Fire of Isengard ?

[The Might]

And as we are talking about this topic I guess it might be good to provide some quotes that also mention Saruman's interest in such things:

...but Curunír went most among Men, and he was subtle in speech and skilled in all the devices of smith-craft.

Also, it makes sense as well since he was a Maiar of Aule:

Curumo, who was chosen by Aule

It does make sense that he enhanced the explosives, but I believe the Orcs were at the time no strangers to such things either, and as suggested in the quote from The Hobbit they took great delight in using such a weapon.