Roman helmets: Imperial Gallic/Italic and Ridge - comparisons and sources

[CaesarAugustus]

I will repeat what I have already written, and you will tell me if this is a general argument or not:

As a matter of fact, the helmet purpose is understood as head protection, against penetration and shock/brain injuries. Thus, the purpose of protective helmets is to prevent head injury by decreasing the amount of impact energy that reaches the head, reducing the severity or probability of injury. So, the quality of the helmet is given by the ability to reduce the severity or probability of injury.

Besides resistance to penetration, the helmet is the initial shock absorber. We can highlight as follow the helmet responsabilities:
- spreading the impact load over a large area of the helmet, therefore reducing the concentrated stresses during an impact that
reaches the head and increasing the amount of energy absorbed, by having a larger area of effective energy absorbing liner;

- prevent helmet penetration by pointed or a sharp object that might otherwise puncture/penetrate the skull;

- absorbing the initial shock in an impact.



We can consider that ridged and italic cover more or less the same area (we can ignore for a while that the italic offer more protection for the neck), so, more or less, the defense against the penetration is the same. But, there is a little problem near the ridge. The ridged is made by flanked pieces, fixed by rivets. This means that near the ridge there is discontinuity and the impact is not distributed equally in different direction. So, the possibility that the metal is bent between rivets is higher than the probability to bend the metal of an italic helmet.

And, problem related, this discontinuity obstruct a correct spread of the shock on a larger area. One think is the ability of an italic helmet to spread the blow on the entire helmet surface, other is the the flanked pieces that circumscribe the impact area on a smaller area, thus reducing the amount of energy absorbed, that is leaving a greater energy to reach the skull, and finally the brain.


This is coming from Materials science theories. You can test what I have written, for example simulating it. Or you can arrange some tests. Or you can go to a technical forum and propose the problem. Here an example of reply:
Discontinuous materials don't transmit forces. They need to be connected somehow and this connection is almost always poorer than a contiguous piece of material. If you take your medieval helmet and make it also monoblock, say via 3D printing the entire thing in one piece, you'd still have a better helmet still than made by joining two plates. Continuity is a prerequisite for forces to transmit from one molecule onto another molecule at the molecular level. This is purely material science.

If you will find a better technical explanation, I will be happy to learn. Personally, I am here to learn.

And consider that I perfectly agree with Renatus, and at the same I continue to do not understand how is possible to liquidate as "nonsense" Goldswothy thesis.

PS a similar approach was known since long time. The spangenhelms was already employed by nomadic races from beyond the Danube, such as the Sarmatians and the related Alans, and depicted in the Trajan's Column.

[CaesarAugustus]

Just to close (if you will finally agree, but I doubt ), there is also a really interesting paper, Evidence from Dura Europos for the origins of late roman Helmets, from Simon James, that de facto confirms what I thought. I suggest you to read it all. 
Roman Spangenhelme therefore preceded rather than evolved from the ridge helmet family. Spangenhelme was known from long time (at least from I century), and used also from Roman Auxiliary men (e.g. archers), while the legionaries continued to use the classic helmets.

It is something to underline: the Spangenhelme was adopted despite the fact that was an helmet that generally was used by auxiliary men, and archers, not from the first line men, and not from the most paid ones.

So, if you are still thinking to "why", hereafter the conclusion from the paper:

