Thaumaturgy is considered a divine gift from Jamba, and a powerful gift at that. Its regulation would be incredibly severe, lest it cause untold damage, offend Jamba, or fall into less than worthy hands.
Only the most trustworthy, sane, loyal, and intelligent individuals would even be considered for training as a Thaumaturge, and I'm willing to bet the tests for entrance would be long, arduous, and rigorous in the extreme. Such individuals would be fiercely loyal to Dracarta, and extremely protective of their secrets.
No Thaumaturge would ever be allowed to just leave Dracarta bearing his caduceus, because that's tantamount to a nuclear scientist just wandering off with some weapon's grade nuclear material. Dracarta just wouldn't let it happen, and if a Thaumaturge went AWOL, you can safely rest assured that they'd spare no expense or effort in recovering him (and the secrets of thaumaturgy he carries). Any Thaumaturge found to be conducting unsanctioned or dangerous use of thaumaturgy had better write his will, because the Dracartans would regard it as both sacrilege, a threat to national security, and a possible threat to their reputation.
As for what Thaumaturgy can actually transform, it is essentially limitless. Purity doesn't enter the equation. If it can transform the water of a lake into a solid block (as we know it can), it would have no trouble with anything. Lake water isn't pure. Likewise, neither is air, and we know that can be altered as well. Accidents involving Thaumaturgy should be horrific indeed, as it liquifies/solidifies/gasifies flesh and bone randomly. The Dracartans take major pains to control its use, and especially safeguard the costly (and all too dangerous) quintessence.
Despite this lethality, it certainly wouldn't be used as a combat weapon. Any stressful situation makes a mishap all that more likely, and you can't afford to take that chance with quintessence. No sane Thaumaturge would dare use it that way, and any insane one would be quickly put down by the authorities.

So, when using Thaumaturgy in your game, consider these simple facts:
1) It is VERY strictly controlled.
2) Thaumaturges are rigorously selected, and also subject to control.
3) It is expensive.
4) Misuse (in any shape or form) would not be tolerated.
5) It is VERY dangerous and volatile indeed, and if used in combat might pose just as great a threat to its user.

Think of Thaumaturgy and quintessence as Dracarta's equivalent of nuclear capability, but with added religious significance, and you're on the right track.