How do we define whether it is pure gold? There are a lot of suggestions in the Inter-net.
Let's name some of them:
1. Try to bite a gold. If it bends, then it’s pure gold.
2. We can possibly identify pure gold with density test.
3. Also it is possible to make a nitric acid test or define if it’s pure gold by its color.
The most correct way to identify pure gold of all given above is to use a density test, but of course not at home. If we decide to compare strength of gold with our tooth, then after that we will probably have to implant an artificial tooth, perhaps made of gold.
Nevertheless we must point out that it is possible to identify the real unmixed gold using a bite test.
When metal is under deformation this can lead only for two variants: destruction or adaptation. In certain range of treatment metal is able to adapt, but the only possibility for that is to transform its structure.
All statements mentioned above are right in presuming that gold is a soft metal. Therefore, it is enough to present a low pressure to deform it, including the pressure made by men's jaw. A bite test is based on twins formation under a pressure of men's jaw. A twin is a defect of crystalline structure that forms as a result of heating or de-formation. During a twinning process a grain (crystallite) is divided into two reflect-ing symmetric parts. The scheme is shown on the fig. 1. Such division occurs in spurts and is accompanied by a typical crackle. And this is it.
It should be noticed that cadmium, bismuth, and tin have the same effect. Tin is easy to access and all of us are able to test it. By the way, silver has no effect of crackle under pressure of tooth. Occasionally we are demonstrated the bite test of a silver coin in movies, anyway the reason for this is pure ignorance.
The usage of bite test was justified at times when coins were made of pure gold. A gold alloy in its turn loses an effect of crackle. The reason for this is that an alloy is stronger comparing with pure gold. Therefore the gold alloy needs much more shear-ing force to change the structure and create twins. Essentially the crackle of some al-loys (not only made of gold) might be heard in the process of tension test. The force of testing machine is up to several ton. It is impossible to perform such pressure by human tooth.
In the past the bite test was the only way for non-specialist to identify the real gold. The hardness of lead is 25-30 MPa and of gold – 22,5-25 MPa. This would make no difference for the bite test. But lead has no effect of crackle. The point is that the re-crystallization temperature of lead is below room temperature. It means that lead with room temperature is in the same condition as aluminium with the temperature above 100оС or alloy above 400оС. With such temperature there is no twin process. With high temperature plastic deformation is going on to mechanism of dislocation glide and dislocation climb.
Figure 1. Shift in crystal lattice in condition of twins deformation: a-a - twin plane, d - lattice parameter; pointer shows a force direction
What the twins look like in metals?
Normally twins look like as strips of various wide. Twins of deformation and heating are narrow and wide respectively. Deformation twins are shown on the fige 2. Due to the different orientations relative to the twinning plane, their etching is going on with a different velocity, so the twins have various etching shades.
If we use a Differential Interference Contrast, twins will be colored differently. The complicated case is shown on the fige 3.
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Figure 2. Twins in brass; light-field image
Figure 3.Twins in brass; Differential Interference Contrast
In the conclusion we have to give a professional advice. At present there are many jeweler’s golden alloys and also many jeweler’s alloys which contain no gold. These both groups have different colors and tints of yellow. Therefore, it is impossible to use the color to identify what you deal with. The same concerns other questionable tests. The answer is testing equipment for chemical composition identification, which we strongly recommend to use.
And the best way is to buy golden jewels in legal stores who take responsibility for what they sell. And wear them with pleasure.