Double Skinned Bell - Do I Need An Expansion Gap Between The 2 Layers?

My latest Bell project is going to have an internal layer of clay brick/firebrick and I want to add an outside layer of slate slabs to match the existing hearth area and add additional mass to the bell.

Can I simply bond the slate to the inner brick layer with mortar or do I need to build in an expansion gap between the two layers? I noticed on the MHA site that they often use a layer of thin card between the two layers of masonry - this burns out over a period of time and leaves an air void that works as an expansion gap.

My concern is that this gap will/may? act as an insulating layer and will prevent the efficient transfer of heat from the inner brick layer of the bell to the outer slate layer.

Any thoughts or advice would be most welcome as always.

Normally, you need an expansion gap.

We've discussed the subject with Peter, at some point. And i suggested to use wire mesh, to fill the gap, so you still have some sort of conduction, besides the usual radiation in the gap. An architect friend said, use corundum grit. But the problem there is packing, which might push the bricks, or slates during the life of the heater.

Thanks for the wire mesh suggestion Satamax.