for those of you that have worked or still do in a restaurant or a big kitchen know about this...but for eveyone else... a grill brick is a square piece (resembling a brick) made of some sort of metallic material to clean industrial grills, it can be easily pulverized as I have found in my eternal quest to find cheaper material to weather my models, when pulverized it can be used all over track vehicles, specially tracks to give a nice finish, and if rubbed against the paint it will act as a very fine sand paper leaving hints of metallic dust....
I hope that this helps somebody