This section of the sculpture includes a steel sheet with a large hole, around 25mm across. The hole has been burnt through the sheet using a flame, as is indicated by the ridges of melt around the edges.
The tool used to cut this hole was most likely an oxy-acetylene cutting torch. These torches use a mixture of oxygen and acetone gas to produce a flame that burns at around 3500°C. This temperature is hot enough to melt steel.
Metalworkers make holes such as this one to allow large steel sheets to be lifted and held aloft using a winch. In this sculpture it may be resonant with a window on a building, or an eye.