Brimstone is also known as sulfur.
It tends to be rather explosive.
In fact, it's contained in many combustible agents.
Why don't you visit the ship's hold? You can reach it by passing through the hatch in the middle of the sub-deck.
Have you examined the kegs?
Take some gunpowder from the kegs on the left side of the screen.
Now add the gunpowder to your Monkey Island Mix.