Cast Earth is a proprietary natural building material developed since the mid-1990s by Harris Lowenhaupt and Michael Frerking based on the earlier Turkish Alker, which is a concrete-like composite with soil of a suitable composition as its bulk component stabilized with about 15% calcined gypsum (plaster of Paris) instead of Portland cement. It can be used to form solid walls that need not be reinforced with a steel frame or timber framing, unless extra seismic reinforcement is necessary. Forms are set up and filled with Cast Earth, which sets quickly and solidly. Once the forms are removed the wall stays sound.