Precious Metal Testing
The video below is from YouTube and shows an instrument for testing precious metal jewellery and giving a reading of each metal that makes up the alloy. Very cool!
Demonstration of how a Thermo Scientific XL2 handheld XRF can analyse gold in a matter of seconds.
A common method of establishing the carat value of a gold alloy involves using acids and noting the colour change in the item being tested. This method ONLY shows the gold content and does not indicate the other metals in the alloy.
An item is rubbed on a test stone that removes some of the metal. As such it is destructive and normally an area of the jewellery that is not seen would be chosen. This leaves a small amount of the metal on the stone.
Two acids are required. Hydrochloric and Nitric. Aqua-Regia is 3 parts hydrochloric acid and 1 part nitric acid.
Basic Tests (Gold)
1. Scratch the item with a file and apply one drop of nitric acid.
If it turns bright green it is either gold filled or gold plated over a base metal.
If it turns creamy pink it is gold filled or plated over silver.
Dark brown is 9ct or 10ct
Light brown is 12ct
14ct and higher will show little or no reaction.
2. You will need test needles (made of 9, 10, 12, 14, 18, 20, 22 & 24ct)
Rub the item on a test stone and rub the test needle (closest to the carat value you think the metal is) next to it. Place one drop of acid (*see below) on each rubbing AT THE SAME TIME. If the colour does not match you need to try a different needle and repeat the test with two fresh rubs.
*For 12ct and below use nitric acid, for 14ct and above use Aqua-Regia.
Basic Tests (Silver)
Scratch your item and apply a drop of nitric acid. Green will indicate silver plated, sterling silver (925) should appear a cloudy cream colour. Mexican jewellery is usually less than 90% (900) silver.
Disclaimer: Acids are dangerous and can cause serious injuries if not used properly.