Hi Jerald,
If I have got your question right your uncle is using the sandstone hearth as the support for the oven floor. A couple of questions first as to the ovens use. What does your uncle want to cook / bake / roast in the oven? This will make a difference as to the amount of thermal mass under the oven-cooking floor.
As to James question, sandstone is a very porous soft stone with large particles of sand making up the sandstone often with air gaps between the particles (thus making it good at water retention). Generally, unless you like a crunchy silica loaf I wouldn’t use it for an oven-baking floor.
Sand has several interesting characteristics One: it conducts and holds heat well Two: it is flexible and can move with expansion and contraction. Three: Before we had our modern day insulating materials it was a reasonable insulator (however, not as good as charcoal)
Alf |