The sculpture was made from the same weathering steel ( also known as Cor @-@ Ten ) as the Angel of the North sculpture , which gradually develops a tightly adhering oxide layer as it is exposed to the elements . This layer inhibits further corrosion by reducing its permeability to water . As part of the design , the spikes swayed slightly in the wind in order to withstand gusts in excess of 100 mph ( 160 km / h ) . At the time of construction a time capsule was placed in one of the spikes of the sculpture , containing children 's poems and paintings , due to be opened circa 2300 . The location of the time capsule after dismantling is currently unknown .