Everything is said in the title. The blend file contains a node tree to simulate burning charcoal.

The node tree can easily be re-purposed to create any pseudo-volumetric material changing from a random point (called Center in the tree) towards the outside in all directions, on 2 axes or on 1 axis only. This is achieved thanks to a special node group which calculates a factor based on the distance from the Center and the dimensions of the object.

Please note, that since the node tree uses the Object coordinates, it doesn't work well with rigged objects. For this material to adapt to rigged objects, you must use the UV coordinates (in 2D only) but the whole node tree is a bit overkill tho. ;)