It seems that the 'traditional' Imperial cavalry and infantry types were displaced at an unknown rate and to an unknown (but large) degree by all-iron Spangenhelme in the second half on the third century. [...] However, these in their turn were already at least partly displaced by new, bipartite ridge helmets by the 320s, as archaeology demonstrates. The latter then seem to become the sole tradition of helmet construction used in the empire throughout the rest of the fourth century. That these bipartite helmets were of Partho-Sassanian origin now seems beyond reasonable doubt in the light of the Dura evidence. However, an important question remains to be addressed. It is not at all obvious why a very long established and highly developed tradition of helmet design was suddenly abandoned to be replaced by transdanubian Spangenhelme and then by orientalising ridge helmets.
Klumbach, who follows Alfôldi in attributing the dissemination of ridge helmets personally to Constantine, sees the employment of Persian-style helmets as part of the orientalisation of court ceremonial and the dress of emperors, officials and state servants, from the time of the Tetrarchy. No doubt this is part of the explanation; one has only to look at Berkasovo I to believe it. However, it seems to me that it is not the barbaric splendour of some of the helmets that is of deepest significance, but the simplicity of design which is common to all, resulting from the elimination of all components requiring high levels of skill to manufacture. Complex forgings such as hinges and one-piece bowls with integral neck guards were excised from the new designs. The simple components of the ridge helmets could be made by relatively unskilled and inexperienced smiths. The downright crudeness of a number of examples suggests that they often were. Even the finest pieces hardly match up to second century standards of construction.
Not only, then, was there a complete change in the design of Roman helmets. There was a simultaneous decline in standards of manufacture. These changes must be seen against the historical background of contemporary developments in the army and
the armaments industry itself.
Little is known for certain about the organisation of the industry which produced the fine helmets of the second and early third centuries AD58. It is assumed that individual craftsmen or small private companies supplied the troops, probably through a variety of mechanisms including commissions from individual soldiers and larger multiple orders from regiments, provincial army commands or the central
government. Whatever the case, presumably these were cash transactions. Armourers will have been paid in specie for their work, and used the coin to cover raw materials and overheads, pay their taxes and support their families. They were particulary dependant on the soundness of the currency as laws controlling sales and possession of armaments restricted their market almost exclusively to the state. The collapse of the coinage from the middle of the third century would have paralysed this system of supply. The army could not afford to buy the weapons, while the armourers could not sell their wares nor buy raw materials. The operating system of the industry,
which had endured for several centuries, collapsed.
The development of a crisis in arms procurement is, in my opinion, the direct cause of the establishment of the state arms factories, or fabricae, which start to appear under the Tetrarchy. It is suggested that from the 260s the state was forced to bypass the
financial crisis and started to maintain the armourers directly, by providing rations and security in return for product, leading to the gradual absorption of the armourers into the Imperial service. This process reached its logical conclusion when Diocletian put it on a regular basis and built new factories to accommodate (and control) them at strategic points across the Empire. It seems that the state wanted quantity production, not fancy quality, hardly surprising when faced with the task of supplying an expanding army suffering high rates of attrition, as Diocletian's surely was. The armourers, now called fabricenses, probably had monthly quotas to fulfill. Against this background, the history of helmet design becomes explicable. Traditional Roman types ceased to be made with the hypothetical rapid collapse of the old industry in the third quarter of the third century. The state, now directly supporting the armourers, naturally wanted cheap, functional and above all quick-toproduce designs. Perhaps the initial result was the general adoption of Danubian type Spangenhelme by the armourers supplying the Illyrian cavalry force of Gallienus, Aurelian and Probus. These troops, many of whom were barbarians from across the Danube, formed the elite of the Tetrarchic armies. Hence the Spangenhelme of Galerius' household troops. Diocletian, who reorganised the army and founded the new arms factories, is most likely to have been responsible for the introduction of the new bipartite ridge helmets. There seems little reason for attributing their dissemination to Gonstantine, as the Berkasovo finds show that they were already established in Licinius' army perhaps as early as AD 314. It is very tempting to link their appearance with the building of the new factories, and to see the opening of the fabricae as both the opportunity and the reason for the introduction of the new, and definitive ridge helmet types. While their Eastern inspiration well in tune with the Tetrarchic switch from Illyrian austerity to oriental splendour, it seems to me that the main motivation was more practical. The Partho-Sassanian prototypes met the requirement for simplicity of manufacture, but were substantially redesigned to meet Roman standards of protection. Hence the addition of plate neck- and cheek guards to all versions. Similarly, the types with a separate brow band, whether this feature was of Danubian or Persian origin or both, were revised. The composite skull was fitted to the outside of the brow band rather than the inside, improving protection by increasing the clearance between the plates and the wearer's head. On the other hand, elimination of hinges and other difficult forgings made them suitable for rapid mass production by even a semi-skilled workforce. The new designs betray much careful thought, as does the distribution of the factories in which they were made. Much more than simply a whim of fashion, the appearance of new style helmets was a result of the 'nationalisation' of the arms industry at the end of the third century. This reinterpretation of the development of Imperial helmet design suggests that there was no simple unilinear sequence. It was part of a much wider network involving several cultural groupings, all with their own traditions of helmet construction, all of which to a greater or lesser degree influenced the other. Thus early Imperial helmets evolved from various currents of Hellenistic, Italian and Gallic design. If radial helmets came to Rome from the Danube and ridge helmets from across the Euphrates, then the Eastern European and Iranian peoples who transmitted them may well have influenced each other via the nomads of Central Asia. Further, the interaction between Rome and her neighbours was not necessarily unidirectional. The reinforcing plate down the front of the Dura helmet appears to owe its inspiration to Roman prototypes of the second and early third centuries! Evidently, we are dealing with a complex web of influences operating over prolonged periods.


