wax/phase change materials buried in the bench

Has anyone tried phase change materials in the bench?

I have a lot of dirty paraffin wax. Melts at about 155F. I've been thinking of putting empty juice cans or masonjars of it between the pipes. It takes in a lot of heat to melt the wax and then gives it back slowly as it hardens.

Hi Bob;  
I haven't heard of anyone trying that yet. I can't see it hurting anything but...
I have to wonder, if it is better than cob and solid rock at releasing heat.
My horizontal pipes are encased in cob. But after that they are under a slate and cob lasagna. It is very good at holding and slowly releasing the heat.

If it were me I think I would go with rock / Cob.
Build a bench and then set the wax containers on top.  Only the ones near the transition area may melt but all would absorb heat and release it.  Sort of an extra layer of mass.

I've seen wax used for automated windows, as they heat up the wax expands, pushing open the window to vent say a greenhouse. So I expect the wax would expand and crack the harder material that surrounds it. Leaving air gaps to account for expansion would severely hinder heat transfer, and whatever is used to contain the wax could crack/break and then the wax could leak wherever gravity takes it. I'd definitely avoid it myself, but maybe I'm misunderstanding how it would behave when heat is applied.

I think I didn't explain myself very well.
My bench is cobb.

I am suggesting that as I add layers of cob, that I will place cans or bottles full of wax in the cob between the pipes.

The wax will melt, absorbing a lot of heat just to melt it, without actually raising the temperature. When the cob is cooling, the wax will harden, releasing the heat that it took to melt it. I fill the containers about 80 % with melted wax. It will shrink as it solidifies. I've put it through a couple of melt/harden/melt cycles and the containers hold up fine. Should they leak, the wax will be contained within the cob.  It might absorb somewhat into the mud, but I don't think it's going to run out in a pool.

As long as it doesn't catch fire...

I have thought about phase change thermal mass, but the $$$ I would have needed for a meaningful amount kept me using stone and cob.