Don't mention it, the first use of this tactic indeed caught the Venetians off guard.
The working Venetian soldiers could only see objects wrapped in blue flames whistling towards them. Once these blue fires hit the ground, they splattered, capable of burning through flesh and skin upon contact. The ensuing smoke made it unbearable to breathe.
Like demons crawling out of hell, the smell of sulfur was everywhere, and it sent Venetians fleeing in panic.
It was Colonel Volbon who urgently addressed the crisis. After taking samples on the spot, he quickly figured out what this "blue fire" was—simply sulfur.
The defenders in the city first broke sulfur ore into fragments, ignited them, and then packed them into clay jars before launching them into the tunnels with counterweight trebuchets.