You can read in that paper all the points that I have already written you in these pages. Diocletian's responsability, quantity over quality and quality decline, the influence of the collapsing economy, and the fact that we are not talking about something new.

[Condottiero Magno]

A paper from 1986 and doesn't cover the superiority of single piece vs multi part helmets, nor does it mention that some of these ridge helmets were better decorated than Gallic/Italic ones! (Bishop & Coulston Roman Military Equipment pp 210-216)

Ridge helmets existed side by side with Niederbieber/Heddernheim type helmets. While spangenhelms are depicted on Trajan's Column, they weren't displaced by ridge helmets, but in fact displaced helmets of bipartite construction by the 6th Century.

[Dan Howard]

Solid bowl helmets do provide better protection than a multi-piece helmet of similar weight - if they are made to the same standard. But they also require substantial more work and skill to create and they are much harder to repair. If you want similar protection from a multi-piece helmet, all you need to do is increase the weight by a fraction. I'm not sure how replacing a (say) 2.5 pound helmet with a 3 pound helmet is evidence of some kind of collapse - especially if the multi-piece process allows the factory to turn out two-three times more helmets for the same cost and production time as a solid-bowl helmet. It suggests to me that they streamlined the process to enable them to equip far more men than previously with more uniform and standardised gear, which is the opposite of a collapse. Enforcing standards and having strict quality control will result in a better multi-piece helmet than a single-bowl helmet that was made without those procedures in place. Sticking with single-bowl helmets would slow down production, increase costs, and make helmets harder to repair, for nothing more substantial than a tiny reduction in a soldier's load.

[CaesarAugustus]

A paper from 1986 and doesn't cover the superiority of single piece vs multi part helmets, nor does it mention that some of these ridge helmets were better decorated than Gallic/Italic ones! (Bishop & Coulston Roman Military Equipment pp 210-216)

Ridge helmets existed side by side with Niederbieber/Heddernheim type helmets. While spangenhelms are depicted on Trajan's Column, they weren't displaced by ridge helmets, but in fact displaced helmets of bipartite construction by the 6th Century.

That superiority is covered by material science, which laws are the same of 2 thousands years ago. For the decoration, we are talking about the protection, this topic is about protection, if you want to discuss about the decoration, please open another topic.

And, spangenhelms helmets and ridge helmets share the same concept, flanked parts united by rivets. It is the same principle, and is subject to the same physical laws and structural issues.

For the year, a valid point is always valid, regardless the year. And, in fact, that paper is far better than comments that are written in these days.

Finally, there is the reply to the ones that are arguing that Romans have adopted the new helmet when they have seen that. This is not true and is witnessed by the adoption of the spangenhelms, that was really well known at least from the first century, but not used before the III century, apart from auxiliary corps. Instead, in the III century Romans adopted it widely. At the same way, Romans well known the Ridge, at least from the Shapur I campaigns, but they did not adopted it for long time, preferring to continue to use they better helmets, the italic one.

---

Solid bowl helmets do provide better protection than a multi-piece helmet of similar weight - if they are made to the same standard. But they also require substantial more work and skill to create and they are much harder to repair. If you want similar protection from a multi-piece helmet, all you need to do is increase the weight by a fraction. I'm not sure how replacing a (say) 2.5 pound helmet with a 3 pound helmet is evidence of some kind of collapse - especially if the multi-piece process allows the factory to turn out two-three times more helmets for the same cost and production time as a solid-bowl helmet. It suggests to me that they streamlined the process to enable them to equip far more men than previously with more uniform and standardised gear, which is the opposite of a collapse. Enforcing standards and having strict quality control will result in a better multi-piece helmet than a single-bowl helmet that was made without those procedures in place. Sticking with single-bowl helmets would slow down production, increase costs, and make helmets harder to repair, for nothing more substantial than a tiny reduction in a soldier's load.

Monoblock helmets require substantial more work and skill to create and they are much harder to repair: true, for sure.
Solid bowl helmets do provide better protection than a multi-piece helmet of similar weight - if they are made to the same standard: basically true.
If you want similar protection from a multi-piece helmet, all you need to do is increase the weight by a fraction: to be proved what is this fraction. And, not negligible factor, if you increase the weight you increase the probability that the head and/or the neck will suffer for trauma in case of impacts, it is not just a tiny reduction in a soldier load. You can reduce weight by reducing the covered area, for example by removing the neck protection, but you're protecting less, so the chances of being hit / injured / killed increase.

