Working with Fire and Paving Slabs ~ Safety Considerations

Since this article about the stove was published in Permaculture Magazine (PM53) I have received several emails asking about the possible dangers of heating paving slabs, particularly with regard to them violently rupturing due to a build up of enclosed air or moisture.

From my own experience I have never had any such trouble. The slabs for my stove lay out in all weathers for several years before building it. In the first six weeks of use a fair amount of water seeped out of them whenever the stove was lit, but never since. Cement is an extremely porous material, wicking up moisture very efficiently, which is why concrete foundations have to be protected from the earth with a totally impermeable membrane. This in turn means that the internal structure of a slab must be spongelike with minute interconnecting airspaces to facilitate this wicking through capillary action.

The amount of heat applied is critical. My stove gets too hot to touch but never so hot that it burns you before you can take your hand away, unlike the metal parts. It takes several hours to warm through, in winter about 6 from stone cold, 3 to 4 in regular use. This I suspect is allowing any expanding air or moisture to escape gradually through the airspaces. But if fierce heat is applied, or too much moisture blocks the escape of air, it cannot escape fast enough and could explode, much like damp rocks around a bonfire. The structure of my stove means that all slabs in direct contact with the fire are double thickness, so any mishap will likely be contained by the outer slab which takes much longer to heat up.

Following some helpful advice from an ex MOD engineer (thanks Ron!) here are some recommendations regarding the storage and use of paving slabs as outlined in the construction of this stove:

New slabs from a garden centre could well have a high moisture content if it is not long since they were manufactured. Old slabs (from a garden path, for instance) would certainly have a high moisture content as they have been in contact with saturated earth for years. Outward appearance is no indication of the internal moisture content.

The slabs should be kept under cover, preferably indoors for a month or two before use. Drying is dependent on surrounding air flow and ambient temperature. Outdoors during the summer would be ok, under cover to keep rain off. Slabs could be leaned against each other, standing on end on wooden blocks to keep them off the ground, allowing clear air flow to both surfaces etc, say for a period of four weeks minimum.

They can be tested (outdoors!) over a similar fire as will be used in the stove itself, by laying two slabs one on top of the other over the bonfire, which most closely mimics the actual finished environment, but would also minimise the dispersal of any fragments should a rupture occur. You could also wrap a slab in clear polythene, leave for at least 24 hours and see if there is any condensation on the polythene. Bear in mind that no moisture is good, but not an absolute assurance that the slab is OK.

Having built the stove with DRY slabs, the stove should be conditioned over as period of time by burning a small fire for a couple of hours to allow the stove to warm up. Leave for 24 hours and repeat a couple of times gradually increasing the time the fire is left burning.

This stove is designed to burn wood, the much higher temperature of coal will likely cause the slabs to crumble over time. If you must burn coal then you could line the firebox with replaceable sacrificial slab pieces or firebricks.

Of course I didn't follow any of these recommendations, my slabs went from outside to being lit in less than two weeks in the middle of winter! BUT I may have just been lucky - so please use your common sense. If you hear hissing - put the fire out and either start with a smaller one or leave for a few weeks to ventilate. Do leave your slabs in a dry environment or in the room they will be used in for at least a month before use and start small. As I say, mine wept quietly for several weeks then stayed dry.



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