What is a "Porous" coin?

samingeorgia · November 23, 2004 11:16PM

I saw two nickels described on the bay lately as "porous" - what is this? How does a coin get that way? The pictures I saw made me think that the coins had been exposed to something caustic.

Thanks in advance.

dthigpen · November 23, 2004 11:23PM

Correct. Coins become porous due to enviromental damage (Acidic compounds, corrosion, etc); if 'natural', generally over long periods of time.

BillJones · November 23, 2004 11:31PM

“Porous” means that the surfaces of the coin have been damaged so that the original smoothness has now been replaced with a rough, uneven layer of corroded metal. Porosity is most often caused by exposure to the elements (for example buried in the ground or lost at sea), but contact with strong chemicals can provide the same result. Porosity is best known as a problem for copper coins, but nickel, silver and even gold coins can suffer similar damage.

If a coin is very rare, it can still command a strong price, even if it is porous. This is most often the case with very rare varieties of early copper coins. Quite often porosity is almost a given for these pieces.

For other metals porosity is usually the kiss of the death with respect to collector values. The values are often considerably reduced, and the coins are usually of little interest to serious collectors.

Weiss · November 23, 2004 11:32PM

Coins found with a metal detector can often have a porous surface. Some soil tends to be acidic and/or basic, fertilizers add to the problem.

longtimecollector · November 23, 2004 11:37PM

To view what porosity looks like, place a pre 82 lincoln in a glass ....... add enough vinegar to just cover the coin. In the morning (pay no attention to the color) examine the surfaces of the coin....... yes, this is really acid etching, but the look of the surface will very closely resemble porosity