The few marketable products from the area were Japanese paper, made from the fibres of the paper mulberry, which occurs naturally in the area, nitre (calcium nitrate) for gunpowder production, and the basic products of sericulture (silkworms and raw silk thread). Paper production declined in the 19th century, and nitre production was brought to an end with the importation of cheap saltpetre from Europe at the same time. The silk industry survived longer, from the late 17th century until the 1970s; its requirement of large enclosed spaces for silkworm beds and storage of mulberry leaves was an important factor in the development of the gassho -style house.