Specific Gravity

Every metal known has a unique weight to mass ratio. The measurement for this weight to mass ratio is called specific gravity. This is one of the oldest and most accurate methods of testing molecular content of a metal.

Many people believe that this test is too complicated and/or requires costly equipment to be properly performed. This is not the case. All that is needed are a few relatively inexpensive pieces of equipment, a calculator, and a little patience. However, the more sensitive and accurate the equipment, the more accurate the result will be.

The following procedures were printed in a past issue of the Numismatist, and also in a later publication of the ANA.

PREPARATION

  • Observe good laboratory techniques as much as possible under existing conditions. Work slowly, carefully and accurately at all times.
  • Place a balance beam scale on the most stable work surface available. A solid desk is less susceptible to vibrations than a folding table.
  • Eliminate stray air currents as much as possible. The scale can be housed in a cardboard box or other suitable container. A piece of plastic
    draped over the opening of the box can very effectively cut air currents that could affect the accuracy of the scale.
  • A container of water with a mouth large enough for one end of the balance beam to enter is necessary. Use pure water whenever possible.
    For best results use steam distilled deionized water. Add a drop or two of a wetting agent such as PhotoFlo 200 or a liquid household detergent.
    This helps prevent the formation of air bubbles, which normally form on the coin or on the suspension device.
  • Always be certain that the scale has been zero balanced before and after any weighing operation or at any time that the scale has been moved
    from one location to another.
  • Always depress the right end of the balance beam after making adjustments for zero balance or after moving poises (weights).
  • Do not allow yourself to jump to any conclusions that might cloud your judgment about the coin being tested.
  • Above all, remember that specific gravity testing is neither an exact science, nor the final word in determining the authenticity of a coin. It is simply
    one tool to be used along with visual examination under a good stereo microscope and measurements of diameter and thickness made with vernier calipers.

TEST PROCEDURES

  • Zero balance the scale exactly and carefully.
  • Place the coin to be checked on the lower pan. Try to stop any swaying of the pan, though it is not essential that the pan be absolutely still.
  • Move the balance poises on the beam to the right until the beam remains at exact zero balance. Start with the heaviest poise and work forward
    until you reach absolute zero using the smallest poise.
  • Read the combines total weight of the poises. This is the weight of the coin in the air, or the dry weight. Mark that weight on a slip of paper.
  • Without moving the poises, remove the coin from the lower pan and place it on the suspension devise. Slowly immerse the coin in the water, being careful that there are no air bubbles trapped on the coin or on the suspension device itself.
  • Move the poises back and forth only far enough to return the beam to zero.
  • Read the new total weight of the poises. This is the weight of the coin in water, or the wet weight. Mark this new weight immediately below the dry
    weight on your slip of paper.
  • Subtract the wet weight from the dry weight and divide the difference into the dry weight. This is the specific gravity of the coin.

 

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