100 sheets of stolen vellum

[Bernd]

I suppose that even books in an imperial library were often lent out, or made available to visitors. Otherwise, what would be the point of creating such libraries?

They were a display of wealth, knowledge and power as well as a collection of valuables (and gifts). Luxury manuscripts (most of the ones that survived the times) were collectibles, not literature to be used frequently. Certainly it was possible to access these works, but not for commoners who couldn't even afford decent parchment. 
Also such works were not copied from libraries. They were compiled in workshops that had text on scrolls and stock imagery, usually on wooden boards (Pinakes). Hence the often substantial differences in text and imagery between copies.

Is BNCF Palatino 766 really Taccola's original notebook?  I imagine that his real notebook would have had all sorts of scribbles, stupid ideas, bad drawings, etc; so he would not have dared to just send it to the Emperor.  I rather think that he hastily made a clean copy for that purpose.

Actually Palatino 766 itself is exactly that, full of doodles and and not befitting for an Emperor. Yet there is no evidence the Emperor ever received a better copy, and most likely he didn't get Palatino 766. But there is strong evidence Taccola did not possess the notebook after he completed it. Exactly because he liked to doodle in his notebooks and amend them. He would have continued to draw in it if he had it and re-use the imagery in his later works. None of that happened. That's what makes De ingeneis III-IV unique. His other works were copied and recompiled all over. Palatino 766 was not - except vaguely in the VM. According to literature, it's believed Taccola was not in possession of Palatino 766 shortly after he finished it, but that it remained in Siena. What happened then for the following centuries until it ended up in BNCF is as much of a question as the whereabouts of the VM. That doesn't mean Palatino 766 was never copied, but these copies certainly were never widely available - or even known to exist. Still I'm not 100% certain Palatino 766 has to be a definitive source for the VM. Taccola heavily copied from Konrad Kyeser, and a Kyeser copy might have also been a source for the VM. But Taccola's organic half-humanist style fits the VM better that known Kyeser copies with their purely medieval strict and straightforward art style.

Vellum and parchment, with iron-gall ink (IGI), were generally used when one wanted the result to be resistant to wear and tear, humidity, and accidental exposure to spill or rain.

So the Author presumably decided to have his paper draft put to vellum because he wanted to preserve its contents for posterity.  Or maybe because he thought that a vellum copy could be sold for a better price than a paper one.

A more intriguing question is why the VMS is not written with IGI.  This fact is clearly shown by the (non)appearance of the ink in the infrared images, and by how easily text and drawings got rubbed off or washed away (as on page You are not allowed to view links. Register or Login to view. and under that "ketchup" stain on f103r).  

Maybe the Scribe had no experience with vellum, and neither he nor the Author knew that he had to use IGI? Or the Scribe used a washable ink on purpose, because he anticipated there would be many mistakes that would have to be corrected?  Or maybe the Author was so poor that he could not even afford the ingredients (oak galls and green vitriol) needed to make IGI?  

Or maybe those ingredients were not available where the vellum copy was made? They should have been easily available in any European town where someone used vellum/parchment -- meaning any town that had a notary, lawyer, monastery, etc.  But outside Europe, where vellum/parchment were not used, it may have been impossible to obtain them.

Yes, I agree. Either an environment where you could neither buy paper, vellum nor IGA - I'm not saying it has to be outside of Europe, but certainly not a wealthy place. Some alpine valley maybe or a crisis-ridden area, of which was no lack in the early 15th century. Or - a deliberate decision to write on poor vellum (hoax?). But then why not use IGA?

I don't think any supposed access to sources is a problem. Access doesn't mean ownership. And even if people, families or institutions owned a number of books, that doesn't mean that they still have the means, ability or desire to buy high-grade materials. It's possible for an individual or institution to have limited financial means, but also have a library: from past acquisitions, inheritance, donations...

I find this very hard to believe. The cost of more decent parchment would not have been prohibitive, not to speak of IGA. Or just use paper! I stand by my point that whoever made the VM had access to a vast and top-level source of imagery. Something of enormous value that does not fit the poor materials of the VM at all. Extremely wealthy collector, university or Royal Court. Which would also fit with Taccola's elusive De Ingeneis III-IV. For a long time my favorite hypothesis was that the author did not get the imagery from books at all - but from the stock image collection of a workshop. Images from a myriad of sources without context which he adapted and / or misinterpreted. But this does not fit with the low-grade parchment and ink quality - and Taccola's copy-less De Ingeneis III-IV.

I believe this is why an expert on parchment could help to answer the key question:
Who made parchment of VM quality and in which environment was such parchment most likely to be found?

My guess from Lisa's comment is that a professional workshop would not even produce VM quality sheets as a by-product or because something during processing went wrong. If the skins used were inferior to begin with, this means the parchment was not produced by a quality parchment maker, rather an amateur. But this is what we should look into. As Fabrizio said, overall parchment quality appears to have declined throughout Europe from Southwest to Northeast