Refining and Assaying Gold, Testing for Fake Gold

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The separating and purifying of precious metals from other metals is called refining.

Smelting is the separation of precious metal from non-metallic impurities.

The Silver Brothers are committed to using correct and accurate evaluation methods of all precious metal purchases from our valued customers.

Our modern  laboratory guarantees that all precious metals are tested using the most suitable methods of evaluation. Gold, silver, platinum, palladium and rhodium are verified (determination of precious metal purity) in various ways to accurately and precisely evaluate each metal’s percentage of purity.

The method known as “fire assay” is the oldest known method of assaying gold and continues to be one of the most accurate. When gold is melted, a sample of the gold is taken, either by drilling through the poured bar or by capillary tube sampling while the gold is in a molten state. The sample is weighed very precisely and the amount recorded. The sample is wrapped in assay lead foil along with a quantity of pure silver. This wrapped ball is placed in the furnace in a cupel (a special kind of disposable crucible). All the non-precious metals are absorbed by the hot cupel. The precious metal forms a button within the cupel. The cupel is removed from the furnace and the button is brushed to remove any remaining bits of the cupel. It is hammered flat, rolled thin and then heated in a porcelain crucible containing a weak nitric acid solution. The acid removes the silver, which is poured off and the silver recovered from solution. The gold is then rinsed in distilled water to remove any residual acid and then dried. The sample of gold is now at least 99.999% pure! The sample is then weighed again. The original weight of the impure sample is divided into the weight of the now pure sample. The result is the assay. In the analysis of platinum group metals, roughly the same procedure is followed, but the final element readings are normally done using ICP.

ICP-OES Inductively coupled plasma optical emissions spectroscopy is a major technique for elemental analysis. The sample to be analyzed, if solid, is normally first dissolved and then mixed with water before being fed into the plasma. Atoms in the plasma emit light (photons) with characteristic wavelengths for each element. This light is recorded by one or more optical spectrometers and when calibrated against standards the technique provides a quantitative analysis of the original sample. ICP instruments comprise of various optical spectrometers, nebulizers, spray chambers, ICP torch and RF generators.

Typical ICP Torch 

X-Ray fluorescence (XRF) spectrometry is a non-destructive analytical technique used to identify and determine the concentrations of elements present in solid, powered and liquid samples. The spectrometer measures the individual component wavelengths of the fluorescent emission produced by a sample when irradiated with X-rays.

The Silver Brothers can accurately determine the purity of most precious metals either by electronic means or by basic scratch and touch stone testing with acid, which compare scratch marks from unknown karat gold items with known karat gold needle marks. We employ the touch stone method which is many centuries old. The touch stone  is a smooth fine grained, slightly abrasive black stone, usually quartz or jasper. It was used as the first method of assaying gold and was used at least as early as 500 BC. Touch stones and karat needles are still used today and can be purchased from jewelry supply houses. They obviously are not anywhere near the accuracy of a fire assay, but still serve as a valuable tool to test the karat value of unknown gold items.

Touchstone set used in the 19th century
for testing the fineness of gold Individuals can test their own gold and silver items with a few basic tools and acid.

Nitric acid can be obtained from most jewelry supply houses and used to determine whether an item is gold or gold-plated on steel or copper. A good strong magnet, available at any hardware store, is useful to test whether the item is just gold-plated over steel. Gold is not magnetic. Gold-filled items are not magnetic, because the gold is usually over copper. Gold-filled items can be tested by filing a deep notch in the surface and putting a drop of nitric acid over the filed notch. If it starts to bubble green color, it is the copper reacting to the acid and it is gold-filled. When nitric acid is applied to 10 KT. gold, it will turn a reddish brown in color. 14 KT. will not change color and the acid will just look like a drop of water on it. From 18 KT. to 24 KT will stay bright on a touch stone, but is hard to tell any difference between the karat value. There are numerous electronic testers on the market that will tell the karat value of gold items and some that can test for platinum. Electronic testers are not nearly as accurate as an actual fire assay, but can serve as a tool to get a basic idea of what an unknown piece of metal is, or is not. Electronic gold testers are available through jewelry supply companies. You can also determine whether a silver colored piece of flatware or jewelry is sterling silver or silver plated over base metal or copper by filing a deep notch in it and applying acid in the notch. If the item is sterling silver it will turn an ashy light gray in color, but if it starts to bubble green at the notch, it is silver plated over copper or other base metal.

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