Your July 4 article on fireworks ("Big bang theory," Page B1), was only half right. The "wick," as you call it, serves the purpose of transmitting a spark to the black powder. It has nothing to do with building pressure. Most importantly, the spark ignites the black powder causing an explosion.
Your article left out the most important part, which is how the shell gets airborne. If the firework or shell is not contained in a mortar tube it won't go anywhere. The principle is the same as a bullet in a gun. The mortar tube confines the explosion and forces the shell up. Without the mortar, you would have an explosion on the ground, and a couple of seconds later, the aerial section of the shell would explode.
Without this piece of information, the article doesn't make much sense.
Terry Chalk, Martinez, Ga.