Tolkien and Black Powder

In LotR:The Two Towers, Saruman breeches the wall at Helm's Deep with black powder (aka gun powder). This point always bothered me but I was finally able to reconcile it.

First, there is no reason that black powder would be incompatible with Middle Earth. It is basically a medieval civilization and black powder was known in Europe through most of the middle ages. Early guns go back as far as the 12th century but they were a poor substitute for bows so they didn't make much of an impression. Early guns were basically a pistol barrel on a sick with little range, accuracy, or penetration. An accurate version became common around 1500 but there was still a debate about the effectiveness of guns a century later.

Cannons developed faster and there were some useful siege guns but they were very expensive. The technology for casting iron was still in the future so there were usually cast from brass or bronze which cost a great deal more.

But there is no hint of any guns or cannon in Tolkien's writing. Is there any justification for including black powder? If you think about it, there is. Tolkien has black powder being used in the first chapter, A Long Expected Party and the context provides the justification.

The first chapter involves Bilbo Baggens's birthday party and he went all out in holding it. A high point of the festivities is the fireworks display provided by Gandolf. Fireworks use black powder.

If Gandolf knows about black powder then it is no stretch to believe that the other wizards also know of it including Saruman. And the wizards were a secretive lot. They were in Middle Earth to stop the rise of Sauron, not to bring knowledge and they may have known that black powder was a potentially disruptive technology. It's best to keep these things to yourself lest the enemy be the first to benefit from it.