Improper Alloy Mix (IAM)
An Improper Alloy Mix error occurs when the metal of a solid-alloy coin is wrong. Error-Ref describes two related forms: the metals are well mixed but in the wrong proportions, which on a copper-alloy cent with too much tin or zinc gives a brassy color, or they are in the right proportions but poorly mixed, which leaves light and dark bands. The two can occur together. Poorly mixed coins with alternating light and dark streaks are known as "woodies" for their wood-grain look. Because the problem is in the metal itself, the color goes deeper than a surface toning.
How Does It Happen?
U.S. coin production begins long before the striking process. The Mint purchases metal strip from commercial suppliers (and historically produced its own strip in-house). Creating coin strip involves melting the component metals together, casting the molten alloy into ingots, and rolling those ingots into strip of the correct thickness.
Each denomination has a precise alloy specification. For example, modern Lincoln cents use copper-plated zinc (97.5% zinc, 2.5% copper). Until 1982, when Philadelphia switched to copper-plated zinc in January and Denver in October, cents were 95% copper: bronze, with the rest tin and zinc (1864–1942 and 1947–1962), or brass with zinc alone (1944–1946 and 1962–1982). Jefferson nickels are 75% copper and 25% nickel.
Improper alloy mix errors arise at several points in this metallurgical chain:
- Incorrect proportions: The component metals are weighed and added to the melt in the wrong ratio. Too much of one element changes the alloy's color, hardness, and striking characteristics.
- Incomplete mixing: Even with the correct proportions, the molten metal must be thoroughly stirred and homogenized. If the melt is poured before complete mixing, the resulting ingot contains pockets of concentrated metal -- copper-rich zones and copper-poor zones distributed unevenly through the material.
- Unmixed metal: In the most extreme cases, Error-Ref notes, the minor metal is not mixed in at all and stays as a separate lump, which it classes as an intrinsic metallic inclusion.
When the improperly mixed ingot is rolled into strip and punched into blanks, the resulting planchets carry the alloy inconsistency through every step of the process. The error is baked into the metal itself.
How to Identify an Improper Alloy Mix
Improper alloy mix coins have characteristics that distinguish them from other color anomalies:
- "Woody" or "woodgrain" streaking: The classic presentation is alternating light and dark streaks across the coin, drawn out in the direction the metal was rolled. On copper-alloy cents Error-Ref notes the lighter bands are enriched in zinc, tin, or both.
- Unusual base color: The overall color differs from normal examples. A copper-alloy cent with too much tin or zinc looks brassy. Error-Ref's example is a 1941 cent of 86% copper, 11% zinc and 3% tin.
- In the metal, not on it: The color difference belongs to the metal, so wear or a rub does not remove it, unlike toning or residue on the surface.
- Both sides: Bands from a poorly mixed strip normally show on both faces; Error-Ref's 1938 cent has mottled bands on obverse and reverse.
- Weight is not a test: An alloy error can come with other planchet problems. Error-Ref's brassy 1941 cent is also overweight at 3.40 grams, because it is also rolled thick.
Improper Alloy Mix vs. Environmental Damage
A common misattribution is confusing environmental toning or chemical exposure with a genuine alloy error. Here is how to distinguish them:
| Feature | Improper Alloy Mix | Environmental Damage |
|---|---|---|
| Depth of color | In the metal itself | On the surface |
| Pattern | Bands or streaks drawn out in the rolling direction | Random splotches, fingerprints, halos |
| Consistency | Usually on both sides | Often one side or a localized area |
Improper Alloy Mix and Laminations
A lamination is metal flaking or peeling away from the surface in thin layers. It's a separate error from an improper alloy mix, but the two often turn up on the same coin: Mike Diamond notes in Coin World that laminations are "both common and expected" on ordinary improper alloy mix coins. The difference is what you see: an alloy mix shows streaks in the color of the metal, while a lamination shows layers lifting, cracking or missing.
Not Every Stripe Is an Alloy Mix
Very narrow, evenly spaced pinstripes, common on 1978-1982 cents, may have a different cause. Diamond points out that these narrowly striped cents never show the laminations usually seen with alloy-mix errors, and suggests the stripes may come from the rolling process instead: grime transferred to the strip by dirty rollers, or lines left by imperfectly smoothed rollers (roller lines). He describes the causes of these stripes as "not fully understood." The classic improper alloy mix, the "woodie," is broad, irregular banding, typical of early Lincoln and Indian Head cents.
Notable Examples
Copper-Zinc Streaked Lincoln Cents
Pre-1982 Lincoln cents (95% copper, alloyed with tin and zinc in the bronze years and zinc alone in the brass years) are where most U.S. alloy mix errors are seen. Error-Ref's examples include a 1938 cent with mottled bands on both faces, a banded 1941 cent, and a 1948 cent crossed by a gray band from an unmixed globule of tin or zinc, which it classes as an intrinsic metallic inclusion.
"Woodgrain" Toned Cents
Poorly mixed alloy can leave alternating light and dark streaks, the "wood grain" pattern that gives "woodies" their name (Error-Ref). Diamond describes the classic woodie as broad, irregular banding, typical of early Lincoln and Indian Head cents.
War Nickel Alloy Variations (1942-1945)
During World War II, the Mint struck Jefferson nickels in a 56% copper, 35% silver, 9% manganese alloy to conserve nickel for the war effort. As with any coin, uneven color on a war nickel counts as an alloy-mix error only if it runs through the metal in bands on both sides; surface toning does not.
Collecting Tips
- Certification helps: Because alloy mix errors can resemble environmental damage, authentication by PCGS or NGC settles what a coin is.
- Start with copper cents: Pre-1982 Lincoln cents and Indian Head cents are where alloy banding is most often found.
- Check for laminations: Peeling or flaking on a streaked coin is consistent with an alloy problem (see above); narrow, even pinstripes with no laminations may be a rolling-mill effect instead.
- Preserve the surfaces: Do not clean improper alloy mix coins. Cleaning removes the evidence that tells metal color from surface toning.