Copper (Cu, atomic number 29) is a soft, malleable element known for its excellent thermal and electrical conductivity. Identifying pure copper is important across various fields, whether for determining scrap metal value, ensuring material quality, or verifying historical authenticity. Distinguishing this reddish-orange metal from its alloys and plated substitutes requires a systematic approach using visual cues and simple physical tests.
Recognizing Copper by Sight and Surface
The most immediate indicator of pure copper is its distinctive color, which appears as a salmon-pink or reddish-orange when the surface is freshly polished or cut. This bright metallic luster is highly reflective, but it does not remain pristine for long when exposed to air. Copper is naturally soft, possessing a low Mohs hardness of about 2.5 to 3, meaning a simple steel nail can easily leave a scratch on its surface.
The color of copper naturally changes over time through oxidation, reacting with oxygen and moisture in the environment. This reaction first causes the bright surface to darken through browns, eventually leading to a deep chocolate-brown finish. If exposed to the elements, copper develops a characteristic blue-green layer called patina, also known as verdigris. This blue-green layer is a strong sign of aged copper and acts as a protective barrier against further corrosion.
Simple Physical Tests for Confirmation
A straightforward and non-destructive way to test a metal is by checking its magnetic properties. Pure copper is non-magnetic, classifying it as diamagnetic, which means it will not be attracted to a standard magnet. If a strong magnet sticks to the object, it is certainly not pure copper and is likely steel, iron, or a copper-plated item with a magnetic core.
Copper is a dense metal, having a specific density of approximately 8.96 grams per cubic centimeter (g/cm³). This high density means a pure copper item will feel surprisingly heavy for its size compared to lighter metals like aluminum. Additionally, tapping the object to listen to the sound it produces can be helpful, as pure copper tends to produce a dull, low-pitched sound when struck.
Distinguishing Copper from Its Alloys
Distinguishing pure copper from its two primary alloys, brass and bronze, is a common challenge. Brass (copper and zinc) is distinctly more yellow than pure copper, often resembling a bright gold color. Bronze (copper and tin) usually has a duller, more reddish-gold or brown tone compared to the reddish-orange of pure copper.
The mechanical properties of these metals offer another key difference, as alloying copper with other elements generally increases its hardness. Pure copper is relatively soft and highly malleable, whereas brass and bronze are significantly harder and more resistant to scratching. A light scratch test in an inconspicuous area can often reveal the softness of pure copper versus the harder surface of its alloys. Patina also forms differently on alloys; while pure copper develops a pronounced blue-green verdigris, brass typically forms a muted brown patina due to its zinc content.
Verifying Identity Using Chemical and Heat Reactions
To check for plating, perform a light scratch test in an inconspicuous area. Copper is often used as a thin plating over cheaper, less valuable metals. If a different-colored metal, such as silver, gray, or white, is visible beneath the reddish-orange layer, the object is copper-plated, not solid copper.
Copper possesses extremely high thermal conductivity, second only to silver among pure metals, meaning it heats up and transfers heat very quickly. Due to its high melting point, pure copper will not melt or discolor easily when exposed to low heat, which can help differentiate it from plastics or metals with lower melting temperatures. A simple, safe chemical reaction can be performed using a paste of salt and household vinegar, a mild acid, which will rapidly clean the tarnish off pure copper. If the metal is plated or a different alloy, this mild chemical reaction may show an uneven cleaning or potential surface damage.