidence is color. When carbon reacts with copper ?
  as copper carbonate ? the resulting substance is blue-green!
  Not brown-black.

  Here is an experiment you can do yourself to prove the villain
  is sulfur, not carbon.  Take any uncirculated coin, bronze or
  silver. The commonest source of sulfur for most people in
  daily life are elastic rubber bands where sulfur was used in its
  manufacture. Place the coin on top of the rubber band so it
  stays in physical contact undisturbed for weeks at a time.
  After months you will see a black line where the continuous
  contact was made, the sulfur reacted with the copper or silver
  to form copper sulfate, or silver sulfate.

  Do something similar with carbon. Place in contact with an
  uncirculated coin any form of carbon ? diamond, coal, pencil
  lead ? and leave for the same time.  Nothing will happen!
  Try to speed up the chemical reaction by introducing oxygen,
  water, heat, pressure or whatever. It will still yield the same
  result, nothing.

  In the finishing of high relief medals, as applying a ?French
  finish,? sulfur is the good guy. An active chemical containing
  sulfur is used to purposefully apply a darkening to the surface
  of bronze or silver medals. With the use of ammonium sulfide
  this takes place in seconds!  Medals totally immersed in this
  chemical must be withdrawn within ten seconds and
  immediately washed with water to stop the chemical action!
  (Then the medals are relieved to produce a two-toned visual
  effect.)

  I am not a chemist or physicist. And if we have any of these
  scientists among our subscribers I would welcome your
  comments."

  Wayne Homren, Editor

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