Let's introduce a bit of conventional research before inventing diffusions of armour styles that didnt exist. I did mention the Dian bracers in the far south from the beginning but to imagine them being worn by East Zhou soldiers is only imagination.
There is an established body of literature on what makes regional styles distinct, or non-Chinese forms, and to suggest as way of a hypothesis that such Dian forms were worn by Chinese is simply invention.
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The problem here is that those survived 'armguards' have deteriorated to such a state that they cannot be convincingly argued that they were indeed armguards or anything else for that matter.
The suits of armour that have survived have infact been recreated extremely well, and if you opened any book on the subject you would see examples of this, and have a consistent story revealed about the appearance of iron and iron armour/weapons in China proper. (Wagner/Dien/Hong/Dong/Rawson etc.)
Iron lamellar armour is found in the late Warring States period. Both leather and iron armours have been found both from East Zhou and Han sites in condition to make conclusions.
Wujiang, I struggle to understand how you can deduce that metal bracers were used at all, or more common than leather armour, when none have ever been found. Ever. Fact.
You seem to have no academic or archaeological or historical basis for your conclusion that presumably the Dian culture armour styles were adopted and worn in the central plains. To make such a statement is totally without any merit, and in opposition to all that I have seen on the subject.
It seems bracers themselves are perhaps absent from the central plains since sleeved leather armours and later metal armours are used instead so the argument may be a red herring.
Excavated evidence shows East Zhou tomb figures with short lamellar sleeves (as shown in 'Chinese armour before the Tang dynasty'), there are actual finds of lamellar sleeves on leather suits & we have lamellar amour in Qin for study (some long lamellar sleeves for charioteers but no bracers).
This is why I felt there was some artistic license in the above example (the spear is a little archiac too, as well as the dhun being unusual and the belthook atypical...otherwise very good as an example of the leather armours (and colour) and basic warriors appearance).
Here is a line drawing from archaeological reconstruction (in both Dien & Hong's text); This is accurate, and based on the armour of the period that was physically found.

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we are left with the sitaution where we know definitively that there were metal armguards in areas within the reaches of the people of Zhou.
This is simply nonsensical reasoning and half true at best. Albert Dien doesnt even mention the Dian armours since they are non-Chinese and he concludes that up untill the Tang there is an entirely unique pattern of armour development in China. The Dian have their own religion, own weapons (primarily decorated battle-axes being notable) as well as useage of bronze neck & arm armour. None of these are found in ancient China.
Only in the Qin period do Chinese occupy this far south and there in NO evidence that there was an influence on Chinese culture in the East Zhou by these 'barbarian' armour and weapons style. Your idea has no basis any any artefact found, weapon or otherwise.
These were a tattooed people & outside Zhou system. They do not influence the central plains culture, but the central plain influenced them (as in Han period rock art showing a shaman wielding a ring pommeled dao and the appearance of Han weapons in graves and the disapearance of Dian styles over time).
see
http://www.chinahistoryforum.com/index.php?showtopic=13653Here is an example of a Dian style forearm armour. Nothing like this has been found in ancient China, despite the fact bronze survives uniquely well.
Hence there was none to be found.

