As a classicist, I can assure you that a great deal of interest has been directed towards the steam "engine," or aeolipile, created by Hero of Alexandria. The creation of the device probably predates Hero, possibly to the 3rd century BCE - but this is not what interests classics or ancient history professors. No, it is that the existence of this device, and the fact that it was never put to any practical use, is proof positive that the Romans did not view technology in the same manner as we do ourselves. No, the Romans were NOT driven by the need to constantly reinvent their universe! The Romans did not see labor saving devices as all important necessities! Hero's steam engine proves that although they had the possibilities of the industrial revolution at their fingertips, the Romans eschewed technology in favor of a more traditional ideal.
What a load of hogswallop.
The above opinion was pressed on me a number of times by my professors, and was argued in a number of texts I was expected to regurgitate. It must be remembered that classicists are as a whole reclusive rather anti-practical lumps, and as such are expected to miss the obvious problem with Hero's "engine." That is, that it isn't an engine at all. It's a toy.
See, what the aeolipile does not do, that which practicality demands, is that it does not process cold water into steam while it is in action. If you add cold water to an aeolipile while it is spinning, it very definitely stops spinning.
It took another 16 centuries for James Watt to satisfactorily improve upon a number of cantankerous steam devices in the 18th century and create an ENGINE. The steam engine, the gentle reader understands, both takes in cold water AND puts out steam, simultaneously. Hero and his contemporaries couldn't figure out how to do that - and that is why the Romans did not embrace the Industrial Revolution.
Yet try and tell a classics professor. Go on. Try.
I won't jump into the function of Watt's steam engine, an improvement on the Newcomen steam engine, a leap forward from Thomas Savery's pump, etc. I won't, because D&D isn't steam punk, and I'm not here to argue that it should be. I would like to point out, however, that the making of steam into water has been around a lot longer than human beings have - the problem for our species has always been the interjection of a great deal of heat into a relatively small amount of water in order to create steam to exact specifications.
Well, it's a problem we have. It's not a problem where a spellcaster is concerned.
Let us consider the spell heat metal. This is a spell which, according to the books, causes metal to glow a dull red - so we're talking about a fair amount of heat. Now ... what stops me from creating an iron flask about the size of a bread loaf, filling it with water, sealing it with a metal plug and dipping that into another layer of metal in order to seal it tightly? If the water within has nowhere to go, and the metal is heated, what is the effect?
For that matter, if I cast shrink (the opposite of enlarge) upon the object, but not upon the water it contains, what is the effect of that?
What if I have clairvoyance, enabling me to see the water, so that I cast either enlarge or shrink on the water, and not on the container? And if I create water, then shrink it, then teleport that water into the container, then dispel magic, what is the effect of that?
What is the effect if all of this is done on a container of considerable thickness which is molded into having an enlongated spout, which is plugged until the pressure is released within. How much pressure can I produce in order to give me how much steam?
If I create whichever steam-producing metal bubble seems most effective, and apply a fire trap, so that it only opens at the spigot-end when a door is opened, how much damage can I cause?
If not a fire trap, how about a ward or an appropriate glyph of some sort?
If I enlarge my metal bubble and put it in the hands of a creature who naturally gives off a level of heat, then create a gate to the plane of water inside the bubble, how much damage will that do?
Are there any particular reasons my character cannot experiment with these things in your world?