Sestertii continued to be struck until the late 3rd century, although there was a marked deterioration in the quality of the metal used and the striking even though portraiture remained strong. Later emperors increasingly relied on melting down older sestertii, a process which led to the zinc component being gradually lost as it burned off in the high temperatures needed to melt copper (zinc melts at 419 °C but boils at 907 °C, copper melts at 1085 °C). The shortfall was made up with bronze and even lead. Later sestertii tend to be darker in appearance as a result and are made from more crudely prepared blanks (see the Hostilian coin on this page).