I went and looked back at the original description of the grenades in question. I was thinking that they were different (more like a Molotov than anything else).
Let me see if I get this right,
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We've got a grenade that looks something like this. (what size though, something as big as a gallon jug, or a coffee mug size?)
We've got two main parts here. First is the payload, which seems to be a slow-burning tar. Then there is the fuse, which is an ever-burning stick, right?
So the idea is to throw the grenade at the enemy, where the spalling will break and cover him with tar, then the fuse will hopefully light the payload giving him a flaming outer coat.
We've decided that these guys can be launched by a staff sling, so it stands to reason that they're of the smaller variety. My major concerns with using this guy would be safety in transport, supplies, accuracy, and damage.
The long burning splints would make the things a lot easier to use (making something to light the igniter would be surprisingly difficult). However, that means that you're marching into battle carrying all of these guys while they're live. If you trip and fall (or get a lucky arrow through your brain) are these going to break on the ground? If they're sturdy enough not to, then you don't have guaranteed fragmentation when you impact the enemy. If they do break when you fall, you end up with fire amongst your own ranks. Plus, depending on the environment (forest or dry plains or hills especially), you risk starting a major fire. Immolating you're entire army wouldn't be good (although it could probably be adapted to suiciding the enemy army as well). Scorched earth FTW or FTL?
2nd, you have a question of supplies. This is actually a problem with all missile troops to a large extent. Does this 100 man unit have a infinite supply of these guys? How many can you carry into battle (especially considering that they present a friendly fire risk)? Personally, it doesn't make sense for melee guys to carry them. If they get close up they don't want to be splashing fire on their comrades. However, if they're carrying these suckers live, then what happens when their opponents realize that hitting the little pots equal the enemy starting on fire? You could order the troops to use them all before they close with the enemy, but that gets us into admitting that supplies are limited.
Next we have problems with accuracy. The description said that the tar is liquid enough to splash onto the enemy, this would make me think that it's going to slosh around in its container. This would have pretty negative effects on its accuracy (changing center of gravity and all). I guess that you could magic it into being a thixotropic liquid, being solid in the container and liquid when it impacts. A nice solid stone is easier to fire than a water balloon (try it some time, compare throwing a baseball to a water balloon). My other concern with the slings would involve the splint starting the sling on fire (preventable with precautions) and dropping the thing when you're reloading. How are they going to be carried? Clipped onto a modern like web gear? Stored in a backpack (juggled?
)? It needs to be considered.
Finally we have a question of damage. For them to be thrown any distance, or fired from a sling, we have to have a small projectile (especially if we consider that they have to be carried). However, this would have considerable effects on the payload (how much tar can be used). I'm not sure how effective (or magical) the tar is going to be. Is it going to burn white hot for a few hours (although this would make the collateral risks much worse). I was assuming that it was going to burn off fairly quickly (in which case metal armor would bleed off the heat somewhat quicker, especially if there was something with a higher specific heat on the inside, like clothing). If it burns longer, then it could be pretty good against armored (really, everyone) troops.
It really depends on how magical these they are.
Personally, if I were going to field these guys, I'd do it differently. It looks like they're being doctrinally used as a javelin or a short bow. I'd make them artillery. Build up some siege weapons. Have a group of twenty or so catapults for example. Have three fourths of them firing big containers of the stuff into the oncoming hostiles. If it's a bigger grenade, it's going to be flying faster, so it'll splash further on impact. After a couple of salvos, use the other five weapons. They fire big burning things of straw (which are carefully lit right before firing, with these catapults a bit separate). The oncoming zefie horde is at this point pretty liberally covered with tar. Once one gets hit by a spark, he starts on fire.
Not even zefie discipline is good enough to prevent the poor sap (let's call him Urist) from panicking. Urist plunders into another charging soldier who is soaked, and he starts up. Chain reaction. Someone lights off the soaked ground. Massive fireball.