The monoblock increase costs, the helmet is harder to repair, there are less people that can do it and so on? Yes, for sure, is what I am already telling.
The point is that previously the empire was able to offer this. Give me the exaggerated comparison, but give the idea: if you have a society that can support a car, you will drive a car, despite the fact that a car is far more expensive than a wagon, it is harder to repair, require skilled mechanical, skilled workers to make it and so on. This is an exaggerated comparison, but it makes the idea.

Following the third century crisis, the society was not any more able to support some quality products. The answer from the imperial infrastructure has been to renounce to this quality.

[Nathan Ross]

Solid bowl helmets... also require substantial more work and skill to create and they are much harder to repair... Enforcing standards and having strict quality control will result in a better multi-piece helmet than a single-bowl helmet that was made without those procedures in place.

That sounds about right.

Going back to Michael's 'Rolls Royce v. Ford' analogy, which is along the same lines, we might compare the old gallic/italic and the new ridge models in the same way:

Principiate: individually hand-made helmets, quality varies considerably, some parts (ie hinges) fiddly and easily broken, and hard to repair or replace. Bowl rather weak and requires cross bracing.

Dominate: factory-produced helmets. Production faster and more frequent. 'Fiddly bits' replaced by simpler straps and buckles. Some models have better facial protection (nasals) and larger cheek pieces. Two-part bowl possibly thicker metal, ridge possibly creates a weak point, but the amount of force needed to break the helmet would probably be fatal to the wearer anyway!

Result: some principiate helmets may have been of better quality overall than dominate models, but many were not. Most ridge helmets, on the other hand, were entirely effective and did what they needed to do, besides being available in far greater numbers. This is why they came to dominate later arms production for c.150 years.

[CaesarAugustus]

Solid bowl helmets... also require substantial more work and skill to create and they are much harder to repair... Enforcing standards and having strict quality control will result in a better multi-piece helmet than a single-bowl helmet that was made without those procedures in place.

That sounds about right.

Going back to Michael's 'Rolls Royce v. Ford' analogy, which is along the same lines, we might compare the old gallic/italic and the new ridge models in the same way:

Principiate: individually hand-made helmets, quality varies considerably, some parts (ie hinges) fiddly and easily broken, and hard to repair or replace. Bowl rather weak and requires cross bracing.

Dominate: factory-produced helmets. Production faster and more frequent. 'Fiddly bits' replaced by simpler straps and buckles. Some models have better facial protection (nasals) and larger cheek pieces. Two-part bowl possibly thicker metal, ridge possibly creates a weak point, but the amount of force needed to break the helmet would probably be fatal to the wearer anyway!

Result: some principiate helmets may have been of better quality overall than dominate models, but many were not. Most ridge helmets, on the other hand, were entirely effective and did what they needed to do, besides being available in far greater numbers. This is why they came to dominate later arms production for c.150 years.

Again, no, non and no. I understand that your points are in crisis, but you have to read

The point is not just the breakage, here what you have to think about:
As a matter of fact, the helmet purpose is understood as head protection, against penetration and shock/brain injuries. Thus, the purpose of protective helmets is to prevent head injury by decreasing the amount of impact energy that reaches the head, reducing the severity or probability of injury. So, the quality of the helmet is given by the ability to reduce the severity or probability of injury.

Besides resistance to penetration, the helmet is the initial shock absorber. We can highlight as follow the helmet responsabilities:
- spreading the impact load over a large area of the helmet, therefore reducing the concentrated stresses during an impact that
reaches the head and increasing the amount of energy absorbed, by having a larger area of effective energy absorbing liner;

- prevent helmet penetration by pointed or a sharp object that might otherwise puncture/penetrate the skull;

- absorbing the initial shock in an impact.

As a matter of fact, smaller plates implies less energy dispersion capacity, so the probability of injuring increases. Material science.

