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How to Test Copper Purity Simply

Posted by Tom Tuttle on June 30, 2017

Copper is a commodity traded on the New York Mercantile Exchange (NYMEX) via its Commodity Exchange (COMEX) division and on the London Metal Exchange (LME). On the day this blog post was authored, copper had experienced a 36.13% swing from high to low value over 52 weeks of trading.

As the single largest material expense in the manufacturing of electric cabling, too often manufacturers will seize the opportunity to substitute an aluminum-copper alloy and a host of copper plated metals in seeking higher profit margins.

Distinguishing the authenticity of a conductor strand that is copper plated requires a simple scrape test. By removing the outer plating with a utility knife, you’ll be able to see the contrasting silver against the copper color. For a thicker gauge wire, simply cutting it in half will reveal the true composition of the strands. But, how do you know if your copper wire is mixed with other metals? By finding the density of the copper strand. Pure copper has a density of 8.92 grams per milliliter.

Density (g/ml) = Weight ÷ Volume

To solve this equation all you need is a volumetric cylinder (preferably of the 25 ml variety), regular tap water, a sample of your copper wire, and a calculator.

Cut a small sample of the wire that is 8” – 12” long. For the thinnest gauge conductor strands you will want to use several separate strands for this experiment—enough to displace water when calculating the volume.

Weigh your sample (while dry) on a scale that measures in grams. You can use a digital scale that measures in pounds or ounces and convert the weight to grams using a simple Google search for “lbs to grams” or “ounces to grams.”

Volume of your copper object:

  1. Coil your sample(s) around a cylindrical object, such as a pencil, pen or marker. You just want it to fit nicely in the graduated cylinder.
  2. Fill the graduated cylinder with enough water to full submerge your sample. Using the bottom of the meniscus (the concave curve the water creates) as your reference point, record the volume of the water.
  3. Gently place the coiled copper wire into the cylinder with the water and record the new volume of the water, then subtract the starting value to arrive at the volume of your sample.

Now that you have all of the necessary information, solve for your sample’s density. Divide the weight of the sample by the volume. Does it equal 8.92? If so, congratulations, you have purchased a quality copper wire. If it varies greatly in either direction from the expected outcome, then you have a copper wire that has been mixed with another metal, which means conductivity, heat disbursement and safety will fall short of your high standards.

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