Really, to argue in the line of reasoning that tribal people wore armour like this therefore despite the lack of any in the Zhou states we can assume they are more common than leather is simply silly. We could then suggest that Chinese wore Sythian-style scale armour since they encountered steppes warriors even though none have been found or that Xiongnu used rotating ranks of crossbowmen. Neither of these are truths either.
Just as there are no Dian swords or battle axes wielded by Yan or Qin warriors there are no Dian bronze bracers worn by these Chinese troops.
(Dian warriors BTW went into battle bare footed, another contrast with Chinese appearances and battle kit).
For a better understanding of the way Dian & central plains cultures influenced I suggest specific commentary, ie;
Early Weapons Systems and Ethnic Identity in the Sino-Vietnamese Frontier
ASPAC ‘96, Edmonton, British Columbia This is clear on the pattern of influence, as are Cheng Dong and Yang Hong.
Archaic/obselete Chinese forms persist in the south (according to 'Art & Archaeology in ancient China') as well as unique regional types exist (such as these bracers).
It is worth noting that certain types of Dian-Yunnan weapons bear a close relationship to bronze age Vietnamese swords and suggest the culture is regional/southern and equally seperate like 'Dong Son' cultural artefacts to the central plains culture.
Some swords I would consider broadly Dian are then labelled 'Vietnamese style' in private collections.
Yang Hong, the other notable commentator on ancient armours only mentions the Dian bracers in a seperate chapter to the east Zhou armours which mentions peripheral cultures and so makes no connection in the development of East Zhou.
...he calls them {Dian} '
minority' armours {in the same way as Ba culture is non-Chinese etc.} and is clear these are from Yunnan and a seperate non-Chinese culture. for example like the Liaoning culture swords of the far North are unrelated to Chinese weapons and eventually these people were subdued and took on Chinese ways (the swords of their culture disapear and Chinese style ge-dagger-axes are found instead in excavations).
Non-Chinese cultures typically adopt Chinese ways, not the other way around. The exceptions to the rule are the influence from the steppes but forget a Dian king tomb find in Yunnan as evidence that contemporary Zhou states wore armour like this too. That is counter-intuitive and devoid of evidence.
Between Dien & Hong they both say;
Dien; ‘
metal armour began to appear in the late Warring States period’ Hong; there is an“
unevenness of production of iron & steel weapons in China’ illustrates the variable quality of iron weapons and armour plates at the time. He emphasizes that apart from finds of bronze helmets and limited iron examples all the other excavated East Zhou armours are leather. He then mentions specific finds of iron weapons and armour as well as noting historical records of iron armed states, i.e Chu, Han, Zhongshan, (but not Qin which used some iron for tools but not weapons).
Cast iron (white iron) was common but Yang Hong's metallurgical analysis of iron armour of the late Warring States period shows it is of variable quality and still in its infancy. This agrees with Donald Wagners 'Iron & Steel in ancient China' as well a commentary on East Zhou iron at the Shaanxi museum by Mr. Wang.
Examples of iron armours are shown in several books. They are all lamellar armours, even in the post Zhou period.
If bracers were to be worn (and perhaps they weren't based on any evidence so far) then the construction would lend itself to leather, as well as the preponderance of armour materials would make leather the likely availible material.
Note; even in the early Han period leather armour is still being used (based on Korean site finds) and not untill bronze weapons are replaced under the expanded iron industry are even iron weapons universally used over bronze. Clearly leather armour was much more important in the preceding Zhou period.
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The leather lamella helment you talked about were a minority {compared to bronze helmets}
This is simply nonsense even if you state it with authority. The finds of bronze helmets are listed from certain sites by Yang Hong and he make the completely opposite conclusion (emphatically) very clearly in his section on East Zhou armour with regards to the importance of leather armour and the rarity of bronze helmets. There are literally only a small sample of bronze helmets to study, and with the survivabilty of bronze this means there were only limited numbers placed in Zhou tombs. At the time of printing of 'Weapons in Ancient China' there were only 2 bronze helmets from West Zhou sites known of, and a handful from East Zhou sites. Don't let artwork in Manga comics fool you. Bronze swords and dagger-axes exist in huge numbers but bronze helmets are very rare objects. The Marquis of Zeng for example had several suits of leather and leather helmets in his one tomb.
Depictions of helmets in East Zhou art suggest a portion of all troops had head gear implying leather was used and hence the scarcity of surviving period armour. Armour ornaments of bronze, which attach to leather armour in Zhou times (as per Cheng Dong & Yang Hong) exist in great numbers & some variation yet bronze helmets are very rare objects.
From the earliest times these bronze helmets were remarkable items for high ranking people.
The idea of leathers importance is basically in consensus with the other sources I mentioned, and the fact the stone lamellar helmets & horse barding at the Qin tomb site are found in the absence of any evidence the Qin had iron armour (or even iron weapons) and hence all Qin armours are likely leather too. (as per Albert Diens paper & J. Rawson and other authors)
Although there are rather rare finds of bronze helmets from the Shang to the East Zhou, and of course I am aware of this since I have mentioned it many times on CHF and posted pictures, at the same time there is no bronze body armour mentioned in an East Zhou context in any publication. This is very clear.
The fact the Chinese didnt cast iron helmets is a little odd, but it is still a fact nonetheless (even in the Han period). Bronze may have had an appeal & lustre iron didnt to justify some officers wearing it. When iron appears it is constructed in another fashion.
The iron helmets are therefore lamellar, and there are no cast bronze bracers or iron bracers in the Zhou-state armies either.
If you want to see what leather helmets and armour look like then there is this (one of several armours found).
They are clear enough to show that the form of armour in leather was also lamellar. The sleeves are clear. The helmet is clear. Most armour of the period was leather (as revealed by decayed traces) and a few allow for reconstruction.
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I would ask you to provide a single piece that is intacted enough for us to agree what it really was.
That you havent been aware of this particular tomb find suggests to me you haven't properly absorbed the present literature and research on ancient Chinese armours. It is from the Marquis' tomb I mentioned earlier.

Many faint traces of leather armour have been found, or occasionally lacquered outer skins from decayed armour, (specific tomb examples named by Yang Hong) but metal armour is much more rare and appears late, even though the traces of surviving metal items would be much clearer had they been there to be found (iron or bronze).
A paradox?
It is only if you choose to believe the opposite of what evidence reveals.
Evidence shows that bracers of bronze are worn by southern non-Chinese peoples in areas outside of the central plains. None occur in ancient China itself since it has a seperate tradition.
Leather is more common in the Zhou-era, and metal body armour arrives late, and as a lamellar form in the central plains, and this a uniquely internal Chinese development.