Gold fill (also called gold overlay or gold-filled) is made by using heat and pressure to apply a layer of karat gold to a base of less costly metal. This produces a surface with karat gold. The minimum layer of karat gold must equal at least 1/20 of the total weight of the item.
What's the Difference between Gold Plate and Gold Fill?
Gold fill is 50 to 100,000 times thicker than normal gold plating, and 17 to 25,000 times thicker than heavy gold electroplate (sometimes stamped 14kt HGP). For more info about GF, see About Metals which includes an illustration of the difference between gold plate and gold fill.