Before 1860 they were copper (quite high purity I believe but obviously not 100% pure). After 1860 they were made of bronze. 1860 to 1923 they were 95% copper, 4% tin and 1% zinc with slight variations. From 1923 to 1942 the composition was fixed at 95.5% copper, 3% tin and 1.5% zinc. 1942 to 1945 they were 97% copper, 0.5% tin and 2.5% zinc. In 1945 they reverted to the pre 1942 composition and in 1959 they switched to the 1942 to 1945 composition, which was used right into decimalisation. (Source: The Bronze Coinage of Great Britain, Michael J Freeman)