All of our firebricks are made from Dupré Minerals' premium quality MicaliteTM vermiculite sheet. Micalite is 100% safe, thermally stable and fire resistant. Compared to heavy traditional refractory clay firebricks this produces a much lighter and much easier to handle firebrick which rarely cracks (unlike the clay type) and which will also provide improved insulation to better protect your stove's bodywork. In fact it's so good that many stove manufacturers now also use it to make the baffle plates designed to deflect the excessive heat and flames inside the stove fire chamber.
For most old stoves which feature clay firebricks our custom cut vermiculite firebricks are realistically probably your only alternative replacement route and most definitely should not be looked at as a compromise or poor substitute for the original clay brick but actually as a much more effective long term solution. Provided you're not too bothered about how the inside of your fire chamber looks then it is definitely OK to use a mix of both clay and vermiculite bricks, only replacing those clay firebricks as and when you need to.
Download the data sheet here.