Gold Assay - (n) a process to measure purity of items containing gold.
For example, the US Mint guarantees that its American Gold Buffalo Coin has a minimum 99.99% purity. Since the price of gold per ounce is based on 1 troy ounce of per gold, this is important that the assay for 1 ounce coins also measures pure gold.
In the following video you will hear from a US Mint chemist describing the precious metals and gold assay process necessary to make sure that all the gold coins minted by the US Mint meet the purity standards they indeed guarantee.
US Mint Gold Assay Process
The practice of gold assaying is commonplace within the gold bullion, gold refining, and gold mining industries.
Understanding gold grades and gold purity through accurate gold assaying is key to profitable gold mining, gold refining, and gold bullion product production.
Gold coin and gold bar content purity also remains essential in the creation of trustworthy gold bullion products and establishing reputable brand or gold bullion hallmarks by both private gold mints and government gold mints respectively.
Gold Assaying Process
There are various gold assaying processes used today, each specific gold assay choice will depend on the type of gold item being checked for purity.
Gold assayers typically begin by taking a sample of the gold being assayed, either by drilling the gold item for shavings or by taking a small sample while it is in a molten state before formation into a gold bar or gold coin.
From there, gold assayers have some different gold assaying options. The most common gold bullion assaying processes are the following two methods.
Fire Gold Assay – (n) the fire assay is one of the oldest and most highly accurate ways to check the gold purity of an item(s). Gold purity is often tested this way, with corresponding tiny amounts of silver and gold melted together in a fire assay porous cupel, in order to verify the amount of non-precious or impure metals in a given sample. This method is time consuming taking about 5 hours to complete.
X-Ray Fluorescence Gold Assaying (XRF Gold Assay) – (n) in XRF gold assaying, the yellow precious metal is irradiated by an X-ray beam. The gold assayer then judges the metal purity depending on the intensity level of the X-rays emitted. This is a fast non-destructive method for testing the gold purity on the exterior of a gold item. This method does not perform well in defense against deep tungsteen inserts implanted with a large gold bullion bar for instance.
Reason for Assaying Gold
Because purity is essential for gold coin or gold bullion bar values, assaying gold contents is therefore critical.
The main reason for assaying gold bullion products is to make sure they meet the standards claimed by their respective issuing mint, meeting the minimum purity requirement for a particular gold coin or gold bar stamping.
Oldest Gold Assaying Ceremonies
One of the world’s oldest gold assaying ceremonies happens in the United Kingdom’s annual Trial of the Pyx, which is a yearly process for assaying both cupronickel circulation coins and bullion products produced by the Royal Mint.
It tests not only coin and bullion product purity levels, but also honesty of measurements and guarantees made. Below is a short video documenting the ceremony and the reasons for its existence.
Gold Assay at the Trial of the Pyx
Gold Bar Assays
Generally, gold bullion bars have an official assay proof stamping or certificate of authenticity card.
As gold bars are often large, highly valuable, and explicitly stamped with high levels of purity (typically .999 fine gold or purer).
The following image shows a Republic Metals 1 kilo gold bars with gold assay card authenticating the 32.15 troy ounce gold bar's content and purity.