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Excerpts from the book:
Theory and Practice of Goldsmithing
White Metals and Alloys
First the sample in question is treated like the colored alloys in the qualitative test, rubbed on the touchstone and tested with the Au 585 acid. The following possibilities arise from this.
Silver and silver alloys
In practice an experienced eye will usually be able to determine by color and luster whether a piece is made of silver, a base white metal such as nickel silver, or a precious white alloy such as white gold or platinum. If this can be determined by eye, it's usual to proceed directly to the qualitative test described below.
streak test with red silver testing acid
As before, the work being tested is scratched in a location where the minor scar will be least noticeable. A drop of silver testing acid is placed on the scratch and the reaction is carefully observed. If the piece being tested is a silver-copper alloy, both metals are dissolved by the sulfuric acid and converted to sulfates. With exposure to potassium dichromate, the silver sulfate is converted to silver dichromate, which has a characteristic blood red color. The test for silver, then, is that an alloy containing silver will turn the brownish red testing acid a strong blood red.
This test is not effective for alloys containing less than 25% silver, though it's worth noting that such alloys have such a yellow color and are so easily tarnished that they are very unlikely to be encountered as jewelry or flatware. Nevertheless, such alloys are sometimes used for technical products, so it is useful to understand the test tube reaction test described below, which can be used to assess low-silver alloys.
Streak test with nitric acid & sodium chloride
This is an alternate test to the one just described and has the advantage of using nitric acid, the same acid used in gold testing. The sample is rubbed on a touchstone to create a streak, which is dabbed with concentrated nitric acid. Both the silver and the non-precious metals present are converted to nitrates. When sodium chloride (table salt) is added, the silver nitrate is converted to silver chloride, which has a characteristic milky color.
The proof for silver is the appearance of a milky or cloudy liquid. If the solution remains clear the alloy contains either a small quantity of silver or no silver.
Reaction in a test tube
This test is used to confirm or deny the presence of silver when its content is below 25%. The procedure is based on the same chemical reaction that occurs in the streak test just described, but because of its larger scale, the reaction is more clearly visible in the following method.
While observing all necessary safety procedures, dissolve about a quarter gram of the questioned alloy in a clear reaction vessel with nitric acid. This is then diluted with distilled water, as usual adding the acid to the water rather than the reverse. A small amount of table salt is stirred into the solution; if silver is present it will form silver chloride, which as noted has a milky color. If the solution remains clear, it contains no silver. To confirm a positive test, add a small amount of ammonia, which should cause the cloudiness to disappear.
It is also possible to test for silver content of an alloy by purely visual analysis. Though this is not strictly scientific, it is possible for an experienced observer with good color sense to determine content to within 30 parts per thousand, or 3%.
The touchstone is used as a neutral palette on which to gauge a variety of streaks. Scrape the questioned object along the stone to create a silver-colored line, then use bits of metal or objects of known composition to create streaks of a similar size running parallel to the sample. Examine the stripes in good light, seeking to find a color match between the unknown piece and a known sample.
This method fails if the unknown sample contains zinc or cadmium, which will brighten the streak. In such cases more sophisticated testing methods must be used to determine silver content.
Fine silver Testing
This test is similar to the reaction in a test tube. A small amount of silver (1 g) is dissolved in 5 ml of nitric acid. The formation of a dark sediment indicates the presence of gold in the alloy. To this add 20 to 30 ml of ammonia until a piece of litmus paper turns blue.
If a dark sediment forms, it contains gold. If neutralization has been reached, a temporary precipitation of silver oxide forms; it is dissolved by the addition of further ammonia. The formation of further precipitate indicates the presence of lead, bismuth, iron or aluminum. If the solution turns blue, it contains copper. Only cadmium and zinc remain undetected with this method.
Platinum, platinum alloys and white gold
The first step is to determine whether the metal in question has platinum content, is a variety of white gold, or an acid-resistant non-precious alloy. Any of these would be indicated by resistance to the Au 585 testing acid (concentrated nitric acid).
The metal streak on the touchstone is first dabbed with Au 750 acid. If it is attacked, the alloy is almost certainly a white gold, probably with a content of less than 660 parts per thousand. If the streak is unaffected, it is then tested with platinum testing acid. Platinum can be dissolved by hot Aqua Regia but is resistant to the same acid at room temperature. Gold, however, will be attacked by Aqua Regia even at room temperature, so action by unheated acid will reveal an alloy as containing gold. A few non-precious alloys (such as certain steels) also remain undissolved in room temperature Aqua Regia, but these can usually be separated out by their appearance.
Table: Streak test using platinum differentiation acid with white alloys:
Extreme safety precautions are required for conducting this test. About half a gram of the alloy in question is boiled in diluted nitric acid (1:1). Silver, nickel and other non-precious metals are dissolved; palladium, gold and platinum remain behind. The metal being tested is then placed in room temperature Aqua Regia. White gold will be cloaked in a dark surface Film. Palladium will be indicated by the acid solution itself becoming darkly colored. Platinum will remain unaffected by the strong acid.
To complete the test, the Aqua Regia is heated. Platinum will dissolve, turning the solution a brownish red. The volume of the solution is reduced by further heating; if the solution contains platinum, a brownish red precipitate of (NH4)2[PtCl6] is formed upon the addition of sal ammoniac.
Quantitative Platinum Test
Truly precise analysis of platinum alloys can only be accomplished in a precious metals testing laboratory; the similarity between metals in the platinum group can easily lead to errors. The variable solubility and dissolution rates of the platinum metals in heated Aqua Regia can however provide a general sense of the composition of platinum-bearing alloys.
As described in gold testing, comparison streaks of an unknown sample and known alloys are rubbed on a small piece of unglazed porcelain plate. Working under a fume hood, a small quantity of Aqua Regia is heated to about 70°C. The streaked ceramic plate is dipped into the hot acid and observed closely to distinguish a similar reaction between the unknown metal and a streak made by a proven sample. Metals that exhibit similar response to the hot acid are probably very close in composition.
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