Please tell me if I am right.
I know "The Aplle that Etches - CrHe 20 Ritual (Covenant p.96)" and I use it to create 100,000 oak galls. According to City and Guild p.140/141, supposing the quality of my creation is Shoddy
100,000 galls = 416 tun circa
1 tun of Shoddy Ink = 10 Pound
So with a simple spell I can sell ink for a total of 4160 (416x10) Pound of Silver? 
Sound good to me but I do not think it is possible...
Sure, if you sell it slowly (over many years) over a wide geographical area. If you try to sell it all at once, three things will happen. Firstly, the price will plummet and it'll become worthless. Secondly, a lot of people with investments in the ink and related trades will come looking for someone to blame for destroying their source of income. Thirdly, you'll find some quaesitores on your door wanting to talk to you about some angry mundanes who're bringing accusations against the Order.
It's a good way to get a nest-egg to provide an income over a long time, but a terrible get-rich-quick scheme.
Fthagn is completely correct (and I say this as the guy who wrote that spell, as I recall.)
Hermetic magic lets you just make valuable stuff. Make as much as you like. You go for it...
Treasure as expressed by money is Simply Not The Point in Ars. You have as much cash as you can swim in, if you want. You just need to pay for it with stories when you spend the cash. We, in the Amber community, call this "Giving you enough rope". Have -all the rope you want-.
Whom do you want to sell 416 tons og galls anyway... at any price...
I would not like to have them delivered for free into my garden...
Assuming you could find enough Buyers to buy that much ink as well.
So you could cripple the ink ecconomy with 1/40 of your stock and still find the ink guilds comming for "a quiet word"*
A
*quiet words in my experience usually involve leg breaking.
I suspect that you'd be better off selling the ink to other covenants that might not have the spell, or a good way to make ink. Additionally, since the spell is a Creo spell, not a Rego, the quality of the oak galls were be perfect, not shoddy.
I wonder - was the spell ever cast a second time in the history of the Order? Considering that a large fraction of all books are circulated via the Redcap network, and that the weight/encumbrance of the ink is just a tiny fraction of the weight of the books, it seems quite likely to me that the first covenant to cast the spell just provided to the rest of the Order all the ink ever used thereafter, for nothing more than a measure of goodwill (and perhaps a couple of books).
The ink's biological, from tannin: if you don't use it it deterioriates, unlike many modern inks which have a shelf life of decades.
Could you cite any source for this? I believed quite the opposite: if properly stored, ink could last centuries - though I really can't find any sources for this belief of mine now that I am looking for them.
And in any case, oak wood can easily last several centuries. Why wouldn't the galls themselves last that long?
An interesting question - oak survives partly because of its structure, partly its chemical composition, the high tannic acid content acting as a preservative. The fact that it has been dried and cured also helps. ::grins:: Wet plant matter of any sort will rot in the right conditions.
That said, it should be a trivial spell to prevent decay in plants, and a few barrels in a ring would do the job nicely.
As for the ink going off, phenolic systems like tannic acid have a nasty tendency to polymerise, add on to anything handy, are very pH sensitive, often show interesting photochemistry and are highly redox active. This isn't to say that a pure sample of an iron tannate wouldn't be stable indefinitely, but it'd need to be under optimal conditions and I very much doubt medieval inks were anything like pure. An Order alchemist could probably make a perfect ink, of course, and there'd doubtless be a good market for it. Then again, it really should be a fairly trivial spell to stop ink going bad. Whilst you'll inevitably eventually warp your ink, that in itself might be of interest to the right magus.
They'd hatch, for one thing. An oak gall is an immune response to a wasp egg.
That being said, I might be wrong...
No, you are right; but the gall can persist beyond the wasp's emergence. Of course, in Mythic Europe cause and effect could be muddled, or absent.
Mark
Well, the spell doesn't require and Animal requisite, and in the same way that certain geese hatch from barnacles, wasps might not emerge from galls.
If you do want to corner the market, you might prefer to use a spell like this:
Blight the Scribe's Livelihood , PeHe20, Ritual
This spell subtly damages and diseases all the galls in a the targetted forest, leaving them visibly unchanged but rendering them incapable of being used to make ink. Whilst any new galls will form healthy, this does effectively destroy a year's potential harvest.
(Base 3, +1 Touch, +4 Boundary)
True, it'll put a few people out of business, but the rest of the market will be quite happy that you've driven up the price of their goods.