Insulating fire bricks are soft and light in weight. They can be easily cut by handheld hack saw or any other hand tool like chisel or even drill bit. Color of insulating bricks varies but usually they come in shades from light brown to white, see pictures. In refractory air is the best insulation and this is why insulating firebricks have excellent insulating properties. Their body is made of tiny air spaces similar to honeycomb effect.
Use of light weight fire brick insulation is broad, mainly in industrial and hobby kilns heated up with either electric spiral elements or gas burners, furnaces, both for hot face lining or outer back-up heat insulation.
Do not confuse lightweight insulating fire bricks with heavy dense firebricks. Insulating bricks are refractory too and of course withstand very high temperatures range BUT for purpose they have very low thermal conductivity and don’t absorb the heat well at all. Those are intentions for heat insulation. For instance if you used them for building of the hot face in a wood fired oven (for cooking) the oven wouldn’t cook, because it wouldn’t store and hold almost any heat. You can however use these insulating fire bricks on the outer side (of the heavy firebrick walls, vault or under the floor bricks and slab) to prevent the soaked in heat in the chamber’s body mass from escaping and achieving superb results. Meaning well insulated oven which will hold the absorbed heat in its mass, where it should be to cook for long time.
Light weight bricks for insulation can be purchased by public in pottery supplies shops.