Testing Precious Metals with Acids
© Courtesy of Lacy West SuppliesThese chemicals are dangerous and should not be used by schools or novice craftspersons without direct supervision by a trained professional. Always add acid to water, not water to acid.
Gold Testing Kit - Acid
Used properly this kit will determine whether or not an object contains precious metals such as gold, silver, platinum, palladium, or any alloy thereof.
It is necessary to obtain Nitric Acid (C.P.), Hydrochloric Acid (C.P.), and distilled water to mix the test solutions. A graduated cylinder is recommended to measure the required proportions. These items may be obtained from laboratory or chemical supply houses in large cities, or from the local drug company in smaller cities. Lacy West sells these and other acids in our showroom, but we will not ship them.
Since the testing procedure requires the use of dangerous acids, special care should be taken not to spill or allow acid to touch the skin. If acid comes in contact with the skin, wash the affected area with a mild soap and water as quickly as possible.
For appraisal and testing of gold, silver, platinum, brass, nickel and German silver. Wood case contains three acid bottles with ground glass stoppers and applicators, test stone, set of gold testing needles ,and salts for making Schwerter's testing fluid. Acid must be purchased locally.
Filling the Acid Bottles
Fill one bottle with Nitric Acid, fill the second bottle with 75% Hydrochloric Acid and 25% Nitric Acid (3 parts Hydrochloric Acid and 1 part Nitric Acid). This formula will make "Aqua-Regia". The shelf life of Aqua Regia is about 3 weeks - make a fresh solution of Aqua Regia every three weeks. The third bottle is provided as a spare.
Care of your Gold Testing Kit
Mark each side of the test stone and always use Nitric Acid on one side and Aqua Regia on the other side. Wash the test stone in clean water after each use. Residual acid from a previous test may yield incorrect results. The testing needles are not plated. The tips are made of gold wire and should have a long life.
Spare Bottle
The spare bottle can be filled with Schwerter Salts. Schwerter's Testing Fluid is made by dissolving all of the Potassium Dichromate Salt in a solution of 3/4 oz Nitric Acid plus 1/4 oz distilled water. Mix well in a glass container to dissolve the salts then fill the small bottle of the kit. This is used for testing gold below 14 karat and detecting base metals.
Testing Gold
Scratch the object with a file and apply a drop of Nitric Acid. If the object turns a bright green it is gold plated or gold filled on base metal. If the object turns a pinkish cream colour it is plated or gold filled on silver. 10 karat gold will turn dark brown. 12 karat gold will turn light brown. 14 karat or higher will have little or no reaction.
A test stone can be used to test a streak of metal laid down from the jewellery, or a small groove can be filed on an inconspicuous place of the jewellery and tested.
Finding the Karat of Gold
* File a clean spot on the article to be tested and rub it on the test stone.
* Rub a test needle nearest the karat you think the article is next to the mark.
* Apply acid to both marks simultaneously. (Nitric Acid for 12 karat or under; Aqua-Regia for 14 karat or higher). If the colour does not match try another needle until it does.
* White and green gold react slowly, but will still leave a brown mark after applying acid.Testing Platinum
Rub the article on the test stone and apply a drop of Aqua-Regia. A platinum mark will remain the same white color with no reaction.
Testing Palladium
Rub the article on the test stone and apply a drop of Aqua-Regia. If it is genuine, the scratch will turn red.
Testing Silver
Scratch the article with a file and apply a drop of Nitric Acid. If it turns green, it is plated. If a cloudy cream, it is sterling or better. Remember, most Mexican silver generally contains less than 90% fine silver.
Testing Below 14 Karat and Base Metals with Schwerter Salts
File a deep notch in the test piece and apply a drop of Schwerter's Solution in the notch. The colour reaction of the solution with the metal will be as follows:
Brass - Dark Brown
Copper - Brown
Nickel - Blue
Palladium - None
Gold - None
Silver Pure - Bright Red
Silver .925 - Dark Red
Silver .800 - Brown
Silver .500 - Green
Lead - Yellow
Tin - YellowPre-Mixed Test Acids
Pre-mixed gold, silver, and platinum test acids are available for those who do not wish to mix and handle acids. Generally a selection of these pre-mixed test acids are kept on hand to test for a variety of karats and metals. These acids can be used on test stones or directly on jewellery.
Using Pre-Mixed Acids with a Test Stone
* Rub the jewellery on the test stone using 4 firm strokes (two forward and two back) so that a very noticeable streak of metal is left on the stone.
* Apply a drop of the pre-mixed acid to the centre of the streak.
If the metal streak noticeably changes colour or disappears, then the metal is less than the karat of the test acid.
If the metal streak changes colour only very slightly, then it has the same karat as the test acid.
If the metal streak does not change at all, then it has a higher karat than the test acid.
* Example: When using a 14K test acid, a 12K gold streak will exhibit a large colour change or completely disappear, a 14K gold streak may have a very slight colour change, and if the gold is higher than 14K it will not change at all.
Using Pre-Mixed Acids on the Object
* Find a place on the piece of jewellery where you can file a small groove without ruining the jewellery - the intent is to penetrate the thin surface layer of plated metal (if it is plated). Try testing behind a link of a chain if you can - beware that it is not uncommon to use karat gold for clasps on chains that are only plated with gold.
* Apply a drop of pre-mixed test acid in the small groove that you filed.
* Note the colour change if any (as above when testing on a stone).
* Most jewellery pieces have areas of wear that can be good places to test. Occasionally the wear is sufficient to remove any plating and testing these areas without filing can confirm that the jewellery is a lower karat than the test acid. If you do try this and there is no colour change, file a small groove and try again to be sure.
