blister on middle congo 1925 token error or heat ?

[MORGENSTERNN]

Hello
I'd like to have your opinion about the blister that appears on that middle congo token
Is it an effect from heat or a defect of the planchet (air/gaz)
The coin is in good grade but the edge present a defect at the level of the bubble : really porous with a kind of crater (expansion from gaz ?)
I read that bubbles can appears if the metal is not perfect as on french holed 10 centimes 1941 stroke in Zn but what about Al?
The blister is maybe due to the pressure when coin is holed ?

[Figleaf]

I am not sure where this token was minted. However, this is a known issue in the Brussels mint. It is seen most often on the holed 5, 10 and 25 centimes pieces of the period 1900-1930. The problem was that the copper and the nickel were insufficiently mixed, so that small lumps of either metal remained. In the best case, they caused brown stripes on the coin's surface, but in worse cases, either the surface started to flake or the metal deformed as on your token. The Paris mint had similar problems on its 1915-1940 coins of 5, 10 and 25 centimes, but usually, they are limited to the brown stripes.

There is no remedy. The only comfort is to know that the problem occurred in the mint, so that the piece qualifies as an error strike.

Peter

[FosseWay]

The problem with brown stripes is widespread - the Royal Mint suffered from it as well in the late 1940s, on the first Cu-Ni circulation UK coins, and again in the 1968-71 period when they were rushing to produce a lot of coins ahead of decimalisation.

[MORGENSTERNN]

Thank you Peter
In my opinion an error coin is most valuable than normal issue because scarcer
so that is a great new for me
the mint was in Paris (Poissy)

[Figleaf]

Errors do not necessarily command a higher price. Some people collect them, others don't. Some will pay a premium, others won't. It may be hard to find a buyer for this piece.

Peter

[MORGENSTERNN]

I am an "error coins collector" so I did not plan to send this one 
thank you again Peter