To satisfy the demand for guns the Queen was forced to look to the Wealdon ironworks for her heavy ordnance. England might be poor in copper, but it was rich in iron ore.
So when did the change to iron occur? Interestingly, though not surprisingly, it took place during the Armada and post-Armada period when the threat of invasion by Spain was at its sharpest and when the need for guns had never been greater or more urgent (s.v. Historical Background). The earliest document on the state of naval armament was that prepared in 1546 (just a year before Henry VIII’s death) by Anthony Anthony, the Navy’s Surveyor of Ordnance. From the so-called Anthony Rolls we learn that the Navy had 256 guns all of bronze.
A typical ship of the Armada period was the Tiger of 1586. It was armed, or intended to be armed, with 4 culverins, 8 demi culverins, 8 sakers, 2 falcons, 2 fowlers and 6 bases – all of bronze.
But in two separate surveys conducted in 1595 (just three years after the Alderney ship) we find the Tiger armed with six demi culverins, 14 sakers and two falcons – all of iron.
A survey of naval-fire power in 1585 recorded 545 big guns of which only two were of iron, but in a survey of 1595 (just three years after the Alderney ship) we find that the weapon-pile had grown by 71 per cent to 931 guns, and that 137 of the increase were of cast iron. Clearly a major shift in technology, product and supply had occurred within the foundries. The armament industry had gone through its first industrial revolution, and it all happened in less than ten years, the period which also saw the arming and sinking of the Alderney ship.
Source:
Bronze to Iron « The Alderney Elizabethan Wreck