Questions?

For questions 1 - 4 I have no idea. On question 5 I've seen some information on what was thought the value of an AE coin could buy. I think if you do an internet search you will find some information. On your last question as pish says beats me alos.

The diadem came in with Diocletian. Perhaps there were no bay trees where he was at the time?

Thanks Antwerpen! You have given some very good answers!
But I always thought Constantine I instituted the diademed not Diocletian?
Also if the Empire was running low on silver for its Spanish mines then how did they issue large issues ofSiliqua in the later 4th century
. Any idea on what the nummi could buy in the mid to late 300's.

Diocletian reorganized the entire imperial system from top to bottom. From the economic reforms (strict price controls) to the very nature of the principate itself. The period of the third century saw an unprecedented series of imperial assassinations and usurpations. Diocletian made the emperor into what Gibbon thought was unlike the western tradition of the emperor as a man to the eastern tradition of the emperor as a god and as such completely unapproachable. Those who approached walk through long corridors of hallways and rings of imperial guards. The soldier emperors of Illyria adopted wearing Asian silk slippers and petitioners prostrated themselves at a safe distance. The pearl diadem was probably another eastern tradition which Rome gradually succumbed to. Those occasional wearers of the laurel were the holdouts who were not going with the flow ! The emperor was gods representative and anybody who challenged god got what was coming to them.
It's worth noting that in this day of loud championing of individual rights and limitation on the power of the 'emperor' what we celebrate is neither Roman tradition nor eastern Biblical belief. The Magna Carta and the limits on supreme power owe to a newer jurisprudence. Queen Elizabeth II traces her line back not to either prophets nor Roman Senators. The wild and wooly immigrants from the north brought with them the odd notion of people having rights as an individual apparat from that of the unit.
And the 'King' receives his authority from all us peasants down below ...... and not some link with the 'guy' upstairs !

Magnentius may not have wanted to muss up his hair?

I read somewhere that the diadem was borrowed from the Greeks.

Too late, off to bed.....

1. By 300 AD, the Romans had basically gotten over their phobia of monarchy, that dominated their earlier history. They were no longer afraid to adopt overtly "royal" symbolism, such as the diadem, a jewel-encrusted ribbon of cloth first used by the Babylonians, then the Persians, then the Seleucid Greeks and Parthians, but up until the time of Diocletian considered too "royal" a symbol to be worn by Romans. I believe this change in culture came about largely because of the military anarchy of the third century, where various factions of the army basically enthroned and deposed emperors almost at random. Adopting the trappings of royalty perhaps gave them some of the "divine right to rule" protections afforded to kings and not to generals.

2. and 3. The system of uniform Empire-wide coinage issued from numerous mints, implemented under the reforms of Diocletian, implies the existence of some kind of official master portraiture, a model which the die-cutters in the various mints throughout the Empire each copied in their own way. If the official portrait had a diadem, then the coins all showed him wearing a diadem; if bare-headed, then the coins were bare-headed, and so forth.

This does not of course answer the deeper question, of why particular emperors chose to be depicted wearing particular headgear. They may have been trying to make a statement, like Jovian II with his long beard, but I don't think we are in any position to make conclusions about the meaning of being bare-headed or laureate instead of diademed.

4. The fineness of the denarius/antoninianus did not rapidly decline during the third century because the Empire "ran out of silver"; it declined because of inflation: the need to make the finite amount of silver available go just a little bit further than it used to. These days, if a government wants to create more money, they can just wave their magic wands and make the printing presses go a little faster. Back then, to make more money, they either had to mine (or steal) more silver, or make the silver in each coin more dilute.

5. One of the few surviving accounts of the monetary system of the Late Roman period is Diocletian's "Edict of Maximum Prices"; read the Wikipedia article on it for more info, and Here's the Google Books version of the Edict itself.

6. Taxes were charged using the money of account of the day - if the Edict quoted above is any indication, the money of account was the "denarius", of which all coins commonly in circulation were multiples of. The government, at least, recognized the official exchange rates when taxes were paid, so you could pay them in whichever form of coinage was most convenient to you.

To a large extent, we simply don't know with any certainty what the denominations of many of the "Late Roman Bronzes" were supposed to be, either in relation to each other, to earlier or later coins, or to the money of account. We have Diocletian's edict which gives a good breakdown of official prices and face values of coins under the Tetrarchy, but we have very little in the way of similar surviving systematic records of the monetary system between the Edict and the "Byzantine" coinage reforms of Anastasius. We find only occasional references to specific coins in the surviving histories: the "centenionalis", presumably so-called because it was slated at being worth 100 denarii, is one example. What this meant for prices is uncertain; as the solid silver argentus was originally also slated at 100 denarii, the denarius had obviously lost a lot of value between AD 298 and AD 340.

People were simply too busy surviving to worry about writing things down for posterity. Or if someone did write such things down, the collective accidents of history have not preserved them.