Conjuring; The Number 12

[Zarleycat at aol.com]

No: HPFGUIDX 12462

I happened to think about the ability to conjure as I was cursing my 
PC at work, after it crashed for the third time. I wished I had some 
sort of magical powers to keep it from doing its usual Friday 
afternoon swoon.  I checked the Archives and didn't see anything on 
point to what I was thinking about, so here goes. 

Wizards apparently have the ability to conjure objects out of thin 
air.  The 2 instances I thought of are in PoA.  First, 
Dumbledore "draws" a chair for Trelawney at Christmas dinner with his 
wand in mid-air and a solid, real chair thumps to the floor for her 
to use. 

(Number 12 reference - there were 12 people at the table until 
Trelawney unexpectedly showed up.)

The second reference is the scene in the Shrieking Shack after Peter 
is revealed and Harry prevents Remus and Sirius from killing him.  
Sirius conjures "heavy manacles from thin air." Although it didn't 
say so, I assume he was still using the unconscious Snape's wand. So, 
here are some questions.

Can all wizards conjure?

If so, why can't Lupin conjure himself some new robes? 

Once an object has been conjured into being, does it last forever or 
is there some sort time frame beyond which the object will disappear?
If the conjured object will eventually disappear, it would certainly 
explain why one wouldn't want to wear conjured clothing!

Conjuring seems to be different from transfiguration - will we see 
Harry, Ron, etal. taking a Conjuring Class in the future?  For that 
matter, will they take a course in Wizard Duelling?

Marianne
who's off to conjure up some dinner...