8 Reales 1770 Potosi - Bubbles?

@dh, we have several experts on these who stop by CCF on occasion. While we wait for their informed replies, I would only add that such bumps could be the result of striking coins with corroded or rusty dies. Such bumps are also sometimes found on cast fakes.

Intense heat can cause bubbles like this (blowtorch or house fire, eg), especially if the coin is plated. Possibly seawater corrosion, but not much damage is visible other than the bubbles, and the damage is on one side. Possibly casting bubbles, though there are far more than usually seen. The coin looks correct otherwise.

I have a few Potosi 4R cobs (1765-1771), which are as black as coal. I suspect that they are Seville Harbor recovery coins. I cleaned a couple of them, and they look like the shield side of this 8R. Here are two of them, an uncleaned one on the left and a cleaned one on the right.

Alloy Errors:
Gas Bubbles
Definition: On rare occasions a pocket of gas forms and expands when a planchet is struck. The heat generated by the strike is deemed responsible for the gas expansion. The expanding gas pushes up the overlying metal, producing a rounded bulge with soft borders. If the roof remains intact, the error is designated an "occluded gas bubble". If the roof explodes from the internal pressure, we call it a "ruptured gas bubble".

If the roof is thin, it will flex or it will be left with a dimple when the tip of a toothpick is pressed into it. If the roof is thick, it may not yield to pressure.

By definition, occluded gas bubbles are generally restricted to solid-alloy issues. While gas bubbles are sometimes seen on clad coins, these always turn out to have been caused by heat applied externally outside the Mint. Occluded gas bubbles should not be confused with blistered plating, the latter being an affliction restricted to copper-plated zinc cents.

So if genuine some RARE form of heat generated striking event - yes unusual making the coin a suspect forgery of casting but it appears genuine. John Lorenzo (Numismatist).

Hi jgenn, sorry but I am not the owner. It was being offered by a private individual on a web marketplace and I was very curious about it. As I said I am not an expert on these coins but I have collected them for years and this mint and era combination is of great interest to me.
Weight info is missing from the article and the only record I could find online from this coin, when it was auctioned by Heritage Europe (17.05.2013, Auction 38 - Session 4), shows no weight either.


This is all I have from the article:



I don't believe to have claimed such thing, but they shouldn't, of course.


I add another detailed picture from the obverse. I still doubt this is a cast fake, but this is just an impression far away from certainty.

I am just a collector and not an authenticator but my vote is those are casting bubbles and I would guess a specific gravity test will show less than 90% silver. I have seen many coins struck from rusted dies and they don't look like this one and I have seen too many cast coins that share this "bubble' anomaly. There are bubbles it appears on the shield side (which is the obverse) located around the "8" and up into the "ET". If you are considering buying this coin, I suggest waiting for a nicer example to come along.