This tactic works only until someone learns of it.
This rather seems to apply to the weaponized transfiguration in general. Compared to identifying and removing Aluminum that is dispersed into the cells of the brain - never mind trying to fix existing neural damage - is far more difficult than defending against the whole class of transfigured gas attacks.
There must be better substances to use!
That particular tactic is, of course, an optimization within the domain "transfigure arbitrary commonplace solid". Did you consider the alternate substances discussed in the comment to which you are replying? Are there some superior options to those that you would suggest? Creating the perfect magical terrorist attack is naturally of great interest.
(The HPMOR discussion thread after this one is here.)
The previous thread is over the 500-comment threshold, so let's start a new Harry Potter and the Methods of Rationality discussion thread. This is the place to discuss Eliezer Yudkowsky's Harry Potter fanfic and anything related to it. The latest chapter as of 09/09/2011 is Ch. 77.
The first 5 discussion threads are on the main page under the harry_potter tag. Threads 6 and on (including this one) are in the discussion section using its separate tag system. Also: one, two, three, four, five, six, seven, eight. The fanfiction.net author page is the central location for information about updates and links to HPMOR-related goodies, and AdeleneDawner has kept an archive of Author's Notes.
As a reminder, it's often useful to start your comment by indicating which chapter you are commenting on.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: