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In the first half of the nineteenth century cast iron and wrought iron were introduced and used for industrial construction in Europe and North America. Steel was not widely used, being considered a rare and expensive building material. Inexpensive steel first began to be produced and used only with the invention of the Bessemer process, in the 1850s. From that period on, metal started to be used as rather popular and useful building material. The famous Eiffel Tower of Paris was constructed of wrought iron in 1889. By that period several steel frame skyscrapers had already been built in the United States. That was the beginning of the new era; a new highly useful and popular construction material had been born and introduced into building industry.