And as we can read in the paper, it is exactly the opposite of what you have written:
Klumbach, who follows Alfôldi in attributing the dissemination of ridge helmets personally to Constantine, sees the employment of Persian-style helmets as part of the orientalisation of court ceremonial and the dress of emperors, officials and state servants, from the time of the Tetrarchy. No doubt this is part of the explanation; one has only to look at Berkasovo I to believe it. However, it seems to me that it is not the barbaric splendour of some of the helmets that is of deepest significance, but the simplicity of design which is common to all, resulting from the elimination of all components requiring high levels of skill to manufacture. Complex forgings such as hinges and one-piece bowls with integral neck guards were excised from the new designs. The simple components of the ridge helmets could be made by relatively unskilled and inexperienced smiths. The downright crudeness of a number of examples suggests that they often were. Even the finest pieces hardly match up to second century standards of construction.
Not only, then, was there a complete change in the design of Roman helmets. There was a simultaneous decline in standards of manufacture. These changes must be seen against the historical background of contemporary developments in the army and the armaments industry itself.
Little is known for certain about the organisation of the industry which produced the fine helmets of the second and early third centuries AD. It is assumed that individual craftsmen or small private companies supplied the troops, probably through a variety of mechanisms including commissions from individual soldiers and larger multiple orders from regiments, provincial army commands or the central
government. Whatever the case, presumably these were cash transactions. Armourers will have been paid in specie for their work, and used the coin to cover raw materials and overheads, pay their taxes and support their families. They were particulary dependant on the soundness of the currency as laws controlling sales and possession of armaments restricted their market almost exclusively to the state. The collapse of the coinage from the middle of the third century would have paralysed this system of supply. The army could not afford to buy the weapons, while the armourers could not sell their wares nor buy raw materials. The operating system of the industry, which had endured for several centuries, collapsed.
The development of a crisis in arms procurement is, in my opinion, the direct cause of the establishment of the state arms factories, or fabricae, which start to appear under the Tetrarchy. It is suggested that from the 260s the state was forced to bypass the
financial crisis and started to maintain the armourers directly, by providing rations and security in return for product, leading to the gradual absorption of the armourers into the Imperial service. This process reached its logical conclusion when Diocletian put it on a regular basis and built new factories to accommodate (and control) them at strategic points across the Empire. It seems that the state wanted quantity production, not fancy quality, hardly surprising when faced with the task of supplying an expanding army suffering high rates of attrition, as Diocletian's surely was. The armourers, now called fabricenses, probably had monthly quotas to fulfill. Against this background, the history of helmet design becomes explicable. Traditional Roman types ceased to be made with the hypothetical rapid collapse of the old industry in the third quarter of the third century. The state, now directly supporting the armourers, naturally wanted cheap, functional and above all quick-toproduce designs. Perhaps the initial result was the general adoption of Danubian type Spangenhelme by the armourers supplying the Illyrian cavalry force of Gallienus, Aurelian and Probus. These troops, many of whom were barbarians from across the Danube, formed the elite of the Tetrarchic armies. Hence the Spangenhelme of Galerius' household troops. Diocletian, who reorganised the army and founded the new arms factories, is most likely to have been responsible for the introduction of the new bipartite ridge helmets. There seems little reason for attributing their dissemination to Gonstantine, as the Berkasovo finds show that they were already established in Licinius' army perhaps as early as AD 314. It is very tempting to link their appearance with the building of the new factories, and to see the opening of the fabricae as both the opportunity and the reason for the introduction of the new, and definitive ridge helmet types. While their Eastern inspiration well in tune with the Tetrarchic switch from Illyrian austerity to oriental splendour, it seems to me that the main motivation was more practical. The Partho-Sassanian prototypes met the requirement for simplicity of manufacture, but were substantially redesigned to meet Roman standards of protection. Hence the addition of plate neck- and cheek guards to all versions. Similarly, the types with a separate brow band, whether this feature was of Danubian or Persian origin or both, were revised. The composite skull was fitted to the outside of the brow band rather than the inside, improving protection by increasing the clearance between the plates and the wearer's head. On the other hand, elimination of hinges and other difficult forgings made them suitable for rapid mass production by even a semi-skilled workforce. The new designs betray much careful thought, as does the distribution of the factories in which they were made. Much more than simply a whim of fashion, the appearance of new style helmets was a result of the 'nationalisation' of the arms industry at the end of the third century. This reinterpretation of the development of Imperial helmet design suggests that there was no simple unilinear sequence. It was part of a much wider network involving several cultural groupings, all with their own traditions of helmet construction, all of which to a greater or lesser degree influenced the other. Thus early Imperial helmets evolved from various currents of Hellenistic, Italian and Gallic design. If radial helmets came to Rome from the Danube and ridge helmets from across the Euphrates, then the Eastern European and Iranian peoples who transmitted them may well have influenced each other via the nomads of Central Asia. Further, the interaction between Rome and her neighbours was not necessarily unidirectional. The reinforcing plate down the front of the Dura helmet appears to owe its inspiration to Roman prototypes of the second and early third centuries! Evidently, we are dealing with a complex web of influences operating over prolonged periods.

It is exactly the opposite: Even the finest pieces hardly match up to second century standards of construction.

That you like or not

[Dan Howard]

Following the third century crisis, the society was not any more able to support some quality products. The answer from the imperial infrastructure has been to renounce to this quality.

Utter rubbish. Fourth century equipment and metallurgy was the best the Romans ever had.

