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i'm no expert on sandstone but all of the the sandstone that i've seen were very weak aggregates. most pieces could be broken by hand. i don't think this would be a very good material for a hearth unless fully supported from beneath. from a heat retention/insulation standpoint it would probably be similar to sand. there have been several threads which have discussed the thermal characteristics of sand and i think the consensus was poor insulation but good heat storage.
One of the theories behind the use of precast refractory floors (and brick) is that they have the right mineral composites to get/stay hot, and that they are porous, so they can extract moisture from dough and convert it into steam. If the floor is too hard or solid, it can't do that.
I think he would put the precast cooking surface on the sandstone instead of cement. I really wanted to know if the layer of insulative cement between the sandstone and cooking surface is needed. I guess I was not very clear.
You will want an insulation layer somewhere in the hearth to keep heat from leaking down. I talked with a builder the other day who build a commercial oven on a 6" concrete hearth, and could feel the bottom of the hearth getting hot to the touch while cooking. His oven was cooling down too fast and his wood costs were higher than he had predicted.
You can insulate directly under the cooking floor in you are only doing pizza and light baking. Otherwise, you would want to insulate under the sandstone hearth to keep the cooking floor and sandstone hot.
Does that make sense?
Jerald, what is the logic behind using sandstone in the hearth?
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)