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<p>[QUOTE=&quot;lrbguy, post: 2873130, member: 88829&quot;]Fair enough.&nbsp; It was not my intent to storm away, but to make my point one last time and drop it.&nbsp; But let me put the facts before you and give you some references along the way.&nbsp; Please be patient, it is an involved topic, but it is not necessary to be a professional chemist to make sense of it.&nbsp; Barry Murphy has it straight, and I have met others in the hobby that do as well.</p><p><br /></p><p>Though not a credentialed expert on all this, I&#039;m no babe in the woods either.&nbsp; That is, I am not a chemist, but my father was a research chemist in a major corporation for his entire career.&nbsp; My training in chemistry began with him when I was&nbsp; 9, but not formally until high school and college. In terms of the topic under question my learning really took off when I was a graduate student at the University of Chicago, where part of the time, I worked in the Oriental Institute Museum artifact registration division, and had lots of conversations with the conservators about how to deal with bronze disease and other threats to our artifacts.&nbsp; By then I was actively collecting ancients and could try solutions directly.&nbsp; I will share what I have learned from them.</p><p><br /></p><p>I&#039;m going to try to put this in terms you guys can follow, but some of it is going to stretch you a bit.&nbsp; Please bear with it.&nbsp; It will help you get started to look at this Wikipedia article: <a href="https://simple.wikipedia.org/wiki/Copper" target="_blank" class="externalLink ProxyLink" data-proxy-href="https://simple.wikipedia.org/wiki/Copper" rel="nofollow">https://simple.wikipedia.org/wiki/Copper</a></p><p><br /></p><p>When a seller offers a bright shiny very &quot;coppery&quot; colored ancient, those of us who know better will avoid it.&nbsp; Why?&nbsp; Because, we say, it has been &quot;stripped&quot; of something down to the &quot;bare metal.&quot;&nbsp; Okay, what&#039;s missing, and how can you tell?&nbsp; First of all, the color is wrong.&nbsp; A well preserved bronze or copper ancient should look reddish brown; maybe greenish, but certainly not like metallic copper.&nbsp; Secondly, some detail is missing; the coin features appear flatter than they should.&nbsp; They are.&nbsp; Something IS missing.&nbsp; We say the coin has been stripped of its &quot;patina,&quot; but what is that?</p><p><br /></p><p>&quot;Patina&quot; is what forms in the conversion of the outermost surfaces of a coin into a non-metallic salt.&nbsp; It results from the interaction over time of the material of the original coin surface with materials around it.&nbsp; Not all chemical reactions at the coin surface will result in products that are stable, so the term &quot;patina&quot; is reserved for the non-metallic conversion products that are stable. Since it is a conversion product, patina becomes part of the fabric of a coin, and its removal should be thought of as a removal of part of the original mass of the coin as it was first made.&nbsp; But because it is a non-metallic conversion component, it does not have the same physical properties as the original coin metal.&nbsp; The most notable differences are color and hardness.&nbsp; For example; whereas metals are malleable, non-metals may be brittle, so an intact fully formed patina is less susceptible to scratching than bare metallic copper would be.&nbsp; But let&#039;s look at color.&nbsp; For bronze coins &quot;patina&quot; may come primarily in two colors and still be a stable coating; reddish brown and azure (a bluish green).&nbsp; Which is preferable is a matter of personal taste.&nbsp; Sestertii with a uniform azure patina have long been regarded as the most elegant.&nbsp; I prefer to see them in a rich reddish brown.&nbsp; De gustibus (et coloribus) non disputandum est.&nbsp; Irrespective of personal preferences, how do we account for this color difference?</p><p><br /></p><p>The &quot;chocolate&quot; surface is what appears when the patina consists of copper oxide.&nbsp; The greenish color is most often the result of copper carbonate.&nbsp; Other stable combinations may also exist, and vary in color tone.&nbsp; Copper salts derive their colors from the &quot;valence&quot; or oxidation state&quot; of the copper that has chemically bonded.&nbsp; Because of the distribution of electrons in the outermost shells of a copper atom, copper may take on one of two &quot;valence states&quot; in forming bonds with other materials; +1 for a single free electron, and +2 for two.&nbsp; It varies dependent upon the nature of the other component to the reaction.&nbsp; Copper oxide, which forms very slowly underground, results from a bond between oxygen and copper in the +1 state.&nbsp; In this state the copper atoms contribute a white color to the mix.&nbsp; Copper carbonate results from copper in the +2 state when the copper atoms contribute a blue color.&nbsp; The precise tones will vary due to other materials that may get into the mix.</p><p><br /></p><p>The natural patina cannot be replaced once it has been stripped off, for the simple reason that some of the metal of the original state of the coin has been lost.&nbsp; Any kind of replacement is a remake of the coin.&nbsp; Toning the outside surface is not a true &quot;repatination&quot; since it does nothing to replace the lost copper atoms.</p><p><br /></p><p>Those are the basics of what goes into the formation of patina.&nbsp; But what is going on when a coin with a nice coating of copper oxide patina starts to form a greenish colored accretion on it?&nbsp; Spray a mist of vinegar on a nicely patinated coin each day for a week and watch what happens.&nbsp; Azure colored dots and spots will form.&nbsp; Doesn&#039;t the patina protect it?&nbsp; The protection a fully patinated coin is going to get from the chemicals around it is dependent upon the activation energy potential of the contaminants.&nbsp; The acetic acid in vinegar has a high enough activation energy so as to interact with copper atoms in the patina and change their valence states from +1 to +2.&nbsp; That will create an azure colored accretion on the coin surface at the outermost level of the patina.&nbsp; For that accretion, and only the accretion, I reserve the term &quot;verdigris.&quot;&nbsp; Since it is formed on the surface of the patina it can be removed without deep damage to the coin surface, but because it is bonded to the surface it may take effort and care must be taken not to go too deep.&nbsp; This is particularly true of mechanical methods of removal.&nbsp; I am still looking for a good chemical solution to that problem.</p><p><br /></p><p>There is one more topic to mention: accretions.&nbsp; These are contaminants which adhere to the coin surface due to chemical fusion.&nbsp; Two nails which overlap in a box, if allowed to get wet, may fuse together as they rust (oxidize).&nbsp; One is an accretion with respect to the other.&nbsp; Foreign matter in the matrix within which a coin has lain for centuries, is often found bonded to the coin.&nbsp; The subject is wide and varied, so my comments are limited to the one dimension of removable accretions versus non-removable.&nbsp; If a patina had formed before the contact was made, then usually the accretion can be removed without too much damage to the coin surface.&nbsp; However, contaminants touching the coin surface before it became fully patinated may find that the accretion cannot easily be removed without damage to the coin.&nbsp; Professional conservators frequently resort to using a hand held grinder tool (e.g. Dremel) to remove bonded accretions or to reduce surface ones.</p><p><br /></p><p>The formation of stable copper salts on the surface of a coin has nothing to do with bronze disease.&nbsp; &quot;Bronze disease is the irreversible and nearly inexorable corrosion process occurring when chlorides come into contact with bronze or other copper-bearing alloys.&quot; - Wikipedia,&nbsp; for verification and further information see: <a href="https://en.wikipedia.org/wiki/Bronze_disease" target="_blank" class="externalLink ProxyLink" data-proxy-href="https://en.wikipedia.org/wiki/Bronze_disease" rel="nofollow">https://en.wikipedia.org/wiki/Bronze_disease</a></p><p><br /></p><p>I don&#039;t want to be a nuisance (I can hear some wag saying, &quot;Too late.&quot;) but more can be said and if you ask me to explain something, I will respond.[/QUOTE]</p><p><br /></p>

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