The flame temperature of thermite is a blistering 4990°C.
Now, in Powder Toy, BRCK is not something strong, like firebrick. Instead, it is more of a solid concrete. A simple guess of the melting point, or at least the semi-liquefying point (concrete is made of a jumble of materials, thus some parts of it melt more then others. Sidewalks will soften in direct heat, you see) of concrete is about 800-1000°C.
@Neospector(View Post) Depends on the kind of brick. Concrete maybe melts at 800-1000 C, but the clay bricks shouldn't melt that easily. And we have concrete ( As Powder ) so brick -should- be clay brick.