Fire bricks - for traditional fireplace (Bilton HG1)

In traditional fireplaces, there are two loose firebricks at the sides and one at the back. Any size would be useful and if too large, they could be cut to size. All three are needed in cold weather, for burning logs from a nature reserve. When funds allow, a log burning stove will be installed. In the nature reserve, lots of cut wood is piled up and left to rot down for wildlife. For the same reason, dying and dead trees are left where safe to do so. To minimise my environmental impact, I would prefer to collect as close to HG1 as possible. Failing that, HG2, HG3 or HG5.