Using gold leaf as a decorative element on buildings has a long history: The Egyptians were the first to figure out how to stretch a single ounce of gold into a thin sheet that could cover 200 square feet. Today, even thinner films of gold are used as a reflective coatings on windows to keep buildings cooler. It's now possible to stretch an ounce of gold to cover up to 1,000 square feet of window surface. Similarly, a gold-palladium alloy is used to braze metals subjected to very high operating temperatures, like jet engines. A thin plating of gold is also used on some parts of fuel cells to reduce corrosion and increase electrical conductivity.