A simple test for gold

Hydrochloric acid is sometimes called muriatic acid when it is in dilute form.  The dilute acid can be used as a simple test for gold.

Sometimes it is a case of not being able to see the forest for the trees, and sometimes I am guilty of this fault among the many others I have. Gold is not called the noble metal because it is used to make crowns, but because there are very few things that can cause it to dissolve. Aqua regia, a mixture of one part of nitric acid and three parts of hydrochloric acid is one of them. It can also be dissolved in a solution sodium cyanide, and a few other compounds.

There are very few minerals that resemble gold alone and a few sulfides that are commonly called fools gold. Some weathered mica can also resemble gold if these substances are treated with acid they will dissolve and emit an odor like rotten eggs. You can use ordinary vinegar to make this test if you want, but muriatic acid works even better. You can buy this acid at a local mason supply store or some times at your local hardware store. Another source of acid can be found in your local auto parts store under the name “battery acid.” Battery acid is a dilute form of sulfuric acid, but any of them will work for this test.

If the material forms than crystals, and it is the color of gold is most probably fools gold. When acid is supplied to this material it will make a gas called hydrogen sulfide is smells just like rotten eggs, you can't miss it. Gold is not affected by any of these acids, nor does it create a smell. This is not to be considered a definitive test for gold, but it sure eliminates in a real hurry many things that do look like gold.

Weathered mica is one of the few things that may not eliminate, but the reaction when acid may bleach the iron staining that makes it look like gold. In any event weathered mica is about nine times lighter than real gold. The only other metals that are not affected by acid are the platinum group metals because they are also considered to be noble metals.

Gold and platinum are often found together so much so that the early Spanish Conquistadors in Peru used platinum as an alloy in their coinage because they had no value for platinum. During the 20th century many of the Spanish coins were dissolve so their platinum content could be recovered.

This test can be used on both gold that is visible to the naked eye, and gold that is only visible under a microscope. It is best to draw a circle around microscopic specimens with a pencil so you can find them again. The graphite in a pencil is not affected by any acid, but don't use a ballpoint pen because the eight may be affected by the acid.

Something else that can be added to your prospecting equipment is a dropper bottle for holding the acid allowing you to dispense it one drop at a time. For many tests all you do this a single drop of acid. This test is also useful for finding out if a stone as limestone or not. In the presence of calcium carbonate it fizzes.  It doesn't affect Quartz, but gold is often found in either mineral. Occasionally it is also found in Jade as flecks of gold.

Flame Tests

Flame tests may not be the most accurate, nor do they work for gold, but they can be really effective for other elements.  Boron is one that imparts a greenish color to a flame.  This gives rise to the story of how the 20 Mule Team Borax Company got started.  It seems a prospector who was down on his luck was prospecting Death Valley, California with his wife when he tried a flame test on some of the dried salts he found in the dessert.  It burned green so he turned to his wife and said, “She burns green Ma, we’re rich!”

A chemist performing a flame test on copper sulfate.  Notice the blue color of the flame.

Flame tests are conducted on a length of thin platinum wire that is first dipped in a dilute solution of hydrochloric acid then in the pulverized sample of the substance to be tested; then it is held into a hot flame.

Depending on the element contained in the sample it colors the flame various colors.  This technique will present a pretty accurate idea of what king of substance you have, but the accuracy of the test can be improves with a spectroscope.

Table 1 shows the colors of the flame test of various elements:

Symbol	Name	Color
As	Arsenic	Blue
B	Boron	Bright green
Ba	Barium	Pale/Apple green
Ca	Calcium	Brick red
Cs	Caesium	Blue-Violet
Cu(I)	Copper(I)	Blue
Cu(II)	Copper(II) (non-halide)	Green
Cu(II)	Copper(II) (halide)	Blue-green
Fe	Iron	Gold
In	Indium	Blue
K	Potassium	Lilac(violet)
Li	Lithium	Pink-Red
Mn (II)	Manganese (II)	Yellowish green
Mg	Magnesium	No colour
Mo	Molybdenum	Yellowish green
Na	Sodium	Intense yellow
P	Phosphorus	Pale bluish green
Pb	Lead	Blue
Rb	Rubidium	Red-violet
Sb	Antimony	Pale green
Se	Selenium	Azure blue
Sr	Strontium	Red
Te	Tellurium	Pale green
Tl	Thallium	Pure green
Zn	Zinc	Bluish green

Using the Purple of Cassius to Check For Gold in the Field

If you have ever gone to church and admired the beautiful stunning colors in the stained glass windows the red in those windows comes from ion sized gold particles disseminated through the glass as it was formed in a furnace.  The same physical characterstics are also seen in the Purple of Cassius.  It is this characteristic of gold that allows us to make a very accurate test for it while in the field.  This test was the standard test for goldsmiths for centuries until it was replaced in the 20^th century by the atomic absorption spectrometer.