[CaesarAugustus]

Following the third century crisis, the society was not any more able to support some quality products. The answer from the imperial infrastructure has been to renounce to this quality.

Utter rubbish. Fourth century equipment and metallurgy was the best the Romans ever had.

The simple components of the ridge helmets could be made by relatively unskilled and inexperienced smiths. The downright crudeness of a number of examples suggests that they often were. Even the finest pieces hardly match up to second century standards of construction.

Not only, then, was there a complete change in the design of Roman helmets. There was a simultaneous decline in standards of manufacture. These changes must be seen against the historical background of contemporary developments in the army and the armaments industry itself.
[...]
The collapse of the coinage from the middle of the third century would have paralysed this system of supply. The army could not afford to buy the weapons, while the armourers could not sell their wares nor buy raw materials. The operating system of the industry, which had endured for several centuries, collapsed.
The development of a crisis in arms procurement is, in my opinion, the direct cause of the establishment of the state arms factories, or fabricae, which start to appear under the Tetrarchy. It is suggested that from the 260s the state was forced to bypass the
financial crisis and started to maintain the armourers directly, by providing rations and security in return for product, leading to the gradual absorption of the armourers into the Imperial service. This process reached its logical conclusion when Diocletian put it on a regular basis and built new factories to accommodate (and control) them at strategic points across the Empire. It seems that the state wanted quantity production, not fancy quality, hardly surprising when faced with the task of supplying an expanding army suffering high rates of attrition, as Diocletian's surely was. 

This is coming from a professor that is still in activity, has studied the excavations of dura europos from which is coming our knowledge about the ridge helmet, and has a resume that you can easily find on internet. This largely defines your sentences free and, as seen so far, incorrect.

[Nathan Ross]

Result: some principiate helmets may have been of better quality overall than dominate models, but many were not. Most ridge helmets, on the other hand, were entirely effective and did what they needed to do, besides being available in far greater numbers. This is why they came to dominate later arms production for c.150 years.

Again, no, non and no. I understand that your points are in crisis,

I'm not sure what point you are disputing here!

Do you believe that all principiate helmets were of better quality?

Or that most ridge helmets were ineffective?

Or that they did not 'dominate later arms production'?

but you have to read

Your 'scientific' theories are unfortunately entirely non-objective and based on opinion alone, as we have no accurate tests available for measurement!

I have indeed read Simon James's 1986 paper - some years ago, in fact - together with his 2004 report on the military equipment from Dura Europos and his 2011 book Rome and the Sword.

His older paper has been highly influential, but more recent work undermines his conclusions somewhat. In particular the redating of the Leiden and Deir el Medineh helmets to the later 5th or 6th centuries, rather than the late 3rd, counts against a theory that they replaced the gallic types before being replaced by the ridge.

There have been a number of later helmet finds since 1986 as well - notably the Koblenz group of 12 (published 1988, Miks) and the Jarak example (2011). There's also been better study of the older finds - most of the helmets from Intercisa have been found to have been originally plated with silver (Coulston, in Sarantis, ed 2013), while other helmets are only known from the stripped gold sheathing - the 'crude' appearance of many of these helmets is often due to only the basic iron core being left behind.

In his 2011 work, James makes no reference to these helmets being 'crude' or ineffective - in fact he refers to some 'splendid examples [which] look more like something out of Arthurian epic or Wagnerian opera than our expectations of Roman military equipment' (Rome and the Sword p.238).

As I've said before, the continuance of these helmets over a long period, their frequent costly decoration (and, as James points out, their apparent use by emperors, who would surely have used the best gear available!) counts against them being in any way 'ineffective' or 'inferior' to previous models in all but stylistic aspects.

[CaesarAugustus]

Again, it is not subjective but objective, it is science material. You can take whatever material science book and you will find that the greater is the area without discontinuity, the greater will be the ability to dissipate the power, so to protect the head, that is what is measuring the quality on an helmet. This is measuring the quality of the helmet, not the decorations. You can create a thread with a poll for the better decoration, here the topic concerns the ability to protect.

And again, you have asked why, the reply is simple and well given. And all the collected points go in the same direction: not only, then, was there a complete change in the design of Roman helmets. There was a simultaneous decline in standards of manufacture. These changes must be seen against the historical background of contemporary developments in the army and the armaments industry itself. And this has lead to helmets that were not matching the standards of the second century.
And again, the usage of the Spangenhelme is well attested in this period, and it was used at least after two centuries that it was already known. Why? Because better? No way. Simply for the disastrous conditions of the empire. It is really simple and is a point that you cannot deny, even if you are trying with dialectics.

