The gold acid test, commonly known as the nitric acid or touchstone test, is a quick, traditional method used to estimate gold purity, measured in karats. The procedure relies on gold’s chemical properties, as it is highly resistant to corrosion and most acids. Despite its wide use, the test can certainly be wrong. Errors stem from two primary areas: procedural mistakes during application and inherent structural limitations of the tested jewelry. This method can easily lead to false positives or negatives, requiring an understanding of its mechanism and limitations for accurate assessment.
The Chemical Basis of the Gold Acid Test
The acid test’s reliability is rooted in gold’s status as a noble metal, meaning it is chemically inert and does not readily react with most substances, including nitric acid. The test uses a series of specialized acid solutions, each formulated to react differently with gold alloys of various purities. A sample is first rubbed against a black abrasive stone, known as a touchstone, leaving a visible metallic streak.
When nitric acid is applied, the reaction—or lack thereof—reveals the metal’s composition. Base metals (such as copper, nickel, or zinc) mixed with gold react vigorously, causing the streak to fizz, change color, or dissolve. Pure gold (24-karat) remains unaffected, while lower-karat gold, which contains a higher percentage of base metals, shows a milder reaction corresponding to its gold content.
Common Sources of Procedural Error
The accuracy of this chemical test can be compromised by errors in technique or material quality. One frequent procedural mistake is improper sample preparation. If the item is not scratched deeply enough on the touchstone, the streak may only contain a thin layer of surface gold, leading to a false positive reading for higher purity.
The physical materials used also introduce inaccuracies. Contamination on the touchstone, such as residue or surface dirt, can interfere with the chemical reaction, causing a misleading result. Likewise, testing acids have a limited shelf life and degrade over time; using expired or improperly concentrated acid yields an incorrect reaction. Finally, the interpretation of the visual reaction is subjective and requires experience. Misinterpreting the resulting effervescence, such as mistaking discoloration for dissolution, leads to an inaccurate determination of the gold’s karat level.
Structural Limitations: Testing Plated and Filled Items
The primary structural limitation of the acid test is that it is inherently a surface analysis method. When the gold piece is rubbed on the touchstone, it only transfers material from the outermost layer, which easily leads to a false positive for items that are not solid gold. This is most apparent when testing gold-plated (GP) or gold-filled (GF) items.
Gold-plated items have an extremely thin layer of gold over a base metal core. A light scratch captures only the plating, suggesting a high-karat reading until the acid burns through to the base metal underneath. Gold-filled items are significantly more misleading. They have a much thicker layer of gold bonded to the core—about 100 times thicker than typical plating. The scratch test may only capture this thick gold sheet, leading the operator to believe the entire item is solid. Testing the item with a scratch deep enough to reach the core is necessary to avoid a false positive, but this permanently damages the piece.
Alternative Methods for Gold Verification
To overcome the limitations of the acid test, professionals rely on several non-destructive verification methods that analyze the metal’s entire composition. Specific Gravity or Density Testing measures the metal’s density by comparing its weight in air to its weight when submerged in water. Since gold has a known, high density of 19.32 grams per cubic centimeter, any significant deviation indicates the presence of lighter metals within the core.
The most definitive modern method is X-Ray Fluorescence (XRF) Scanning. An XRF analyzer directs X-rays at the item, causing the elements within the metal to emit unique fluorescent X-rays. The instrument reads these emissions to determine the exact elemental composition and percentage of the alloy, providing a precise karat value without causing any damage. Magnetism Testing is a simple screening tool because pure gold is non-magnetic. If a strong magnet attracts the item, it confirms the presence of ferromagnetic base metals like iron or steel, signaling a fake or heavily alloyed piece.