Due to the peripheral influence of the masonry heater in Western Europe and North America, and the relatively modern and constantly evolving refractory industry, specific information regarding the correct use of materials for the construction of masonry stoves, is non existent. There are for the same reason no refractory products specifically designed for use in the construction of masonry stoves.
At this point rather than follow an established protocol, professional stove builders and auto constructors alike, are obliged to wade through a soup of random technical information, and conflicting experiences, in order to choose what they hope will be the correct materials.
Though the Internet is the eventual key to the problem it is also to an extent, exasperating the dilemma of choice for the auto constructor.
The most important question in assessment of materials is durability over time.
If the difference of efficiency of various refractory materials is accepted as minimal, the important question becomes longevity.
With the names and composition of refractory materials constantly changing due to an evolving competitive industry and its marketing strategies, the assessment of specific brands of materials over time is all but impossible.
Most professionals and auto-constructors will have an answer when asked about the longevity of a masonry heater, though this will often be based on hearsay.
In North America the trade has been practiced for only thirty years. Even if the pioneer professionals are still using the materials that they began with, which is doubtful, how many are revisiting their original projects in order to assess the the condition of the materials used ?
Besides the reports by Norbert Senf:-
http://heatkit.com/research/callback-ruppell.htm Observations 28 years later.
http://heatkit.com/research/callback-inksett.htm Observations 17 years later
there have been no assessments published regarding the condition of a masonry stove after two decades of regular use.
The following information is the result of my own experience in the trade and as such, is only that. After 18 years continuous work in the field, and cross-referencing with other professionals, it is important to say that I have only a limited knowledge regarding the choice of materials for core construction.
Three things influence the choice of brick. Prospective durability over time, regularity of dimensions, and cost effectiveness.
I have observed that the only place in which there has been destruction of the surface of materials in the cores I have built has been on the rear and side walls of the fire box. The other areas of the core are relatively intact, even after many years of continual use. I once accepted that the enemy of the longevity of a masonry heater was thermal shock. Though thermal cycling is obviously not good for a heater, if this were a major problem, degradation of materials would not only occur in the fire box but throughout the core, particularly in the firebox ceiling and fire tube.
It has become evident that the actual enemy of longevity is chemical attack particularly the effect of carbon monoxide. The indicator of this is that surface destruction of materials is confined to the areas of the firebox wall that are closest to the wood load. These are the hottest areas of the core and the first to be washed by both carbon monoxide from the combustion process, and outgassing chemicals from the wood load.
There is also the possible influence of gas abrasion, and direct abrasion from the wood load.
The type of destruction seen ie vitrification or clinkering and disintegration of the materials surface are indicative of gas attack.
Taking this into account, in these areas, it is better to use a dense high alumina brick ie medium or high duty, than a porous low duty brick. Porous brick may resist thermal shock better than denser, high aluminum brick, but the denser brick will stand up better to chemical attack, and so should be used in the fire box. A denser high duty brick will also have a higher capacity of accumulation and superior transfer efficiency, and so if affordable should be used throughout the core.
Dimensional regularity is desirable in a brick, as a true brick will be much easier to lay, and use less refractory mortar than an irregular brick. Having to lay deformed or under, or oversized brick will necessitate thick joints. As joint thickness is a serious consideration, irregular brick are best avoided.
In general I have found that high and medium duty brick tend to be truer that low duty or generic brick.
High duty brick can be much more expensive than low duty brick. For the professional undertaking repeated construction this is an important issue, though for the one time constructor it should not be given too much consideration.
I often use high duty brick throughout the core due to their uniformity, and the delivery logistics of employing the same rather than multiple classes of brick.
Though in terms of necessity I am confident to use high duty brick only in the fire box, including its floor and corbelled ceiling, and low duty brick for all other portions of the core.
When a particularly true, dense, solid facing brick is available I do not hesitate to use it for the bottom two thirds of the side channels, and the first four courses of the core, below the hearth.
In this example the first 4 courses of the core and 12 courses of the side channels are in clay brick.
This open forum discussion within the MHA will emphasise the complexities of choosing a suitable refractory brick.
TD sheet for a typical high duty brick
France Non Spall (PDF)