Both the points, the inductive and the deductive one converge. And this is well reflected by the disastrous management of the borders of the empire followed Constantin. Already with the sons, the borders were broken on the Rhine and in the East. Very far from being a florid situation.

[Nathan Ross]

Again, it is not subjective but objective, it is science material.

I think any scientist would admit that without an object of study and the ability to accurately measure and compare it, there can be no 'science material'.

Romans, on the other did, did have both types of helmet - and they chose the ridge type, for various reasons. But unless we believe that they were poor, or disorganised (and they were clearly neither) or suffering from some mass outbreak of stupidity or incompetence, there would be no logical reason for them to mass produce an ineffective helmet, and go on doing so for over a century. That is the historical fact from which this discussion must proceed.

There was a simultaneous decline in standards of manufacture.

One aspect of scholarship in the last 20 or 30 years has been a considerable reinterpretation of what 'decline' is, and how we might measure it.

the usage of the Spangenhelme is well attested in this period

By the regular Roman army? I don't think so. Suggested, maybe, but not attested. No helmet of this type has been found in a military context securely dated before the 6th century.

you are trying with dialectics.

I'm afraid I don't know what you mean by that!

the borders were broken on the Rhine and in the East. Very far from being a florid situation.

[Edit] - we should also consider that the borders collapsed massively in the 3rd century during the 'crisis' period. The Roman armies that were annihilated at Abrittus and Barbalissos, and who let the barbarians devastate Gaul and Thrace, were equipped on the principiate model with single-bowl helmets.

Twenty years later, Diocletian had rebuilt the Roman army and it was entirely effective in defeating every enemy that Rome faced on every front. He restored the frontiers, and twenty years later still Constantine expanded them by reconquering part of Dacia.

On a performance-analysis level, the army of the dominate beats the army of the later principiate hands down!

[CaesarAugustus]

Come on, read what I have written. It is a general law, it is science material. You can take whatever material science book and you will find that the greater is the area without discontinuity, the greater will be the ability to dissipate the power, so to protect the head, that is what is measuring the quality on an helmet. In case of discontinuity you block this ability, so you increase the probability that the soldier is injured. And this is the reason for which they are doing monoblock helmets now.

Then, Spangenhelme and other helmets worn by archers on Trajan's Column. So, Romans perfectly knew it and they have not adopted for legionaries for centuries. It is useless that you try everytime to speak of other, this was well known and not used for long time. And there is independent evidence that Spangenhelme very like the Der-el-Medineh type were used by Roman troops, apparently cavalry, around AD 300. The reliefs on the Arch of Galerius at Thessalonika show this quite clearly. Then, this has been abandoned for a while and the ridge has been distributed in a homogeneous way. Probably, it was the compromise between the Spangenhelme and the II century mono-block design. So, as you an see, the Spangenhelme was adopted despite it has been discarded for long time.

All is pointing to the same direction, II century helmets provided a better protection.

[Xenophon]

Hey everyone! Long time lurker, but if registering and jumping on a year old forum and 'beating a dead horse' is not the most fashionable thing to do I don't know what is .
I am admittedly biased and surprised by the flak CaesarAugustus has received regarding this problem and frankly a little amused at the defensiveness for a long dead civilization. I am by no means a Roman historian and hardly qualify to even an Armchair level of expertise; however, I have engineering degree and figured I'd chime in.
If we were forced to physically test every problem we came across, I believe engineers would make oodles less money then they do. It is a science by its own right based off of "assumptions" and "educated guesses" to a certain precision. If I drop a ball, I assume rightfully that 100% of the time gravity is going to make it fall. I think CaesarAugustus accurately provides the structural science to a point almost perfectly; it makes perfect sense that a ridged helmet would be inferior in structural strength to a singular bowl (generally speaking). Welding vs Casting for Tanks comes to mind as a suitable analogy; casting turrets maintains a superior structural integrity as oppose to welding, as atomically the fused pieces extend across the whole cross section that would be welded on only either side of the cross section (surface area vs sides). Yet, we see both trended actively in the Second world war, in some cases even for the same model tank (t-34). It would be totally possible to throw some models of a Ridged helmet and a suitable Italic/Gallic counterpart into a FEM program, maybe Solidworks etc, and simulate a Impact force on multiple points of either helmet, but I think it is a totally fair assumption that the ridged helmet would lose out at some point (who knows though, it may be well above any impact force a human could produce reasonably). It is not necessary to test this case.

