Understand though I can put stones on top of my radiator where I can’t with a bucket. I’ll have a think and see what I could do.I see. The point of spacebus was that it is more efficient to move cold dense air to the stove than warm air from the stove.
Not to belabor my point, but the stone will be hot too (dogs). Moreover, water has a volumetric heat capacity of 4.18 J/cm3 K whereas soap stone has about 3 J/cm3 K. This means that moving the same volume of water )and at the same temp) upstairs gets you about 40% more heat upstairs. AND by only carrying 1/3 of the weight.
So, carrying 1/3 of the mass gives you 40 % more heat. And you'll likely have to pay for soap stone when water is comparably cheap (almost free for the quantity you would be using).
You're free to do as you wish, but I wanted to make clear that at the same temperature (and same danger level), the physics dictates that the most efficient solution is not soap stone. Convenience is of course an aspect that is hard to quantify.
Thanks for all your help and patience.