This is almost too broad for me to even think about. Armour and Weapons -are- military material culture. The study of armour and weapons is the study of these objects and how they were used, viewed and produced. Documentary and artistic evidence is a way of adding context and understanding them, but in the end it's all a study of things.

I guess I'll focus one aspect that I'm pretty interested in, which is the way in which modern preconceptions about aesthetics and purpose can confuse the ways we look at armour. This can also be a kind of case study in how one can contextualize armour and evaluate it in context.

Namely, in the modern world, there's a decorative/useful divide. People are used to seeing things as either practical, or just for show. The genesis of this way of viewing things is outside of my specialty, but anyway, in the modern world many useful objects (especially in warfare) don't have overt decoration.

So when people look at armour, or talk to me about it, they will look at decoration and assume that something was just for show if it is extremely decorated. You also encounter this idea in c general histories. But this isn't true. Claude Blair and others observed decades ago that many decorated armours appear to be fully functional - they have lance rests, glancing surfaces to resist blows, and are made impressively thick. Beyond that, many of them are not specialized tournament armours (whose form is dictated by the rules of the tournament) but adaptable armours for battle, providing a combination of mobility and all-around protection. The modern metallurgical analysis of Alan Williams confirms that many of these armours, as gilded as they are, were made of hardened steel of medium carbon content, presumably to better resist blows.

For example, The armour of the Earl of Cumberland in the metropolitan museum of art, circa 1585. A very late example of a garniture (full harness and exchangeable accessories) for field and tilt, this armour, despite the guilding and bluing, is quenched and hardened to an excess of 350 VPH (for reference, unhardened wrought iron is a bit harder than 100 VPH). Though in hindsight we think of the latter 16th century as the twilight of full armour, and though armour -was- qualitatively in decline (in general), in that very decade some commentators had blamed the death of Sir Philip Sidney in 1584 on his neglecting to wear his cuises (thigh armour) - the idea of armour as -protection- was still very much in people's minds, even as people discarded it for economic and practical reasons (the growing weight of 'proof' armour). This armour, under the bluing and gilding, was made to be able to protect the earl, even if he never ended up using it as such.

The decoration on an armour isn't simply there to look good, either. Representing one's place in the social hieararchy is an important part of being a nobleman, and an even more important part of being a king. An armour of Charles the V was a highly developed piece of early modern military hardware, but it was -also- a symbolic representation of his role as one of the great (if not the greatest) rulers in Christendom - Emperor of the Romans, King of Castille and Leon, Naples, Aragon and Sicily, etc. etc, defender of Christendom against heretics and the Turk. The oppulence of the armour demonstrated the Emperor's wealth, its religious motifs demonstrated his piety as a Christian monarch, and its heraldic motifs showed him as the legitimate heir of his ancestors and forebears.

Now, were their armours made in the 16th century for use in parades and ceremonies? There seem to be. But just being lavishly decorated isn't necessarily evidence of that.

It is perhaps best to view armour not through a lens of practical vs. decorative, but to view -all- of its elements as fulfilling a function, either for defense, or for social or political purposes

EDIT: to be more explicit about -method-:

As in the above post, I think a multidisciplinary approach is key to placing armour in context.

Analyzing the physical properties of surviving armours (metallurgy, weight, physical form) can offer hints to their purpose, as it does for the Earl's armour above--hardening, decent steel, glancing surfaces and a thickness of 2mm or more all make armour more protective, for instance. A lack of hardening or sculpted forms that weaken the metal and catch blows are less protective and could indicate an armour whose primary purpose was not protection. But even this 'scientific' evidence is uncertain. Some incredibly sculpted, embossed, unhardened Italian 16th century armours are nonetheless made of decently thick medium carbon steel. Is this because the armours were intended, at least partially, to protect, or could there be another purpose, such as ensuring that the embossed decoration was not easily damaged? It is hard to say.

Art history is essential for helping us understand the symbolism of decorative motifs and the kind of allusions an armour might be making. Does this, armour evoke classical heroes, or emphasize Christian symbolism? How do patterns an etcher uses compare to contemporary decorative schemes in other media?

Finally military history is essential to placing the armour in its context on the battlefield and the tournament field. Between 1300 and 1600 people fighting in plate armour fought in many different modes. Considering the differences between the way 15th century English and Italian men at arms fought can give context into the way their armour is designed - the copious reinforcing pieces and overlapping parts of an Italian armour might be more useful when fighting on horseback, while the smaller pauldrons and deep fault (skirt) of Enlgish armours could provide more mobility on foot. But it is easy to fall into a kind of fallacy where every stylistic difference is explained by a difference in the way different soldiers fought. I have seen it argued that the flutes if late 15th century German armour were arrow-resistant. This does not strike me as a likely purpose, given the small period of time when extravagantly fluted armours were popular, and the fairly narrow geographic region in which they were used.

So all of these approaches can have pitfalls. I think that avoiding too narrow a focus is important in avoiding tunnel vision - if I know that English armour was not heavily fluted, and yet late 15th century English knights faced a large number of arrows, I might not assume that the flutes of German armour were arrow resistant, to pick a very easy example.

Sources:

The Art of Power: Armour and Royal Portraits of Imperial Spain - Soler del Campo, Alvaro. - This catalog of the exhibition of the same name discussing the symbolic meaning of armour in Hapsburg Spain

The Knight and the Blast Furnace - Alan Williams

This lecture by Dr. Tobias Capwell of the Wallace Collection gets at the way in which 'ornament' isn't necessarily impractical, and compares the armours of kings to depictions of martial saints (Maurice, George, etc) in armour.

A question about the ordering of fields. There seem to be ongoing arguments over very ancient artifacts that come from places where, for example, Homo sapiens and Neanderthals coexisted. Many anthropologists and the historians who take their cues from them insist that cultural artifacts and particularly art must be from H. sap because Neanderthals lacked the ability to think symbolically. But this seems to be just the latest of many beliefs about Neanderthals that have turned out to be wrong. Language was always thought to be beyond them, until they were found to have the FOXP2 gene.

So are we seeing a reordering of disciplines? Geneticists such as Svante Pääbo have suggested that it's time for older, more guesswork-oriented fields (anthropology, linguistics, even archaeology) to swallow hard and accept the genome as the best available source. But most historians I know were educated at a time when these older fields were the cutting edge. Will we hang on too long to outdated paradigms? What do people think?

I'm a little intimidated by the proposed questions because "how do we know if our interpretations of an artifact are in line with the original conception(s) of that artifact?" is probably the most abiding question in archaeological theory since the 80's.

Without digressing on all of archaeological theory, I'd like to ask the historians that work with material culture (fashion historians maybe? some of the military historians? anyone else?) how they approach the topic. As a historical archaeologist I generally treat the archaeological record (so the material culture) as my primary data and use the historical record as secondary data to help provide some of that cultural context that is so often a difficulty for archaeologists to decipher from the material record alone. For the historians who are treating the documentary record as primary how do you use to artifacts to inform on the documents, rather than the other way around as I do? Just curious at some approaches.