Why would Rome arm their troops with "Inferior" helmets? This terminology is poisonous to the discussion. Why did Rome arm their troops with "less good" helmets is better. I think it follows suit, why would the Soviets weld tanks instead of cast them? Why did the Germans migrate towards the MG-42 instead of the MG-34?
These could be subjects of discussions in their own right, and my simplification is nearly sin, but they are all heavily associated with cost. Personally, I believe it is for this reason the Lorica segmentata seemed to have phased out too (mail is so standardized in comparison), though this is a can of worms I am ill equipped or educated to handle . It seems rather clear to me that this is a primary reason for the Ridged helmets appearance and adoption, because they were good enough at a lower cost. Not inferior, just good enough.

It is cheaper in fact to strap 2-4 hemispherical pieces of metal together than to first shape and anneal repeatedly into a full helmet. I am unfamiliar with the Roman construction, but it appears the Italic/Gallic types were fashioned in a similar way to the shaping and annealing methods the greeks used. However, afterwards with the adoption of Ridges helmets, "...helmet-bowls were designed out of two or more different pieces of iron, allowing for increased rigidity and a simplified manufacturing process"(Source below, pg 57). Note, not for increased strength. As for the usage of decorations as a indicator of the wealth of the Romans at the time; Shakos and Pickelhaubes are extremely ornate AND useless. The correlation cannot be mistaken for causation.
Instead of this being the real issue of discussion, the rather repetitive issue in this forum is whether or not the Army of Rome during/after the third century is up to snuff with the expectations we have of them. I simplify it ten fold, but I assume with the title "Crisis of the third century", the luxury of cost was seldom on the minds of those arming troops. That seems to be the real issue.
My apologies for mistakes/inadequacy speaking to anyone's specific points.
Source for more reading and used above: 
(HISTORICAL EVOLUTION OF ROMAN INFANTRY ARMS AND ARMOR,
https://web.wpi.edu/Pubs/E-project/Avail..._ARMOR.pdf)

[Crispianus]

"It would be totally possible to throw some models of a Ridged helmet and a suitable Italic/Gallic counterpart into a FEM program, maybe Solidworks etc, and simulate a Impact force on multiple points of either helmet, but I think it is a totally fair assumption that the ridged helmet would lose out at some point (who knows though, it may be well above any impact force a human could produce reasonably). It is not necessary to test this case."

Well that really is the crux of the matter, all things being equal, the probem is they arn't.
At the time as we would understand it there was no standardisation (this doesn't actually occur untill well into the industrial age) in material thicknesses, composition, strengths/ hardness / toughness etc, surviving helmets whether one piece or multi piece vary by degrees, so any armorer would be going with physical results alone and as a result different helmets would give varying protection independent of construction.......
Any modern tests would have to take into account the lowest and the highest common denominator in surviving helmets, and its likely some one piece and some multi piece examples would fail the acid test, however as long as they are:

"above any impact force a human could produce reasonably"  They would prove to be an effective protection.

EG: the one piece helmets worn during Trajans campaign required strengthening (some were done in the field), later helmets had this additional protection as standard.

Ridge helmets are said to be based on Iron Persian examples and its quite possible considering the above that these proved equal or superior at less cost/ease of manufacture hence their adoption, materials are likely the main cost factor at the time since once you have plate, to produce a basic helmet of any type takes a similar amount of work dependant on its detail complexity, manufacturing material by hand (or more likely with trip hammers) is very time consuming with the iron needing several processes, remember that charcoal is the main source of fuel and you'd need a lot of it as well likely more for one piece helmets.

larger pieces of hand produced iron plate are more likely to have faults resulting in failure when producing a one piece helmet, though this material could be recycled in multi piece types or iron scales etc, so nothing except the labour which would likely be done by slaves would actually go to waste.

In short if you want to make a helmet to protect you against more force you increase the thickness or quality of the materials or add additional external protection, construction plays a part as does shape, a conical helmet as in the original Persian style(and some ridge helmets) would prove more effective to many attacks then a dome whether one piece or multi piece, but the effective protection is entirely dependant upon the thickness and quality of the material used and any heat treatment or other process that was known at the time, such as Carborisation.

Unfortunatly we have far too little material detail on most surviving helmets (there are a lot of reasons for this), so untill more is available this will simply remain speculation..... but its probably a good idea to look up manufacturing and blacksmithing techniques for iron from the period, especially where this includes studies on surviving items of all types.

Eg: Phosphorus if a high enough content is present in the ore can have a significant effect on the hardness of the iron, making it unnecassary to use carborisation although the two combined can produce significant results, on the other hand one piece objects usually require a less hard material for ease of hand working..