Numbers 1 and 2 are the most convenient to use, but any or all can be made up and kept in glass test tubes ready for use. It will be realized that all varieties of quartz, whose specific gravity is about 2.66, will float on bromoform; many similarly colored stones will sink. Some of the liquids are poisonous to handle, others are unstable, so certain precautions must be taken in their use.
Another method which involves the use of an ordinary jeweler’s balance and very little other apparatus will determine the specific gravity of a stone fairly accurately. This method is based on the well known principle that, when a solid is totally immersed in water, the apparent loss in weight is equal to the weight of water displaced. The stone must first be weighed, and then weighed again while totally submerged in water. The difference between these two weights, the apparent loss, is the weight of water displaced. This figure, divided into the weight in air, gives the specific gravity of the stone.
Any delicate balance can be easily adapted. For weighing in water, a piece of thin wire coiled to act as a holder or cage for the stone may be used. This can be suspended from one arm of the balance so that the cage reaches into a small beaker of water. The beaker must stand free of the balance, and the weight of the wire must also be ascertained.
For instance, the following figures will be given as an example, carat weights being used.
Weight of wire. .80 carats.
Weight of wire and stone. 13.65 carats.
Weight of stone. 13.65 - .80 = = 12.85 carats.
Weight of wire partly immersed. .72 carats.
Weight of wire and stone partly immersed. 10.38 carats.
Weight of stone in water. 10.38 — .72 = = 9.66 carats.
Loss of weight of stone in water. 12.85-9.66 = 3.19 carats.
Specific gravity of stone. 12.85
= 4.02.
3-i9
If desired, a counterpoise of the same weight as the wire may be used, and if another of the same weight as the scale pan on the side from which the cage is suspended is used, this scale pan may be dispensed with altogether. Weighing in water will thus be facilitated.
In order to obtain more accurate results, certain additional measures should be taken. Distilled water at a temperature of 60 ° F should be used, and the weight of the atmosphere might be taken into account. Surface tension also produces small errors; the high surface tension of water, which makes it cling to the wire cage and also causes bubbles, makes results inaccurate. In order to avoid this, other liquids are sometimes used. Alcohol, toluol (sp.g. .87), carbon tetra-chloride (sp.g. 1.59), and ethylene dibro-mide (sp.g. 2.18) are among these, and at the laboratory of the Precious Stone Section of the London Chamber of Commerce, Anderson and Payne have recommended the use of the last. When using any of these liquids, it is necessary to allow for the density of the liquid at the temperature when the experiment is being carried out. Tables showing details of such necessary corrections may be obtained. But for ordinary practical purposes, the method first described should give sufficiently accurate results.
Specific gravity may be ascertained without damaging most stones since very few are affected by water. Turquoise, opal, and porous stones should not be tested in liquids other than water. There are balances which have been specially devised for taking specific gravities, and among these are the aerometer, Walker’s balance, and Jolly’s spring balance. For small fragments or powdered minerals, a specific gravity bottle may be used. Reference to these is made in a later chapter.
Leave a Reply