Historical records also mention his elder brother, Jang Seong-bal, who was born in 1344 and whose grave is located at Ui-seong in North Gyeongsang Province.

[7] Yeong-sil's mother was a government-registered courtesan, making Jang a government-owned nobi – a type of Joseon's lowest class.

[8] Jang's fame gained him entry into the royal court in Hanseong (present-day Seoul), where select commoners displayed their talents before the king and his advisers.

When he returned in 1434, Jang created Korea's first self-striking water clock, the Jagyeokru (자격루), which would mark the hour automatically with the sounds of a bell, gong, and drum, and was used to keep the standard of time in Joseon.

The collision would cause the larger balls to travel down a lower pipe and hit a giant cymbal, announcing the time to the community.

A ball would then land on another container, which was part of a complex system of levers and pulleys that moved wooden figures to indicate the time visually.

Jang, Ichun, Kimjo, and other scientists made Korea's first sundial, the Angbu-ilgu (앙부일구/仰釜日晷),[22] which meant "pot-shaped sun clock staring at the sky".

[23] Angbu-ilgu was bronze in composition, and consisted of a bowl marked with 13 meters to indicate time and 4 legs jointed by a cross at the base.

[23] No sundials from the Joseon dynasty made during King Sejong's reign still exist today; none are known to have survived past the Japanese invasions of Korea (1592–1598) (임진왜란).

Since Jang used to be a gwanno (관노/官奴) (a man-slave in government employ), he had already acquired much knowledge about metal working, and also knew the geography of the area.

Jang surveyed the available metals and their characteristics, and presented his research to Sejong and the generals, contributing to the development of Korean weaponry.

[24] The Korean economy during the Joseon dynasty was agriculturally based and vulnerable to prolonged or consecutively occurring droughts; therefore, there was a need for better ways to manage water.