Compound armour was a non-alloyed attempt to combine the benefits of two different metals—the hardness of steel with the toughness of iron—that would stand up to intense and repeated punishment in battle.
There had been several attempts to improve on iron with the addition of harder steels on the face, but these all failed for the same reason as the earlier laminate experiments; the ability for the armour to spread sideways into its softer backing allowed it to be penetrated more easily.
The decisive winner was a new soft steel from the French firm of Schneider et Cie, but this proved to be prone to breakage when stressed, making it less useful in naval applications.
The problem of welding them together was solved independently by two Sheffield engineers, A. Wilson of John Brown & Company and J. D. Ellis of Cammell Laird.
Two major reasons for this were the introduction of forged chrome-steel shot in 1886 and the discovery of nickel-steel alloys in 1889 which proved particularly effective as armour plate.