Alloy errors

It is common for coins to have imperfections in the composition of the metal alloy, although extreme cases are less frequent. These variations in the alloy’s composition can arise during the casting process in the foundry.
To obtain planchets that comply with the issue decree, it is essential to dose the different metals that make up the alloy in the correct proportions. If the metals are amalgamated in incorrect doses during the melting phase, several variations may occur. These variations include changes in the color, specific weight, magnetic attraction, and porosity of the alloy.
When coins are minted on planchets with imperfections in the alloy, several variations can occur that result in noticeable differences. These differences include:
  1. Differences in the reflection of light on the coin’s field. The field of the coin may be opaque in some places, including the outline, leading to uneven light reflection.
  2. Differences in the color of the coin. The field of the coin may have different colors, sometimes appearing homogeneous and other times streaked or spotted. These colors may vary according to the inclination of the coin with respect to the position of light.
  3. Differences in the specific weight of the coin. When the heavier metal used for the preparation of a specific alloy is present in greater or lesser quantities, it can lead to a difference in the specific weight of the coin.
  4. Differences in the magnetic attraction of the coin. Ferromagnetic coins have very intense elementary magnetic moments and are strongly attracted to a magnet. Paramagnetic coins have rather weak elementary magnetic moments and are weakly attracted to a magnet. Diamagnetic coins have magnetic moments equal to zero, and are weakly repelled by a magnet.
  5. Differences in the porosity of the alloy. The surface of the coin can be porous, or the constituent metals can be dosed in the right proportions but poorly mixed. These imperfections are manifested by clearly visible dark and light streaks on both sides of the coin, including the border. In some cases, these imperfections can occur together.
When it comes to alloy mixing imperfections in coins, the most severe type occurs when the minor metal element is completely un-mixed. This particular flaw is known as intrinsic metal inclusion. It should be noted that this type of defect is relatively common in coins produced in the United States. Some examples of alloy-related coin errors include planchet errors, which occur when the metal disc used to make the coin is improperly prepared or composed of the wrong materials
On rare occasions, a gas pocket may form within the coin during the striking process, which expands as a result of the heat generated during the beating. This expansion of the gas causes the overlying metal to bulge outwards, resulting in a planchet swelling with soft edges. If the roof of the gas pocket remains intact, the flaw is referred to as an occluded gas bubble. In cases where the internal pressure causes the roof to burst, the imperfection is known as a broken gas bubble. When the roof is thin, it will bend or leave a visible dent if pressed with the tip of a toothpick. Conversely, if the roof is thick, it will not give in to pressure.