silver testing solution

The use of testing solutions, often referred to as acid testing, is common among scrap buyers and coin dealers, and affordable enough to use at home.  Like many simple tests, this method cannot precisely determine metal purity, but it can be used to categorize a sample into a purity range.

If applied directly to a coin or bar, testing solutions will dull the finish and leave an unsightly mark.  It is also very difficult to observe the reaction of gold testing solutions if applied directly to the sample.

testing solution on genuine silver roundThe dull spot on this genuine silver round is the result of applying silver testing solution directly to the surface.

Instead it is best to use a testing stone.  Carefully rub an edge of the piece to be tested on the stone, leaving a visible streak of metal.  Be sure to press firmly as most counterfeits are thinly plated with pure silver or gold and it is important to reach the underlying metal.  The amount of material removed from the bar or round will be negligible.

testing stoneMetal streaks on testing stone.

Apply a drop of the appropriate testing solution to the streak on the stone and observe the reaction.  Silver testing solution typically comes in only one strength, and the purity of the silver is approximated according to the color the solution turns when it is applied to the sample.  If the sample does not dissolve and the solution turns bright red, the sample is high purity silver.  Dark red to a reddish-brown color suggests coin or sterling silver (.800 - .925).  If the solution turns blue or green, the sample likely contains copper or brass.  If the sample dissolves very quickly, or the solution turns dark brown or yellow, it is likely a base metal.  Experience and experimentation can lead to more precise interpretations, but for bullion purposes, silver testing solution is best used as a pass/fail test.  Bright red being the expected result for high purity silver, and anything else indicating a problem.

testing solution applied to testing stoneSilver testing solution applied to metal streaks on testing stone.

Only a small amount of testing solution is required.  More was used here to make the reaction easier to photograph.  Note that when using a testing stone, the solution will only change color directly over the metal streak.  When applied directly to a sample piece, the entire drop of solution will change color.

silver testing solution turns red on genuine silverSilver testing solution turns bright red on high-purity silver.

silver testing solution turns other colors on low purity or fake silverThe silver testing solution turned blue on this counterfeit piece which consists of thin silver plating over a brass core.

Be sure to thoroughly clean the stone between tests to avoid contaminating your next sample.  You can use a few drops of 22kt gold testing solution to dissolve most metals before rinsing the stone with water and patting it dry with a clean cloth or paper towel.  Rubbing the stone with a paper towel may leave particles on the textured surface.

Testing solutions for gold work differently.  The sample is tested with a series of solutions of varying strength, each identified by a karat number.  If the solution dissolves the sample or changes color significantly, it is less pure than the karat specified on the solution.  If the solution does not dissolve the sample, but changes color slightly, the purity of the sample is approximately equal to that solution's karat value.  If the solution does not dissolve the sample or change color at all, then the purity is greater than the specified karat.  Gold testing solutions are commonly sold in a set of 10kt, 14kt, 18kt, and 22kt strengths.  The 22kt strength is the most useful in bullion applications, as most bullion gold is equal to or more pure than .9167 fine.

Typical set of gold testing solutions.

The 22kt solution did not change color or dissolve this gold streak, indicating it is more than .9167 fine.

It is important to note that testing solutions have a shelf life.  Silver testing solution will darken and turn more opaque in as little as 2-3 months, making it difficult to detect important color changes when testing.  Be sure to buy from reputable dealers with fresh stock, and use name brand solutions such as those from JSP.  Gold testing solutions should remain viable longer, but it is a good idea to periodically compare results to samples of known purity to make sure the solution is performing as expected.  Always store testing solutions in a cool, dark environment and ensure the caps are sealed completely.

fresh and aged silver testing solution
Fresh silver testing solution (left) is translucent light red. Aged solution (right) is darker and more opaque.

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