Metal testing is a process used to verify the material composition of an object, which can be necessary for confirming its value, ensuring its suitability for a specific application, or guaranteeing its safety. The choice of testing method depends heavily on the objective, whether one is seeking to identify the base metal, confirm the purity of an alloy, or screen for harmful trace elements. Various techniques exist, ranging from simple non-destructive observations to more complex chemical reactions and precise quantitative measurements.
Basic Non-Destructive Physical Tests
Simple physical examinations provide a quick, initial assessment of a metal without causing any damage to the object. One of the easiest non-destructive checks involves testing for magnetic attraction, as pure precious metals like gold, silver, and most platinum alloys are not ferromagnetic. If a strong magnet attracts the item, it often indicates the presence of a base metal like iron, nickel, or cobalt, which are commonly used in fake or plated items.
Visual inspection offers further clues, such as assessing the metal’s color, sheen, and the nature of any oxidation. For example, a pure copper piece exhibits an orange-red hue and may develop a green patina over time, while cast iron typically presents a dark gray appearance. The general heft or feel of the object can also provide an initial indication of density.
A basic file test provides a rough, non-quantitative measure of a metal’s resistance to abrasion. This involves using a known piece of steel or standardized hardness files to see if the metal can be easily scratched. If a file with a known Rockwell hardness rating fails to mark the surface, it suggests the material is harder than that rating, helping to narrow down the possible metal or alloy type.
Chemical and Scratch Tests for Identification
Chemical testing relies on a metal’s characteristic reaction with specific acids to determine its purity or identity, particularly for gold and silver. This process begins with the streak test, where the object is rubbed across an unglazed ceramic plate, or “streak plate,” to leave a visible powder residue. True gold leaves a yellow-gold streak, while common imitations like pyrite, or “fool’s gold,” leave a greenish-black streak, helping to quickly differentiate them.
The definitive acid test, often called the touchstone test, involves rubbing the metal onto a black basalt stone to create a faint streak. A drop of nitric acid or a mixture of nitric and hydrochloric acid is then applied directly to this streak. Because pure gold resists all but the strongest acids, the purer the metal, the less it will react or dissolve when exposed to a specific concentration of acid.
Different concentrations of nitric acid are formulated to target specific karat levels, such as 14K or 18K gold. If the streak disappears completely or turns a highly effervescent green, it indicates the presence of a lower-purity alloy or a base metal. Due to the corrosive nature of the reagents used, safety is paramount, requiring the use of protective gloves and goggles, and the test must be performed in a well-ventilated area to avoid inhaling toxic fumes.
Determining Density and Specific Gravity
Determining a metal’s density provides a quantitative method for verifying its identity or purity, as every element has a unique density value. Density is calculated by dividing an object’s mass by its volume, with units typically expressed as grams per cubic centimeter. For irregularly shaped metal objects, the volume is determined using the principle of water displacement, first described by Archimedes.
The procedure requires a precise scale to first measure the object’s mass in air. Next, the object is completely submerged in water within a container, such as a graduated cylinder, and the resulting rise in the water level measures the volume of the object. Alternatively, a specialized balance can measure the difference between the object’s weight in air and its apparent weight when submerged in water.
Specific gravity is a related, unitless measurement that compares the density of the metal to the density of water. This value is particularly useful for authenticating precious metals, as their high specific gravities are difficult to mimic with common base metals. For instance, the specific gravity of pure gold is approximately 19.32, which is significantly higher than that of most common metals, making the measurement a reliable indicator of authenticity.
Testing for Toxic Contaminants
Testing for toxic metals addresses safety concerns rather than focusing on the material’s identity or value. These tests are primarily designed to detect the presence of common health hazards such as lead, arsenic, mercury, and cadmium in items like vintage jewelry, children’s toys, or drinking water. The objective is simply to confirm if a harmful substance exists, not to analyze the composition of the base metal.
Specialized kits are used for this screening, often employing a simple swab or reagent-based system that produces a color change upon contact with the toxic metal. For instance, lead swab tests are used to check painted surfaces, while arsenic kits often use a chemical reaction to provide a reading in parts per million (ppm) within minutes. These kits offer a quick, non-invasive method for homeowners to screen for dangerous contaminants.