[Gedrin]

Here's what I've got for the steam bomb after reading the responses here:

Manufacture:
Requires Magery 1 with training in metalworking and the following spells: Seek Earth, Shape Earth, Earth to Stone, 3 Other Earth Spells and Essential Earth. One Water, one Fire, one Air and Find Weakness is required for a supervisor who should have Find Weakness at 15.

The weapon body is a unibody sphere of essential metal. Internally, a mesh or other radiator crosses through the middle. Protrusions in the form of inward facing spikes are arranged about the sphere in a way that increases the likelihood one will be struck on impact, but will not interfere with launch. I think the protrusions are probably unnecessary, but I've included them as military over-design. The weapon contains 200 liters of water and weighs about 250kg, and is about 70 cm across.

Cost to create: ~30 pts of power, in work unit chunks of 6 or 8. (IMC it's Threshold Magery, so spreading this cost out is useful, and there's a promotion path (Shape Earth, Earth to Stone, Essential, Find Weakness).
The weapon has a Malf of 14, but without a supervisor this is reduced to 12. Carelessness or poor discipline on the behalf of the crews or manufacturers should also be punished.


Field Weapon Tech:
Requires Magery 0, artillery training, and Measurement and Purify Air (for removing accidental detonation steam). The tech's job is to handle the weapon with assistants (muscle) and to act as accurate rangefinder for his crew.

Launcher:
Counterweight trebuchet. Some of these things used projectiles of truly wacky size. It wouldn't be unreasonable to have Mega bombs (as in 1000kg) and hurl them with the largest of such machines for sieges. I'm only looking at 250kg and that should offer good range and RoF given a late 1300's tech base. I'm going to posit 200m for the standard 250kg projectile.

Use:
The weapon is rested on a sled and heated in a pit relatively near a catapult. The weapon tech uses a variety of conventional means (known ignition and melting points of test materials) to monitor the temperature. Measurement is used for a final verification. The weapon is used within a margin of error between critical steam and structural failure at launch. The weapon is launched, impacts cracking the vessel, and releases a cloud of steam approximately 5 yards across. Concussion and shrapnel are minimal (3d cr ex, [1d]).
However, the cloud of steam is lethal, dealing 2d damage the first round, ignoring armor (natural DR applies). All effected targets must make 2 HT checks. Failing the first results in 1d damage and blinds. Failing the second results in 1d damage and 1 HT from scalded lungs. The hot steam persists for up to a minute, depending on weather conditions. After the first round, damage is reduced to 1d-1, no damage is done from failing the HT checks, but their penalties (blind and -1 HT) still apply. Note, if the target holds his breath he can avoid the lung damage, but will suffer -1 FP from Suffocation under combat conditions.
While lethal, the primary use of the weapon is to disrupt enemy combat formations of close packed infantry (such as phalanxes).


Innovations:
Some things might be worth having, or are alternate ways to pursue the weapon:

Alchemical Turkey Timer: A material that gives a visual indicator when the weapon is ready. My first thought is a paint that changes color or a clay that curls up when proper temp is reached. This makes it easier and more reliable to time weapon readiness. You might still want to have the weapon tech trained with Measurement as a verification, but he'd have more power available to range the weapon. If I could pick one invention to pair with the weapon, this would be it.

Water Cracking: A Spell Stone with the Create Water spell is embedded into the weapon, such that it breaks when the weapon impacts. The heated sphere is shocked with exposure to the water and cracks. Water cracking would enable different vessel designs, possibly increasing shrapnel damage. It might also be a better way to ensure shattering. Honestly, I think the pressurized, heated, metal egg is going to break after being hurled a hundred yards either way. Create water has the lowest prereqs, but is 2 points (40 for 'Stone). Frost, while one extra prerequisite, is cheaper in the long run, would effect the entire vessel at once, and might be better for those reasons.

Enhanced Throwers: Better catapult or thrower designs could let you use simpler materials than Essential Metal. The tolerance testing would need to be done again, but that might be worth saving the training cost for Essential Earth and the increase in rate of production from not requiring the power to fuel that spell. Additionally, throwers might have higher RoF and support heavier projectiles at given materials costs (using Essential Wood and the like).

Self Propelled Steam Bombs: This is madness, but someone will try it. Probably a PC.

Heated Steam Bombs: The Heat spell is used to arm and detonate the weapon. These weapons don't crack from impact, but from over-pressure. Yield is higher, but the deployment Energy cost is much greater as well. Additionally, timing could be a problem. Possibly an application for sapper weapons or mines (providing you could risk getting the weapon tech close enough to cast the spell). A 1000kg steam bomb isn't any more expensive to Heat than a 250kg one. Training program is harder for weapon techs, but would also include spells useful for dealing with fire hazards in the field.