In the development of bigger greenhouses, cast iron became the conventional support framework. As both exhibition halls and greenhouses were nearly always made of cast iron (occasionally in conjunction with wrought iron) in the nineteenth century, the concept's popularity generated numerous imitators. The use of this material for supports continues today in greenhouse design. The first recorded use of cast iron for architectural purposes was in 1593 by Christopher Wren who used it to build an observatory on Greenwich Park near London. He would not be the last architect to use this material since it was easy to work with and inexpensive. By the 1820s, cast iron had become a popular choice for outdoor furniture such as benches and chairs. In fact, many urban centers around the country adopted cast-iron furniture as their default style because of its durability and cost effectiveness. Cast iron is also heat-resistant enough to be used inside homes as well; for example, Thomas Edison invented a cast-iron stove that was sold commercially from 1872 to 1975. The use of cast iron for supports continued into the twentieth century with some innovative designs coming out of Europe. In 1955, Gunnar Birkerts designed and built what is now considered the first modern greenhouse using mainly steel supports and glass panels. However, most contemporary greenhouses still use cast iron or wood because of their affordability and longevity.