In my book, Masonry Heaters: Designing, Building and Living with a Piece of the Sun, I introduced the concept of an heirloom masonry heater. I characterize an heirloom heater as a unique heater, designed for a specific house, and designed to be as long lived as possible.
Though masonry construction, by its very nature, is very long lived, it is certainly not indestructible. And masonry heaters in particular are not just your typical masonry construction. They are purposely designed to be subjected to repeated cycles of extreme heat followed by hours of cooling, only to be hit by the extreme heat again. This is very stressful on hard, brittle materials made out of fired clay. These extreme cycles are, or can be, just as destructive as the well-known "freeze-thaw" cycles that masonry can be subjected to in the outdoors.
Heating anything makes it expand. It matters not whether you are talking about something as light as air or as dense as concrete. Heat makes things grow. Something that is heated gets bigger; when it cools, it gets smaller again. What's even worse is when one surface of a material gets substantially hotter than another surface of the same material. The differential expansion - the fact that one part of the same piece of material is expanding faster than other parts - can split a single piece into many pieces. Parts of masonry heaters are subjected to this kind of stress all the time.
The key to making an heirloom heater - one that will last for generations - is to understand how the building blocks of a masonry heater must always expand and contract. "Expand and contract" is the same as "open and close"; "bigger and smaller"; "grow and shrink". This is a lot like the way humans breathe. The chest expands with air, then deflates, expands, contracts. The muscles of the body do the same thing. They flex, then relax, flex, relax. In good heater building, we take note that what these systems have in common is materials of one kind or another that allow them to expand and contract without coming apart. In the case of muscles, the body has connective tissue such as tendons and ligaments that bind the muscle to bone and help hold everything in place as it moves with expansion and contraction. A really good heirloom heater uses those same principles.
Some masonry heaters are just built with common bricks and mortar. These items are quite strong for most purposes in construction. For example, they are very good at bearing lots of weight. But mortar is relatively weak as a connective tissue. It has no flex to it like the tendon of a muscle and not very good marks in tensile strength (the ability to resist tension - a pulling apart stress). So sometimes bricks and mortar will crack when subjected to the stresses of a masonry heater. Well, this is not an issue if the heater is built with some kind of mechanical tendon to hold everything together. But, most bricks are not laid that way. This is why what I call an heirloom masonry heater is not built with common bricks and mortar.
Instead, an heirloom heater is built with refractory (heat resistant) materials. In addition, we bind the heat resistant materials together, literally, with physical tendons that help hold everything together. The materials can expand and contract, but the tendons make sure everything remains where it belongs. This prevents cracks between materials, should they occur, from growing ever larger, which, inevitably, would tear the heater apart. The use of mechanical tendons means we can also use mortars that naturally have a little more "give" or flex than harder mortars. We want the heater to be able stretch and come right back to its original size without doing damage to itself. This is one aspect of an heirloom masonry heater.