Historically, chromed bikes were "triple plated", which means they were plated first with copper, then nickel, and finally chrome. After the chrome plating, the frames were baked in an oven for several hours to minimize hydrogen embrittlement.

The added weight of a chromed frame over a painted one wasn't due as much the final chrome plating (which was relatively thin) as it was to the relatively thick copper layer used as a sort of primer to help the nickel layer adhere to the steel frame. According to the CyclArt website, improvements in the nickel plating process have eliminated the need for the initial copper layer, so today's chromed frames are lighter than those of yesteryear. One advantage of triple plating, though, is that the base copper layer minimized pitting over the years.

http://www.cyclart.com/questions.html#chrome1