In the manufacture of black powder one of the processes is a screening or sorting of the granulars of powder by size. Sort of like grading the size of chicken eggs. It is not generally a good practise to mix different grades (size of the gradulars) of powder together, because of the different burn rates, unless an explosion is what you want in the first place. You wouldn't use a mixture of powders to fire a cannon. But you might use such a mixture for the bursting charge inside the shell the cannon fired.