Is Your $20 – 1870 Double Eagle Real? Authentication Guide for Collectors

The Authentication Crisis in Classic Gold Coins

With counterfeits flooding the market, knowing the specific diagnostic points for this item is critical. The $20 Liberty Head Double Eagle from 1870 represents one of the most targeted coins by counterfeiters due to its historical significance and numismatic value. This authentication guide will walk you through the essential steps to verify your 1870 Double Eagle’s authenticity, focusing on weight, magnetic properties, die markers, common fakes, and testing methods.

Historical Context of the 1870 Double Eagle

The 1870 $20 Liberty Head Double Eagle was produced during a fascinating period in American numismatics. This coin was minted in three locations that year: Philadelphia (no mint mark), San Francisco (S), and Carson City (CC). The Double Eagle series, authorized by the Gold Coinage Act of 1849, represented the highest denomination in U.S. coinage at the time.

The 1870-S is particularly significant as it was produced at the newly established San Francisco Mint, which had begun operations in 1854. Carson City had opened in 1870, making coins from this mint mark especially desirable to collectors. The Philadelphia issue, while lacking a mint mark, was produced in the largest quantities of the three mints.

Weight Verification: The First Line of Defense

Authentic 1870 Double Eagles should weigh precisely 33.436 grams. This weight is non-negotiable and represents one of the most reliable authentication methods. Counterfeiters often get the weight wrong, either making coins too light or too heavy.

To properly verify weight:

• Use a precision digital scale accurate to at least 0.001 grams
• Weigh the coin multiple times to ensure consistency
• Account for any encrustation or debris that might affect the measurement
• Compare against a known genuine specimen if possible

Weight variations beyond ±0.1 grams should raise immediate red flags. Many modern counterfeits are made from tungsten cores with gold plating, which can achieve correct dimensions but often have incorrect weight due to density differences.

Magnetic Properties Testing

Authentic 1870 Double Eagles were struck from 90% gold and 10% copper alloy. This composition should render the coin non-magnetic. A simple magnet test can quickly eliminate many counterfeits.

Testing procedure:

• Use a strong neodymium magnet
• Hold the magnet near the coin’s surface (don’t touch)
• Observe any magnetic attraction
• Rotate the coin to test different areas

Any magnetic attraction indicates a counterfeit, as gold and copper are non-ferrous metals. Some sophisticated fakes use magnetic steel cores, making this test essential even for seemingly genuine coins.

Die Markers and Design Specifics

The 1870 Double Eagle has several distinctive die characteristics that can help authenticate it:

Mint Mark Identification

As discussed in the forum, the 1870 was minted in Philadelphia, San Francisco, and Carson City. The mint mark appears on the reverse, below the eagle. For authentication purposes:

• Philadelphia issues have no mint mark
• San Francisco issues display a clear “S”
• Carson City issues show a “CC”

The forum discussion correctly identified that process of elimination helps determine mint marks when they’re unclear. A single letter indicates San Francisco, while the double “CC” is distinctive for Carson City.

Key Design Elements

Examine these specific features under magnification:

• The date should show clear, sharp numerals with proper spacing
• Liberty’s coronet should have clear lettering reading “LIBERTY”
• The stars around Liberty should be evenly spaced and well-defined
• The reverse eagle should show detailed feathering
• The motto “IN GOD WE TRUST” (added in 1866) should be crisp and complete

Common Counterfeits and How to Spot Them

Understanding the most prevalent counterfeit types helps you avoid purchasing fakes:

Cast Counterfeits

These are made by creating molds from genuine coins. They often show:

• Lack of sharp detail, especially in recessed areas
• Pitting or porosity on the surface
• Incorrect weight (usually too light)
• Soft, indistinct lettering

Struck Counterfeits

More sophisticated fakes struck from transfer dies can be harder to detect:

• May have correct weight but wrong composition
• Often show slight design distortions
• Can have incorrect die markers or anomalies
• May lack the proper mint luster

Altered Date/Rim Repairs

Some counterfeits involve altering dates on common-date coins or repairing damaged rims:

• Examine date numerals under magnification for tool marks
• Check for unnatural metal flow around the date
• Look for inconsistencies in patina or wear patterns

Professional Testing Methods

For high-value specimens, consider these additional authentication methods:

Specific Gravity Test

This test verifies the coin’s density matches that of 90% gold alloy:

1. Weigh the coin in air
2. Weigh it suspended in water
3. Calculate specific gravity using the formula
4. Compare to the known specific gravity of 17.18 for this alloy

Sigma Metalytics Testing

Professional bullion testing devices can verify the coin’s metal content without damage:

• Penetration testing shows metal consistency through the coin
• Can detect tungsten cores or other non-gold interiors
• Provides digital readout of metal composition

XRF Analysis

X-ray fluorescence provides detailed elemental analysis:

• Confirms 90% gold, 10% copper composition
• Detects surface plating on non-gold cores
• Provides percentage breakdown of all elements present

Visual Authentication Checklist

When examining your 1870 Double Eagle, systematically check these elements:

Authentication Flow: Start with weight verification → magnetic test → visual examination → magnification inspection → professional testing if needed.

Surface Examination

Under proper lighting:

• Check for natural mint luster
• Examine edge reeding for consistency
• Look for signs of casting (bubbles, irregularities)
• Verify the coin’s diameter (34mm for Double Eagles)

Magnification Inspection

Using 10x loupe or microscope:

• Examine fine details in Liberty’s hair and coronet
• Check the eagle’s feather detail on reverse
• Look for die cracks or polishing marks (can indicate authenticity)
• Verify mint mark style matches known genuine examples

Conclusion: The Value of Proper Authentication

The 1870 $20 Liberty Head Double Eagle represents a significant piece of American numismatic history. Whether you’re a serious collector or investor, proper authentication is essential before any purchase or sale. The combination of weight verification, magnetic testing, die marker examination, and awareness of common counterfeits provides a solid foundation for authentication.

Remember that even experienced collectors can be fooled by sophisticated counterfeits. When in doubt, seek professional authentication from reputable third-party grading services. The peace of mind and protection of your investment is well worth the cost of expert verification.

For the 1870 Double Eagle specifically, the forum discussion demonstrates how community knowledge and process of elimination can help identify mint marks and potential issues. However, physical authentication should always accompany visual identification, especially for coins of this value and historical importance.

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