One of the first things many authenticators look for on U.S. coins they examine is the presence of metal flow. This characteristic provides a good indication that the coin was made using dies.

Forty or so years ago, this was virtually enough proof that a coin was genuine because at that time, most counterfeits were made by casting.

Even by the early 1970s, years after die-struck fakes had appeared, the majority of struck counterfeit coins showed little evidence of metal flow lines. In fact, most struck counterfeits of the day were similar to castings, having weak, mushy details.

It's not that simple anymore. Authenticating a coin by metal flow alone is down-right ignorant. That's because, over time, struck counterfeits began to show traces of flow lines, and even duplicated much of the die polishing lines found on genuine specimens. Today, there are many die-struck counterfeits that almost defy detection.

Metal flow lines occur as the dies become worn. When a planchet is struck between two dies to make a coin, its metal is squeezed, causing it to flow into the recesses of the die's design. As more coins are produced, tiny grooves are carved out of the dies from the flow of planchet metal over their surface. These grooves gradually get deeper and produce tiny raised lines on the coin's surface, mostly radiating outward from a central point of the coin. Flow lines are just that, so sometimes we can even see subtle changes in their direction around parts of a coin's design. Coins made with fresh, new dies or polished dies such as those used to make our modern proof coins do not commonly show flow lines unless examined microscopically or if kept in service too long.

The best place to look for metal flow lines on a coin is near its rim. Usually, on larger coins, this characteristic becomes more obvious due to the increased pressure needed to strike them. On the 1926-S nickel illustrated, you can see the light reflecting from the flow lines at the bottom edge of the micrograph. Trace the lines upward toward the "C" and "E" of CENT until they disappear. We can say with certainty that this is a die-struck coin. Next, we can say that this coin is most certainly genuine due to the quality of its surface, the sharpness of its relief and the microscopic flow lines that disappear into the field.

If a modern counterfeiter were to use this coin as a model to make fake dies, the larger flow lines near the rim could be picked up and transferred, but the less obvious flow lines as we move away from the rim are too tiny and are too low in relief to be transferred. That is one reason there is often a difference in the surface characteristics between a genuine coin and a fake specimen even though both were struck using dies. Because a genuine coin has more microscopic detail, it will appear to have more "life" to its surface.

In spite of the quality of die-struck fakes, metal flow lines are still of some use to authenticators and graders. They provide us with a radial grid on the surface of a coin. Any disturbance to this "grid" may indicate that tooling, repairs or even rub from circulation have marred the coin's surface.

Let's go back to the 1926-S nickel photo. There should be metal flow lines going up the base of the mintmark on a genuine coin; yet, none are visible on this specimen. It's as if the "S" was sitting on top of the field rather than being a part of it. This is the case here. This photo is of a 1926 Philadelphia nickel with an added "S" mintmark.

If you encounter a coin such as this with a disturbance to its metal flow on or around its mintmark, you should have it checked by one of the major grading services.