I don't know how generally this works but I've found that I can burn my lumps of myrrh and frankincense directly. I put them in the empty aluminum cup left when a tea candle had all burned up. Then I just held a lighted match in direct contact with the resin until it caught. I only got the smell and the smoke of the incense, nothing extra from charcoal. For safety I worked on a hotpad, on a glass topped table, in the middle of the room, so I also didn't see smoke on any walls or fabrics. If the fire goes out while some material remains unburned, I just leave it until next time when I add some fresh resin to light. In a confined space the fire goes nicely from one lump to the next.
"No elegance is possible without it...perfume is a part of you." Gabrielle "Coco" Chanel
I've been making incense for years too, but I never knew about the putting ash on the charcoal bit.
Nice tip, thanks for that.
I often find myself rushing over to the incense burner and removing the blackened pieces when the incense has finished, as too much of the 'after smell' is a bit chokey.
Another good thing to burn is fruit tree woods, and mixed with Frankincense, they smell really nice.
Like others have said, Myrrh is best in smaller ratios when burned with Frankincense, as it can be a bit overpowering, but I do like it on it's own too in small amounts.
I usually break the charcoal blocks into quarters too, as the room fills with smoke long before the charcoal has finished, but that's in an average sized room.