- How to calculate Specific Gravity - Direct Weighing Method
- How to calculate Specific Gravity - Heavy Liquids
When a solid is placed in a liquid of lower density, it sinks. On the other hand, if its density is lower than that of the liquid, it floats, as does wood in water. If a stone has an S.G. equal to the density of the liquid into which it is dropped, it remains suspended in the liquid. This would have no particular significance if it were not for the availability of a number of liquids with densities up to several times that of water. The advantage of a heavy liquid is that it is so simple and free from the potentiality of arithmetical error. In addition, it provides a very fast method.
Although the obvious findings of the heavy liquids are simply whether the stone has a higher or lower S.G. than the liquid, more information may be gained. As a stone sinks in a liquid, and idea of how much more dense it is than the liquid is furnished by observing how rapidly it sinks. This is affected also by the shape of the stone. For example, a large flat stone will sink more slowly in the flat position than in an upended position. When a stone floats, the percentage above and below the liquid represents the relationship between the two S.G's. In other words, if the stone has 9/10 of its volume beneath the surface, it was an S.G. equal to 9/10 that of the liquid. The exact ratio can be estimated only roughly by eye, but it serves as a guide.
Unfortunately, the heavy-liquid method has a major limitation (in addition to the one that any S.G. method suffers: the restriction to loose stones only), and that is the practical upper limit for safe liquids at 3.32. Above that figure, those that are readily available are either opaque suspensions of finely divided solids in a heavy liquid or Clerici's solution. The latter is poisonous and corrosive, so it is dangerous unless used with caution.
| Table of specific gravities of various liquids | |
| Liquid | Specific Gravity |
| Saturated salt solution | 1.15 |
| Carbon tetrachloride | 1.59 |
| Bromoform | 2.89 |
| Methylene iodide | 3.32 |
| Thallium formate and malonate (Clerici's solution) | 4.65 |
| Thallium silver nitrate | 5.00 |
A very useful set of heavy liquids consists of three small bottles or beakers, each containing about 25 cc. of a heavy liquid. The first bottle contains methylene iodide (3.32), the second contains bromoform (2.89), and the third bottle contains bromoform diluted with xylene (2.62). By dropping the unknown stone first into one and then into the other Liquid (being careful to wipe the stone thoroughly to avoid contamination of one liquid by another), it is possible to place it into one of the following four broad groups:
- Floats in 2.62 Liquid.
- Sinks in 2.62 liquid and floats in 2.89 liquid.
- Sinks in 2.89 Liquid and floats in 3.2 liquid.
- Sinks in 3.32 liquid.
If desired, additional liquids having intermediate S.G.'s may be prepared by diluting pure methylene iodide or bromoform with different amounts of xylene and using the following gems as indicators to calibrate the liquids; moonstone (2.56), quartz (2.66), aquamarine (2.72), nephrite (2.95), tourmaline (3.06), kunzite (3.18). The methylene iodide or bromoform is diluted until one of these gems remains suspended in the liquid. However, since xylene evaporate more rapidly than either of the other chemicals, the bottles should be kept tightly closed if the liquids are to remain constant. In addition, the indicators should be left in the bottles at all times so that an immediate visual check of the value of the liquids can be made.
Because of the low cost and the rapidity with which reasonably accurate determinations can be made, a set of heavy liquids should be one of the first acquisitions for any gem-testing laboratory. Of course, for a wider range of S.G.'s, the direct weighing method with the diamond balance is necessary.
