What is the buildup on coins?

[Barabus]

 What is the buildup on coins that its obvious that it is not part of the coin,but it has the same color as copper and when its scratched it shines like new copper?I can't do anything with them,you can't tell when you reach the true coin surface.Is it chemical deposits from other coins breaking down and depositing on other coins?

[Ghengis_Jon]

Post a scan of one of your coins in question, and we'll all have a look!

[wileyc]

    Here is what I think downindixie means, I have several coins with this. A seemingly metal encrustation obviously above the surface of the coin by several mm. On this coin pictured, the reverse of I think (in my humble opinion) a common Constantine era ae4 is the burnished top of this metal encrustation. It is the two sort of circular blobs at 1100. One may note underlying letters at 1300. I took a dremel to the surface of the blob, found the metal, and put it back in the DW for me to think about another day. I hestate to get to agressive as despite it most likely being a common coin I rather not destroy it in the pursuit of the encrustation.

  Sort of curious as to what others have to say

cordially
CW

[Barabus]

 Yes,that is the buildup I am talking about.I buy large uncleaned coins and I am getting a lot of them with this stuff.

[awl]

I took a dremel to the surface of the blob, found the metal,

What kind of dremel head are you using brass or steel. If it is brass there is a good chance that the brass just rubbed off on the coin if you did not see the metallic substance before using the dremel.

[wileyc]

Awl
    Yes, actually on reflection that might be true. When I pick at these under a microscope they tend to be extremely hard encrustations but not true metal, they take on the gloss after I use a brass brush. I have never been brave enough for a steel brush. I sort of fear the brass brush and dremel but do consent its usefullness in some instances

CW

[Barabus]

 I don't own a brass brush! This stuff is evident from the start.I try to remove the encrustation before any serious cleaning with a straight pin and craft knife.The copper color is there before brushing with a bronze brush.If you pick at the lumps and break some loose,it is copper color inside the lump.

[bruce61813]

    Here is what I think downindixie means, I have several coins with this. A seemingly metal encrustation obviously above the surface of the coin by several mm.

cordially
CW

What you are calling 'build up' is the original surface of the coin.  These have been eaten by corrosion, both your and Downindixie's, so they are are in fairly bad shape.

Bruce

[wileyc]

Bruce,


     so I am seeking clarification here......... on the top coin for example, what is the encrustation is so much remarkably higher than the surface layer of the coin, there is clearly fairly intact lettering with patina well below the "surface" of this encrustation surface, which seems to have no assiciation with where I might assume a portrait to be aside from it being a good 2-3mm higher than the lettering.
    In my humble short experience of cleaning coins it is hard for me to think it is the coin surface (it may well be I have been wrong many times in my life, a point my lovely bride will gently point out). will the surface of a coin during corrision bubble up higher than the orginal surface??if so how far??

just curious

cw

[SC]

I have wondered about this too.  Not having a sceintific background I am at a loss for the proper terms.  But it sure appears that some coins get covered in stuff (from nice "desert patina" sand to nasty "concrete") which leaves a normal surface to the coin if you can get the stuff off.  Other coins get eaten away.  Yet others seem to "swell" up like the ones in this thread. 

This latter process reminds me more of what happens to some iron objects when they corrode.  I have some late mediavel era spurs which have "swelled" up to twice their normal thickness in places.  I have also seen rusted iron blades which have been cut in half where you can see that the actucal object has dissappeared - leaving a hole or a soft rust - yet a thick hard rust shell thicker than the original item formed around it.  I wonder if the process is ever similar for cupro-alloy objects.

Science types???

Shawn 

[Barabus]

 I thought that in a hoard, coins were deteriorating and the material wash being deposited on coins in the botttom of the hoard. To me this would explain the buildup on the coins on the bottom of the pile!

[bruce61813]

I will try to explain, but there are several factors involved here. First, when the flan is struck two thing happen, some of the metal moves radially towards the rim, the other thing is that parts of the flan are compressed and the local density changes.  Some where in my collection is a Byzantine Follies that has had about 2 mm of a 4 mm thick flan eaten away, but the image and legend are still very visible. It is as if the coin was struck for 15 very thin sheets of bronze that were stuck together.

  It is possible that the corrosion has swelled the underlying flan and lifted the surface, but just because the lower has oxidation, does not mean that was the original surface, it just indicates the current surface. when I get home tonight, I will look at the picture and see if I can point out what I am talking about.

It is not always possible to identify 'top' or 'bottom' coins. The entry of oxygen rich water may well cause the top coins to corrode faster or more than the lower one, as the pH may change before the water gets to the lower ones, or there may be another metal that alter the oxidation.

Bruce

[Barabus]

Well some of the raised spots,the smaller ones,I can pick up with a sraight pin and the coins surface shows underneath.To me it appears to be build up that seems to be the same material that coins are made of.The larger ones are much harder to get off with a pin,but I have been successful in removing the edge of a clump and beneath it appears to be the coin surface.

[Joe Sermarini]

I frequently find coins are encrusted with metal apparently from other coins that have been destroyed.  This is very common with silver.  Usually it will flake off in chunks (at least they look like chunks under a microscope).