Assaying is the scientific procedure used to determine the exact content and composition of precious metals within a material. This process is necessary across the gold industry to verify authenticity and establish a fair market value for raw ore, finished jewelry, and bullion bars. Gold rarely exists in a perfectly pure state and is often alloyed with other metals like copper or silver to enhance durability or alter color. Accurate measurement ensures that the stated purity level matches the actual metallic content, which is a fundamental requirement for trade, investment, and quality control. The methodology chosen depends on the material being tested, balancing precision with practicality.
The Traditional Method of Fire Assaying
Fire assay, also known as cupellation, is the most accurate technique for determining gold content, often called the “gold standard.” This method is destructive, requiring a small portion of the material, but it offers unmatched precision for high-value bullion and complex ore samples. The process begins with meticulous sample preparation, mixing a precisely weighed portion of the material with chemical fluxes and lead oxide (litharge).
This mixture is placed into a ceramic crucible and heated in a specialized furnace, typically between 1100°C and 1150°C. During this fusion stage, the fluxes melt and dissolve non-metallic components, forming a glassy waste material called slag. Simultaneously, the molten lead collects the precious metals—gold and silver—separating them from the base metals due to density differences.
Once cooled, the metallic portion solidifies into a small lead button, which is separated from the slag. This lead button is then transferred to a porous cupel, a container made of bone ash or magnesia, for cupellation. The cupel is reheated in a furnace, causing the lead to oxidize into lead oxide.
The porous cupel absorbs the resulting lead oxide and other base metal oxides, leaving behind a tiny, purified bead of precious metals, known as the doré button (gold and silver). To isolate the gold, the doré button is subjected to parting. This involves treating the bead with nitric acid, which dissolves the silver but leaves the gold untouched. The remaining pure gold is rinsed, dried, and weighed on a high-precision balance to calculate the exact percentage of gold present. Results often achieve accuracy within 0.01%.
Instrumental Techniques for Gold Purity
Modern technology offers rapid, non-destructive alternatives to fire assay, widely used for finished products like jewelry and bullion. The most common is X-Ray Fluorescence (XRF) spectroscopy, which provides elemental analysis without damaging the item. XRF works by bombarding the sample’s surface with a beam of high-energy X-rays.
When atoms in the sample absorb the primary X-ray energy, electrons in their inner orbital shells are temporarily displaced. To regain stability, electrons from higher energy shells drop down, emitting a characteristic burst of secondary X-rays (fluorescence). Every element, including gold, emits X-rays at unique energy levels, which an XRF detector measures and quantifies.
This process allows for an immediate, non-destructive breakdown of the gold alloy’s composition by percentage. However, XRF is limited by surface sensitivity, meaning it analyzes only the outer layers of the material. If an item is gold-plated or has a non-uniform composition, the reading may not reflect the true bulk purity.
For analyzing liquid samples derived from dissolved ore or concentrates, laboratories use high-precision methods like Atomic Absorption Spectroscopy (AAS) or Inductively Coupled Plasma (ICP) techniques. These instrumental methods measure gold concentrations down to parts per million (PPM) or parts per billion (PPB). While requiring the sample to be in solution, they provide highly sensitive elemental analysis, useful in mining exploration.
Understanding Assaying Results
The purity determined by assaying is communicated using industry-standard measurements defining the proportion of pure gold. The two most common systems are Karat and Fineness. Karat is an older system expressing purity in parts out of 24. For instance, 24 Karat (24K) is considered pure, while 18 Karat signifies 18 out of 24 parts are pure gold (75% purity).
Fineness, or millesimal fineness, is the standard for investment-grade gold and bullion, offering a more precise measurement. It expresses purity in parts per thousand (‰). A fineness of 999 indicates 99.9% pure gold, and 999.9 fineness is the highest standard accepted internationally as four-nines fine gold.
For raw materials like gold ore, results are typically reported in Parts Per Million (PPM) or grams per metric ton (g/t). A PPM value indicates the number of gold particles found for every million particles of the total sample. These units are used by mining companies to estimate the economic viability of a deposit, representing trace amounts of gold within the bulk rock.