Coin Detail

[sjv]

The worst thing about find old coppers ( pre 1800's and large cent) is that sometimes it looks good when you dig it up,but by the time you get it home the dirt has flaked off and the coin looks alot more worn. Why is it that some coins have good patina (which hold the coins detail) and some are just covered in dirt? Is it something to do with soil type?

[The Beep Goes On]

A little googling provided the following info...

The metal (copper) develops adherent protective coatings, initially of oxide, but subsequently thickening to give a familiar green patina on roofs and the dark brownish color of bronze statuary.

There are two types of copper corrosion: uniform and nonuniform. Both types are caused by certain characteristics of water chemistry, including low pH, high alkalinity, and the presence of sulfates or nitrates. Uniform corrosion is identified by the presence of a relatively uniform deposition of copper corrosion by-products across the inner surface of a pipe wall and is typically associated with elevated copper levels at our taps. Nonuniform corrosion, or pitting, is the isolated development of corrosion cells across the inner surface of a pipe wall. Although pitting corrosion is seldom associated with elevated levels of copper at our taps, excessive pitting corrosion can lead to “pinhole” leaks in the pipe, which could result in water damage and mold growth.

The corrosion of copper by oxidation on exposure to the air takes place very slowly, the metal becoming soon coated with a skin of carbonate commonly called verdigris, though that name is correctly applied to a basic acetate of copper. This familiar film on the surface of exposed copper constitutes a protection against further oxidation. Extensive tests have been made of the corrosion resistance of copper and its alloys to various atmospheres. Various alloys were exposed in rural, industrial, and marine atmospheres for periods of up to twenty years. From data accumulated in these tests and the calibrations of relative corrosivity of the test sites, a fairly clear picture can be obtained of the corrosion behavior of copper. In addition to the corrosion penetration rates, one must be mindful of dezincification of brasses and selective attack on some bronzes, as well as SCC illustrated by season cracking of brass. These types of corrosion contribute to the failure of the material in mechanical respects without significant weight changes or losses in thickness.

Where copper is used as flashing on roofs, corrosion has been encountered at the edge of the shingles as a continuous groove. This effect is more pronounced when the atmosphere contains both chlorides and sulfides, and with wood shingles as compared with roofs of other composition.

Copper compounds are known in several oxidation states, usually +2, where they often impart blue or green colors to natural minerals such as turquoise and have been used historically widely as pigments. Copper metal architectural structures and statuary eventually corrode to acquire a characteristic green patina. Copper as both metal and pigmented salt, has a significant presence in decorative art.

Copper and its alloys are not exposed to the atmosphere in great quantities when compared with steel. However, this material brings aesthetic value to building construction, in addition to excellent corrosion resistance. The black and then green patina formed on the surface provides an attractive decorative finish, while sealing the metal from further corrosion. As a consequence, some copper has been used for roofs, gutters, and as flashings on wood or composition shingled roofs. If the green patina on copper alloys is desired for aesthetic reasons, pre-treatment of the surface with appropriate passivating solutions is recommended. If oxidation by sulfur compounds precedes the desired reaction, the surface will present only a dark brown color for many years.


So, depending on soil chemicals, fertilizer use and water action (and pH, alkalinity, etc.) you can end up with diferent types of corrosion. The passive corrosion resistance (or patina) is a good thing...sometimes I leave the patina on the find instead of cleaning it.

HH!
Beep

[coinnut]

Thanks, Beep! I never read that one, but it gives a great overview of how copper handles everything from rain (acid rain), minerals, polution, moisture, etc... Add to the fact that our government bought large quantities of copper and all of it could not be the same grade. It's a very complicated chemical issue. Sometimes you can tell that a coin will clean up well, but most times we just blindly hope it will lol