Especially if you plan on using it to capture enemy secrets, there is another expansion necessary: some simply means of disenchantment (fast, not necessarily cheap). On the one side, once you have learned the secret, you will want to prevent accidental writing on your copy that would immediately show on the other side. On the other side, once somebody knows of this, paper and all mediums of information will start to be suspected, and important documents will be surely disenchanted shortly before writing, just in case.
And so starts another magical arms race... :)
On Chaosmark's idea: would the burning of one paper result in some changes on the other side? Maybe even temporary, but there should be some hint that this happened... the paper could turn brownish for a while, etc. Go to Comment
A very good basic enchantment. Of course once this item becomes known, it will cause two things to happen.
Visible Watermarking for government documents. Contracting companies will do the same. This will make it difficult to "forge" things with the enchanted paper.
Wax seals will be used, as those can not be duplicated by the enchantment (or will just show as a splat). And Notaries will become a requirement, as they will list what they have certificed and why. ( http://en.wikipedia.org/wiki/Notary_public for information about these people). The Notary has a record of what they witnessed. Thus they can eliminate some of the semi-fraudulent processes.
Still, it is a great "fantasy" radio. The price is not that outragous and would provide huge advantages in government, warfare, and large scale businesses over those who do not have it. Even expensive, it would easily pay for itself.
Communication processes are great. And if Books of Law were made with this paper, and tied to a few dozen other books of the same time, you could have instant distribution of laws, warrents, and such to the outlying providences.
From this item, Ledgers of Governence would be created. Right Sided Pages for sending out to the outlying location, the left sided pages for inbound information. Thus laws, tax records, appointments, warrents, and such could be recorded and sent instantly.
Of course one thing. If what is done to one page, is done to another, then what about burning one page? Will the other burn as well? Go to Comment
So the shreading of this age will be painting it over with black ink. If the ink used was flamable, even the better.
Actually many an early modern era espionage mission was done by France and England to get the other's official paper, much the same way people try to steal Money Blank paper. With it, you could make official documents. With this technology though, you would not only have to steal the heavily guarded paper, but put it back so someone will use it (after enchanting it).
If this paper imaging becomes "common", then they will start enchanting certain paperweights with a detection spell keyed to it. Thus the copied paper can be detected easily. Technology, counter technology, it is a natural progression.
Aside: There is a spooktale where the KGB tagged certain military office's paper supply with a radioactive material, so when they sent orders out on it they could trace where the orders went (finding the spies associated with the office via geiger counter). Do not know how reliable this one is, but it sounds good. Go to Comment
A useful means of mystical communication. Another use might be watchtowers on the outskirts of the country. When an invading force is first seen, the watchers write out a warning, then burn their linked-logbook, as the messages have already been imprinted within the matching book inside the capital. This instant communication gives the soldiers an extra edge, because they do not need to return to the capital or anywhere else to give warning; instead they can focus on survival, and hindering the enemy's progress. Go to Comment
A great addition to any espionage game, obviously goes without saying, but I'll say it anyway. Hmm, this is a pricey enterprise, creating these sheets, but then again, royal courts and governments could theoretically mass produce these in droves, having teams of alchemists working around the clock, and procuring the necessary components ad nauseam.
Hmm, here's a plot. You can have an entire nation overthrow another one, with this innovation, assuming the latter nation does not have the secret paste recipe or awareness of the process. A potentially humbling tide turner in a great war.
The list goes on. Thieves Guild intrigue, unscrupulous monks copying the religious texts of another secret order, on and on.
Your point about countermeasures is well made, and could help reduce it's impact. However a powerful nation could get around some of the measures by forging or stealing watermarked paper and then treating it.
The wax seals and Notary are an excellent countermeasure.
Regarding damage transferral, it was not stated explictly, but only minor alterations such as writing will transfer, otherwise the use as a 'backup' copy is not possible. So, I'd say physical damage (tearing, etc) do not transfer at all, and total destruction like burning will simply cause the other sheet to become somewhat scorched but readable. Go to Comment
An ideal companion item for a spy deep within an enemy is the use of a magical quill which writes illusionary text. In reality it writes a plain-text message which due to the paper not transmitting magic, comes through clear on the other side. If someone were to see the local copy, they would see the illusionary text which would appear as something unimportant.
Spells which accomplish the same task are also useful. Go to Comment
A lesser powered (No magic capturing) version of the Book of Transcription made with this type of paper would be even more powerful, being a 'bug' with unlimited range, though the book would be hideously expensive.
Eg. For a 100 page book with standard (8"x11" - yes I know that is not a medival standard) size pages, you are looking at expending the equivilent of 550lbs of gold to produce the necessary twin paper. This is not even touching the cost of making it into a book of transcription.
Anyone of power would be well advised to periodically checking their library for books radiating enchantment they are not expecting. Could provide much gainful employment for lesser wizards of a trustworthy sort :) Go to Comment