Shrinkage. A reduction in size particularly in cast objects caused by the cooling of the metal from the original size of the mold. Among the properties of each metal is its rate of contraction (molecules are drawn closer together during cooling usually in uniform rate, but uneven contraction voids are known to exist). In numismatics the most popular casting metal – bronze – can shrink form 1 1/2 to 2% in diameter. For this reason a second generation cast, or any succeeding cast (called an after-cast), can be differentiated from an original by its slightly smaller diameter.

It must be remembered in all cases of shrinkage, however, that if a mold is made from the original medal the mold itself shrinks as it cools. A cast made from that mold would exhibit the combined shrinkage of both mold and cast. Thus there is no precise percentage that a second, or subsequent generation cast, can differ from the original item. Along with the shrinkage there is some loss of definition and sharpness of detail – largely due to the experience of the caster – but the most obvious criterion is the smaller diameter. See cast medals.

Measurements should be carefully made. For medallic items it is best to measure from outside rim across the widest part, to that outside rim (not necessarily from edge to edge). A method of eluding shrinkage measurement is to have a wider diameter piece (easy to accomplish in casting) that could measure greater than the original!

Foundry workers use a rule of thumb of 3/16-inch shrinkage per linear foot – or 1.56% – for copper, bronze or thin brass. Thicker castings shrink somewhat less than thinner castings. Since cast medals are among thinner castings their shrinkage tends toward the high degree of shrinkage. See Chart.

Shrinkage

Casting Metal Per foot Percent

Pure tin. . . . . . . . . . . . . 1/32-inch 0.26%

Cast iron, malleable iron . . . 1/8-inch 1.04%

Britannia metal . . . . . . . . . 11/64-inch 1.43%

Copper, bronze, thin brass or

nickel casting alloys . . . . . 3/16-inch 1.56%

Pure aluminum . . . . . . . . . . 13/64-inch 1.69%

Zinc, lead. . . . . . . . . . . . 5/16-inch 2.60%

References:

O46 {1987} Pollard; "Surface Characteristics of Renaissance Medals and Their Interpreation" by Arthur Beale, pp 27-33.