We are frequently asked by architects or designers if it is possible to make a firebox using soapstone slabs rather than our soapstone firebrick. They want to avoid joints and have a monolithic look.
The answer is yes, but with some major caveats and considerations.
Differential thermal expansion is the most critical issue. Basically, if a single slab of stone gets very hot in one area, but remains relatively cool in another, the hot area wants to expand, while the cool area wants to stay inert. Result: a crack. This can be addressed if you are dealing with a predictable heat source like a gas log unit, but it’s a different matter with an unpredictable wood burning fireplace. Soapstone bricks, properly jointed using refractory mortar, cope much better with differential thermal expansion.
Also, its a bad idea to make the firebox floor and the hearth slab one solid piece. This is usually done for looks and to ease the cleaning of wood ash. But you will almost certainly get a crack where the hot firebox floor ends and the hearth begins. So better to put a nice straight sawn joint there from the get-go.
All the above said, true soapstone has a relatively low thermal coefficient of expansion, compared to other stones like granite or marble. It wants to expand and contract less when its heated or cooled. If it must be a slab firebox, soapstone is the best choice.
If you make the soapstone slabs thick enough, you can reduce the chances of a slab firebox cracking. We recommend 2 ½” thick cut to size slabs bought from The Vermont Soapstone Company ( www.vermontsoapstone.com ). They also offer even thicker slabs on a special order basis. They ship nationwide.
In most areas of the country the only soapstone slabs available are 1 ¼” thick, intended for use on countertops. This is too thin for a slab firebox, unless it’s a low temperature gas log set.
It’s also worth noting that much of the “soapstone” sold for kitchen countertops in the US and Canada is not, in fact, true Soapstone at all. Its often a different kind of rock, like a schist, slate, serpentine, basalt etc. These stones may look like soapstone, but their mineral composition is quite different and so they have very different properties when exposed to heat.
Also, many countertop slabs have a plastic resin and/or fiberglass mesh applied to them, on one or two sides, to fill cracks and holes and make them stronger. These resins should not be exposed to high heat.
A word about veining: The classic soapstone look of a dark gray stone with a white vein cutting through it is very dramatic, but in a firebox it can spell trouble. That’s because the veins often have a different mineral makeup than the stone around them. And remember: different minerals = different thermal expansion.
If a soapstone slab is going to crack when heated, it will usually crack on a vein. So try and pick slabs with the minimum of “cross-cutting veins”. Natural variations of light and dark colored stone are not a problem, we call this “texture” and it is typical of the soapstone we use for our firebrick. It makes it beautiful and shows it is natural stone. But “lightning bolt” veins should always be avoided inside a firebox. They can be used for the fireplace surround or facing, usually with no problem. But inside the firebox they will crack.
Finally, there is the question of cost. Our soapstone bricks are made from small blocks and quarry off-cuts, so they are (relatively) inexpensive. Slabs on the other hand are made from prime, grade A blocks. They also require special packaging, handling and shipment. So by the square foot, slabs are going to be much more expensive than bricks.
If you have further questions, please Contact Us !