First we need to define 'full' plate armour. Plate armour gets its start in the 13th century in the form of a pair (or 'coat') of plates for the torso, followed by limited limb protection. I write about the very early history of plate armour here. In the 14th century the extent and sophistication of plate armour worn by soldiers expands enormously, as does the availability of that plate armour. While in 1300 mail was the primary defense, supplemented with plate, in 1400 plate armour is the primary defense, supplemented by mail. In some places in Europe, like Italy, the mail shirt was still worn under mail, and would continue to be worn for decades more after plate armour reached its 'fullness.' In other places, such as England, even in the 3rd quarter of the 14th century men at arms were discarding full mail shirts and wearing supplemental pieces of mail instead - sleeves to protect the arms, gussets and voiders for the underarms, 'pisans' or 'standards' for the neck and 'paunces' or skirts for the lower torso, crotch and upper thigh. This corresponds to a preference to more complete limb armour, and hints at very complete torso protection as well. In addition, the English men at arms from Crecy onwards often fought on foot in full armour, so saving weight was important. So even well before 1400 men at arms were using plate armour as their primary defense.

But that isn't the whole story - plate armour was continuing to develop in the later 14th and early 15th century, and this is what people mean when they say that 'full' plate armour wasn't developed until 1420. Some men at arms at the very end of the 15th century wear an uncovered single-piece breastplate, like that on the Pistoia Altarpiece. But many breastplates in the later 14th century were covered, like This breastplate in Munich. Though this is an aesthetic distinction, covered breastplates don't fit our image of the 'knight in shining armour'. In addition, until around 1400 many breastplates were in two parts in the front, and were joined with straps (a style some people today call 'corazinna' though I have seen little evidence that the term was used at the time - they may have called it a 'brigandine' like the 15th century front-opening garment of small plates). Most importantly, the backplate didn't develop until some time after 1400 - until then the back was either protected with mail or with smaller metal plates, as it had been by 'pairs of plates' earlier in the 14th century. In addition, the neck was often protected by a mail 'aventail' hung from the helmet until the beginning of the 15th century, when plate throat defenses become more common with the introduction of the 'great basinet'. Personally, I think it is too narrow to say that 'full plate armour didn't develop until 1420' - plate armour was the primary defense from the third quarter of the 14th century onwards in many places, even though some parts of the full plate harness were still being developed after 1420. But this is a matter of definitions.

On the other side of the timeline, the decline of full plate armour is far more complex than armour becoming obsolete when firearms became more common in the 15th century. On the contrary, the later 15th and 16th century was arguably the high point of plate armours quality and quantity in Europe - the metallurgy of the best armours was at its most refined, and their construction was technically impressive in the way that it combined rigidity and flexibility. Meanwhile partial armours for the infantry were more available than ever before, as munition breastplates replace brigandines and cloth armour for infantryman. Men at Arms were still used on the battlefield of the early 16th century, and used decisively - the charge of the French gendarmes into the rear of the Imperial army at Ravenna won the battle for the French. The reasons for armour's continued relevance are several - first, firearms were not nearly as prevalent as they would become later in the 16th century - in 1500 arquebusiers made up a minority of armies, while in 1600 they were a majority. Secondly, the 'pike and shot' tactics that allowed firearms to be used in open field battles had not yet been perfected. Finally, firearms and gunpowder were much less powerful than they would become later in the 16th century as heavy, long-barrelled muskets were developed and 'corned' gunpowder was introduced. In addition, gunpowder becomes much cheaper over the course of the 16th century as the secret of manufacturing saltpetre in 'nitre beds' by sprinkling urine on manure (yes, really) spreads.

Rather than being rendered obsolete with the introduction of gunpowder, full plate armour declines in use over the 16th century, beginning around 1530. Full plate harnesses are being made and apparently worn into the later part of the 16th century, though in the second half of the century more and more soldiers wear partial armour. There are a number of factors to this very complex development, and the increasing power of firearms and the increasing weight of armour is only part of this story, though I write about this at length here and discuss the successes and failures of making bulletproof armour here. In addition to the technological devleopments in gunpowder and heavier breastplates 'of proof' requiring soldiers to discard the extra weight of limb armour, there are a lot of indications that more and more soldiers preferred not to fight as men at arms for economic reasons - it was cheaper to outfit oneself as a pistol-armed 'Reiter' or partially armoured 'demi-lancer' rather than a knight.

Thus both the introduction and the decline of plate armour is not clear cut, and the reasons for armour's eventual decline are not a simple matter of improved technology making previous technology obsolete immediately (though the introduction of gunpowder is a critical part of the story). I think this is a good illustration of how technological development occurs - it is not a simple story of gunpowder as a 'killer app' or the gun as a world-changing gadget, but of a complex interaction of multiple technological developments with social, political and economic developments.

Sources: Alan Williams - The Knight and the Blast Furnace

Matthias Pfaffenbichler - Armourers

Claude Blair - European Armour c. 1066-1700

Thom Richardson - The medieval inventories of the Tower Armouries 1320-1410