Hydrostatic Balance

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The hydrostatic balance is used to determine the specific gravity of a gemstone. Although it is a fairly simple apparatus to operate it can be time consuming and one would need to put the results in a calculation, something not all gemmologists enjoy doing.


Specific gravity (SG) is a constant ratio of an object's weight compared to an equal volume of water. When an object in totally immersed in water it will experience an upward trust that is equal to the volume of water which is displaced by the object. This will make the object appear lighter in water.
The first to observe this was Archimedes while investigating a possible fraud with a crown for King Hiero of Syracuse (present day Sicili).

Archimedes and Archimedes' Law

When King Hiero commisioned a new gold crown a certain volume of gold was provided to a goldsmith, who manufactured the crown to the liking of the king. Soon after rumours started that the goldsmith may have been frauduleus and added silver to the gold, making a nice profit for himself on the side.
King Hiero asked Archimedes to investigate and one day while taking a bath, Archimedes noticed that his body mass caused the water in the bath to overflow. In his excitement over finding a possible answer to the problem he ran home naked, shouting "eureka" (I've found it out).

He immersed equal volumes of gold and silver in water and by observing the amount of water that was displaced, he noticed that the mass of gold displaced less water than the silver. He then immersed the crown and through a series of calculations he was able to determine how much gold and silver was used to create the crown. The fraud was detected and this was the first time in history SG was used.

Archimedes' Law (or the Law of Buoyancy) states that: the upward force on an immersed object is equal to the weight of the displaced fluid.

Types of hydrostatic balances

In gemmology 4 different types of hydrostatic balances are used.

  • The two-pan scales
  • The one-pan scales
  • Spring balances
  • Direct reading scales

Two-pan scales

The two-pan scales are the most time consuming scales to use. Some jewelry stores still use them but little of them are used for determining SG of stones. Mostly because the counterweights in general are not small enough to be of any help and they really need to be calibrated every few years.

The principle of the two-pan scale is very useful for explaining the hydrostatic balance though.


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