Says the man married to the jar? You cant be a sucker to the effortless way and an expert to the way that is proper. I however, am an expert roux making aficionado dedicated to the means necessary to please a pallet. I would not know the first thing about using roux from a jar. Now, you are not entirely wrong. A preferred and accepted way would be to use a cast iron vessel for preparation but it is by no means the only way. I have heard folks bake their roux in the oven using glass. I use a heavy pot and I have used cast iron as well as a dutch oven. Roux is not exclusive to cast iron and I am almost certain that shit in a jar is made from something other than "the right pot".
And you have found the magic bean. Cast, magnalite, glass but it needs to be heavy. More importantly, heavy on the bottom to distribute heat properly. Try it in one of those cheap shit 1/2 lb jobs and you have a chore on your hands
Dude, @shane0911, you and me in traffic bro would not end well. Someone would get hurt and it would be either of us.