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Clad, Cladding

Clad, Cladding.  Bonding together thin layers of metal of differing alloys to form a sandwich construction. Technically the outer or exposed layers are termed the clad layer, or lamina, the internal layer is the core. This is done in every instance for the physical characteristics of the outer or clad composition, as for the color, hardness, appearance, and electric sensitivity of this surface. The use of the core is generally one of lower cost than the clad metal. The line separating the core from the clad metal, called the boundry, it can be observed on the edge of U.S. coins minted after 1965 (but not those of some European coins, as Sweden, where upsetting is designed to cover the core on the edge).

Medals were first clad as early as 1789. In that year Barton's metal was used for the George III Recovery Medal by J.P. Droz (Brown 311) struck at Matthew Boulton's factory. Barton's metal was formed by rolling strips of silver (or gold) on a copper core with adhesion by fusion.

In 1964 the United States enacted a coinage law that required U.S. coins to be made of clad metal. Faced with a drastic coin shortage, a world-wide silver shortage and rising metal prices, the Treasury Department chose to turn to a copper nickel clad metal over a copper core for all former silver coins, dimes, quarters and half dollars.

This was a brilliant solution. Not only did it meet all the criteria of a circulating coin composition (including the demanding needs of the vending machine industry) but also considered the metal salvage of this composition. Skeleton scrap of this clad composition could be melted and easily reformulated back into a copper nickel formulation.

How clad strip is made. Ingots of the correct composition are rolled repeatedly to a thickness several times the thickness of the intended coin. They are metal cleaned by pickling. The layers are then placed in a rolling mill in proper sequence, the core between the two outer layers. They can be roll bonded if the metal strips are heated, then rolling is done under heat and pressure. If they are cold rolled they can be bonded by explosion bonding. The strips are then successively rolled until the required thickness is obtained.

Since January 1994 clad strips of copper nickel lamina on copper core are supplied to the United States Mints (Philadelphia and Denver) by PMX Industries, Cedar Rapids, Iowa, which uses the roll bonded method. Each strip weights between 3,000 and 7,000 pounds and are approximately 1,000 feet long. The strips have the following dimensions:

Strip             Width                    Thickness

50 Cent . . 12  9/16-inch (319.08mm)  .0685-inch (1.740mm)

25 Cent . . 12 13/16-inch (325.43mm)  .0545-inch (1.384mm)

10 Cent . . 12 11/16-inch (322.26mm)  .041 -inch (1.041mm)

The strip must be manufactured within a permitted tolerance of .0015-inch (.038mm) thickness for clad strip of any denomination.

The mints blank the strips and return the skeleton scrap strips to the supplier. The supplier repossesses the scrap by adding fresh nickel to the formulation to the correct formula (75% copper 25% nickel) for new copper nickel clad layers.

The mints blank about 75% of the strip (73% at Philadelphia Mint) and return the balance. The blanks are upset by upsetting machines then struck in coining presses within the mint.

More recently, the U.S. Mint has been contracting the blanking to private industry as well as the formulation and rolling. Thus economies are achieved by not having to ship the skeleton scrap for reprocessing. The mint receives blanks for upsetting and striking within the mint.

Clad coinage characteristics.  When clad blanks are struck, particularly during coining, the surface displacement comes only from the outer lamina on each side. The inner core is unaffected. On solid blanks metal is drawn from deep within the blank in addition to surface movement into the die cavities. The boundary on clad blanks literally acts as an edge beyond which no metal flow occurs in the core. This is quite evident when the lamina becomes separated from the core. To a degree, clad compositions effectively eliminate suction or ghosting in struck pieces.

 

Clad anomalies.  Potential for a new group of errors occurred once a clad composition was chosen for national coinage. The debonding of the clad lamina layer, created a new class of lamination errors. One anomaly can occur before the coin is struck – causing a lightweight blank and very poor detail – or another after the coin is struck, with clad metal sometimes still in place, more often separated. The separating of the clad layer is called peeling.

Should the separated lamina flake off and fall onto another blank while in the coining press it could be impressed into another coin; often this is a letter (from

the high stress area of the legend) and is called a dropped letter. If the flake is still attached to the coin it is called a clamshell by collectors, the point where it is still attached is called the hinge.

 

Reference:                                                                                                                          

B12  {1969} Young.

CH82 {1998} Doty, p 36 (Barton's metal).

excerpted with permission from

An Encyclopedia of Coin and Medal Technology

For Artists, Makers, Collectors and Curators

COMPILED AND WRITTEN BY D. WAYNE JOHNSON

Roger W. Burdette, Editor


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