I vaguely remember something along those lines.
Are these threads ringing a bell?
Those weren't quite what I was looking for, but they led me to others, etc.
OK, Elements:
Mechanically elements (as evocation) are all capable of the same four actions. Attack, Block, Maneuver and Counterspell. But how they do them will differ in the following ways.
Firstly, different pre-existing aspects will interact with elements in different ways. For example, a fire spell could be boosted by invoking the "Soaked in gasoline" aspect, while a water or air spell could not. The "Stormy weather" aspect could be invoked to create a more powerful water or air spell, however a fire spell may actually be compelled against.
Secondly elements themselves can be treated as aspects or be the source of declarations concerning the element. A great example of this is if I throw up my "Dome of force" and you go "Aha! Force does not protect you from the heat of my flamethrower!" This is essentially you compelling the aspect "Dome of Force" (or invoking your "flamethrower" which compels my "Dome of Force") to bypass my block. This also allows us to look at the elements themselves and compel the ways that they are different. Fire is great at messing with energy, but it's not good at physical objects so much. Most fire blocks could be compelled to allow physical objects (like bullets) to pass.
Thirdly (and this comes up less often than others) elements interact with Toughness powers. Something may be immune to fire (but not other elements), or may have a Catch of fire.
Lastly if you look at the section on modifying difficulties on YS311-312 it talks about increasing the difficulty of actions by two shifts if they are rushed, hampered by movement, or (and this is the important one) ill prepared. I would say that fire is definitely "Ill prepared" to move something or freeze something. It's possible (using vacuums or diverting heat elsewhere) but it's harder because the element is not suited to the task. This gives us a 2 shift increase in difficulty. Normally (since there's only one difficulty in the evocation action) this would apply to the control roll, but as Fred has discussed before, it doesn't really make a huge difference where those shifts go. They could increase the amount of power required (increasing the conviction difficulty), they could increase the opponent's defense roll (increasing the to-hit difficulty), throw those two shifts where you feel they make the most sense.