This is how they prospected for gold and other metals in 15th century Germany   Georgius Agricola

The Purple of Cassius was discovered in 1685 by Andeas Cassius, an alchemist who described it in the work De auro.  It was also described in a book published in the Italian City of Bologna as an artist’s color.  The alchemists of that day that quickly realized that it could be used as a sensitive test for gold.

Besides affording a sensitive test for gold the Purple of Cassius has found much use as an artist’s color. As noted earlier it is also used to produce red glass. The Purple of Cassius is also painted on glass before it is fired leaving the glass with a thin layer of fused-on gold.

When you are prospecting for gold in hard rock or any other place the purple of Cassius is the ideal test to take into the field because the various chemicals you need are capable of being carried in a small space without taking up too much room. It is also so sensitive that the prospector could get a rough idea of how rich the gold deposit is. This is determined by the color of the reaction that could range from a light pink to a dark purple. The darker the purple is the richer is the gold ore.

This is how they mined during the 15th century.   Georgius Agricola

Another advantage of using this test in the field is the ease with which you can mix these chemicals used to test with. There are several simple formulas for doing this but one of the most common is two parts of concentrated nitric acid in one part of concentrated hydrochloric acid. The result is a very powerful acid called “Aqua Regia” one of the few substances that will dissolve Gold.  The other ingredient used in the test is tin chloride solution you can either purchase it alone or you can make it yourself by dissolving the metal tin in some hydrochloric acid.

Beakers like those used in the Purple of Cassius test.

When you are dissolving gold in this acid mixture slosh the material being tested around in the acid to dissolve any gold that may be present.  In itself this is a sort of test as the aqua regia will turn yellow if there is any gold present. This is not too accurate though so don't depend on it as a test.

To perform the test fill a beaker with deionized water followed by a few drops of the gold bearing aqua regia into the water followed by a few drops of the tin chloride solution. The purple of Cassius will then precipitate out of the solution in a depending on the quantity of gold present the precipitation can range from a light pink to a deep purple color. Remember, the deeper purple color there is the more gold there is present.

Telling gold from fools gold!

The problem of identifying real gold in the field to separate it from fools gold often arise. There are several minerals that mimic gold in appearance like iron pyrite and weathered biotite mica that both have a gold-like appearance.

The easiest field test for gold is simple its density is 19 grams per cubic centimeter that is far denser then any of its mimics. Some genius in antiquity developed the “touchstone” test for gold based on what color its streak is when left on a dark colored stone. In practice this is usually a piece of black slate. Gold will leave a gold-colored streak on this stone its mimics will not. For ages this was the definitive test for gold until it was replaced by the “Purple of Cassius” test during 1685 by Andreas Cassius.

Another ancient test used frequently was based on the fact gold is malleable. You can bend it, and it keeps its bend. You can also bend weathered biotite, but unlike gold it quickly snaps back into shape. The other method based on malleable gold is hammering it into a flat sheet all of those minerals that mmimic gold are brittle and will break-up into a powder if hammered.

The acid test works because gold is unaffected by all strong acids except aqua regia, all the other minerals that mimic gold are affected by acids and are dissolved. A possible exception to this is weathered biotite mica.

The Purple of Cassius’s history goes back to the fourteenth century where it was described as an artist’s pigment however Andreas Cassius gets the credit for its invention. The principle behind the purple of Cassius is the ability of Aqua Regia a two to one mixture of concentrated nitric and hydrochloric acids to dissolve gold.

Just by its dissolution in aqua regia gold produces a rough-and-ready test because the gold charged aqua regia assumes a golden-yellow color. This test is not accurate enough however to be definitive. The purple of Cassius is however. Until the invention of the modern science of spectrometry developed during the nineteenth and twentieth century’s the purple of Cassius was the recognized definitive test for gold.

The test works by adding a few drops of the gold charged aqua regia to 100 ml of clean water, then adding twice as many drops of a solution of tin chloride to the solution. The tin will cause the gold to precipitate from the solution forming a colloid that ranges in color from pink thru dark purple. The darker the color the more gold is present.

The science of “spectroscopy” has supplanted most of the chemical tests for gold in the laboratory, but not in the field where because the various manifestations of the spectroscope are just to bulky. Many of the old chemical tests for gold and other minerals are still being used.

References:

The creation of color in eighteenth century Europe, Gutenburg.org,
http://www.gutenberg-e.org/lowengard/C_Chap33.html

Acid test (Gold), Wikipedia, http://en.wikipedia.org/wiki/Acid_test_(gold)

Spectroscopy, Wikipedia, http://en.wikipedia.org/wiki/File:Light_dispersion_conceptual_waves.gif