1860s replacement of the British copper coinage

Beginning in 1860 and continuing for several years, Britain replaced its copper coinage with bronze pieces. The copper coins had been struck since 1797 in various sizes, all of which were seen as too large. Over time, the copper metal wore or oxidised, or had advertising punched into it, and there were also counterfeits and foreign coins in circulation.

https://en.wikipedia.org/wiki/1860s_replacement_of_the_British_copper_coinage