Although the 2000 edition of the Coincraft catalogue of British coins says the wartime pause was due to a surplus of pennies, numismatist Kerry Rodgers, in his 2016 article on the currency emergency in Fiji during the Second World War, cited a colonial legislative record to the effect that it was to conserve copper for munitions. Beginning with the 1942 production of 1940-dated pennies, the composition was altered, as tin was a critical war material, rare because most of it came from Malaya, so pennies from then until later in 1945 are 97 percent copper, 0.5 percent tin, and 2.5 percent zinc. Such coins tarnish to a colour different than the prewar coins, and they were treated with sodium thiosulphate to give them a darker tone. That substance, dubbed "hypo", had been used to darken all 1934 pennies before issue and most of the 1935 mintage, "Hypo" would be used again in 1946, discolouring the coins to discourage the hoarding of new pennies. Although the prewar alloy was restored in 1945, the Mint reversed itself in 1959, made pennies out of that wartime composition for the remainder of the coin's pre-decimal history without chemical treatment, and continued its use for the new decimal bronze pieces.