Heavy Liquids

From The Gemology Project
Revision as of 05:09, 9 October 2006 by Doos (talk | contribs) (Calibration of heavy liquids)
Jump to: navigation, search

Heavy liquids are substances in liquid form that have a known specific gravity (SG) and are used as a means of quick seperation of stones with similar appearance. In general these liquids are highly toxic and precautions should be taken when used.

Basic

Heavy liquids are stored in the dark, to prevent light from influencing their physical properties. They should be stored closed with a stopper to prevent evaporation.
Heavy liquids.jpg

Safety warning

Before using heavy liquids it must be stressed that one should avoid all contact with the skin and that they are non-potable. Always use tweezers to insert the gemstones and when in contact with the skin, you should thouroughly wash. When in contact with eyes, you should rinse your eye with streaming water and contact your medical Dr. immediatly.

Always use heavy liquids in a well ventilated environment and they should be kept away from children and pets.

Use of heavy liquids

The use of heavy liquids is fairly straightforward. When a stone is immersed into the liquid it will either float, suspent or sink.

When the SG of the liquid is known, take the gemstone in your tweezers and gently lower the stone in the liquid. Observe the stone while looking through the side of the bottle and take the stone out again with your tweezers.
Carefully rinse the stone and tweezers under streaming water (you might want to take precautions to not let the stone go down the sewers). Or you could rinse it in a large beaker filled with clean water.

After your observations you should be able to tell if the stone is of higher, lower or equal SG than the liquid used.

  • When the stone floats in the liquid, it will have a lesser SG than the liquid.
  • When the stone is suspending in the liquid, it will be of equal SG as the liquid.
  • When the stone sinks, it will have a higher SG than the liquid.

One can also observe to which degree a stone will sink or float after suspending it in the liquid.
For instance when the stone sinks rapidly, it will have a far greater SG than the liquid.

Types of heavy liquids

In gemology generaly 4 heavy liquids are used.

  • Methylene Iodide - SG = 3.33
  • Methylene Iodide diluted with Toluol - SG = 3.05
  • Bromoform - SG = 2.85
  • Bromoform diluted with Toluol - SG = 2.65

In addidion Clerici Solution with a SG of 4.2 is used in laboratories which can be diluted with water to bring it down to SG = 4 or SG = 3.52 for instance. This solution is extremely toxic and should only be used in a well equipped laboratory with additional safety precautions as an acid chamber.
Dr. W. Wm. Hanneman suggested the use of Sodium Polytungstate as a non-toxic alternative for most heavy liquids.

Aside from these there is also a water/salt solution for Amber.

Methylene Iodide

Methylene Iodide (CH2I2) is also named di-idiomethane and has a SG of about 3.33.

Yellow Topaz (3.5) will sink in this liquid whilst Citrine (2.65) will float.

Methylene Iodide diluted with Toluol

Methylene Iodide diluted with Toluol has an SG of 3.05.
This liquid can easily seperate Tourmaline (3.05) from Topaz (3.5) as the first will suspent while Topaz will sink in this liquid.

Bromoform

Undiluted Bromoform has a SG of 2.85 and can be calibrated to lower SG with the addition of Toluol.

Bromoform diluted with Toluol

Diluted Bromoform with an SG of 2.65 and is the indicator for Quartz.

Clerici Solution

This highly toxic liquid has a SG of 4.2 and can be diltued with water to calibrate it for Corundum (4) or Diamond (3.52).

Sodium Polytungstate

Sodium Polytungstate is a commercial, non-toxic and odor free alternative for most heavy liquids.
It is sold as a white powder to which water is added to form a solution. The upper value of the saturation is 3.10 and with added water it can go down to whatever prefered SG.

Salty water

When 50 grams of table salt is saturated in 0.28 liters of water a solution with density of 1.13 is created. This lets Amber (SG = 1.08) float in it whilst some simulants (mostly - but not all - plastics) sink.

Calibration of heavy liquids

As evaporation, temperatures and light conditions may effect the SG of the liquids, it is recommended to have calibration stones at hand so one can periodically adjust the solutions.
When the calibration stone suspents in the liquid it will be up-to-date, otherwise one should adjust the solution. Differences in temperatures can highly influence the calibration.

In general one calibrates a heavy liquid by dropping a calibration stone slowly in to a liquid and diluting it drop by drop till the stone of known SG suspents in it.

Sources

  • Gemmology 3rd edition (2006) - Peter Read
  • Guide to Affordable Gemology (2001) - Dr. W. Wm. Hanneman Ph.D.
  • Journal of Gemmology volume 22 no. 6 (april 1991) - Gem-A
  • Gems, Their Sources, Descriptions and Identification 4th edition (1990) - R. Webster, B.W. Anderson