OK, to figure out your unit wieght of cinders, take and times your specific gravity by the wieght of a cubic foot of water, (62.4 lbs) and you get the unit weight of the cinders, but there is actually more to it.
You need a container of a known volume. At least a quarter of a cubic foot, and then weigh the material inside the container, subtract the container weight, and then divide but the volume of the container. The material needs to be oven dryed out first, or you will be weighing water as well.
If you do not know the specific gravity of the material, you will need to figure that our first. It is a little more complicated than a simple unit weight.