 With the US entry into World War II in 1941 , copper and tin , which were both used in the cent , were in short supply . Experiments were carried out by several corporations under contract from the Mint ; they tested various metallic and non @-@ metallic substances , including fiber , tempered glass , and several types of plastic . These experiments used various designs , since actual Lincoln cent dies could not leave government custody . As the experiments proceeded , production of bronze cents was cut back drastically in July 1942 , and ceased in December . On December 18 , 1942 , Congress gave the mint authorization to change the composition of the cent for a three @-@ year period , and five days later , Treasury Secretary Henry Morgenthau announced that the coin would be made out of zinc @-@ coated steel . Zinc and iron form an electromagnetic " couple " ; the two metals soon corrode when in contact with each other in a damp atmosphere . The public soon complained that the new coins were becoming spotted and stained . Another common complaint was confusion with the dime , and some letters suggested that a hole be punched in the center of the new coins . Morgenthau responded that the new pieces would soon become darker , and that the Mint would be willing to darken them if it could figure out a suitable process . 
