I have two magical stones. The first one—let's call it fire stone—is a stone that able to increase its temperature equal to that of a normal fire. The second one—as you might already guess, is the ice stone—is the stone that is able to decrease its temperature to equal to a normal water ice.
These two stones would have a room temperature which is normal but, when induced with magic from the user, they would do their job then revert back to normal temperature in the matter of seconds. Also, the stones will shrink with every application of magic.
Some additional considerations:
The society would use these stones mainly to boil or freeze liquid. For example, to use as steam engine fuel or to keep beverages stay hot or cold, etc.
The magic works with physical contact, but there also exists an aluminum-like material that could "channel" magic and act like wire. Let's call it 'channelium'.
The stones are easy to shape and non-poisonous.
PREFERRED: Method/design that doesn't use electricity.
Back to the question in the title, I'd been thinking, perhaps we could use the vacuum flask (thermos) design. Perhaps we could build the thermos cap with channelium that would then have the stones attached in it, so we could replace the stone easily. But it would raise another concern with channelium, now it is too hot/cold to touch! I don't even know about the possible problems that might occur with the real thermos design, pressure maybe?
So, what another properties should I give to the channelium beside channeling magic? Are there another plausible approaches to design the fire & ice stone temperature container, with or without channelium?
EDIT: I don't know about fire temperature difference from the different fuels, but it could be more or less like 'coal fire', since that is what they use for steam engines in the "industrial revolution", isn't it?
EDIT2: To avoid future confusion; channelium needs physical contact in order to connect the magic. So basically, if your hand touches channelium and then the channelium touches the magical stone, the magical energy will travel from your hand to the channelium and then to the magical stone in an instant.