What is gold filled wire?

In most of my gold jewelry designs, I use 14k gold filled wire. I use it because it is tarnish resistant, looks like gold, and costs a lot less than pure gold. But what exactly is gold filled wire?

Gold filled wire is made by forming a tube of gold (10, 12, or 14kt) and filling it with a base metal (usually brass). The gold content is 5%, or 1/20th of the total metal content of the wire. This is not the same as gold plated – gold filled contains 100 times more gold than does gold plated. Gold filled wire is strong, durable, resistant to tarnish, and should last a lifetime without showing signs of wear.

Peruvian Opal Earrings on Handmade Gold Filled Hoops