Firebrick v Insulating Firebrick

Nowadays they are called heavy and dense Firebricksbut old masters still call them fire clay bricks just because they are made of simple fireclay (which actually is the most ordinary mud.) Fire clay can be easily located out in the nature but it must containing the right refractory properties, suitable content ratio of silica and alumina. Some shops call these bricksfireplace bricks. They are used for instance for building cooking chamber in wood fired ovens, for creating fireplaces, all sorts of fire boxes and wood heaters’ lining, linings in a small or the hugest industrial furnaces, you name it. 

Fire clay bricks are very heavy/dense having low porosity and even on various re-heating, they will last for very/very long time.

Some may confuse them with insulating lightweight firebricks, those are used in different applications. Dense firebricks can be cut only with diamond wheel attached to high speed handheld angle grinders, on an ordinary building brick saw or sliding drop saw for cutting bricks. Fire bricks can be chopped in half easily by using brick chisel and a couple of hits with a heavier hammer. It is fun and quick but may you want to achieve precise nice cuts hire a trade machine or buy yourself at least small size grinder. Before cutting soak the brick in water by dipping it in a bucket of water or in a wheelbarrow if you had too many. Leave the bricks in that water for 5 minutes minimum. 

The cutting diamond wheel will last you for a long time if the firebricks are cut wet plus you won’t breath in any dust, and of course cutting gets a lot easier and faster!
What firebricks type to use?

When it comes to fire-bricks and dense refractory products composition content often Alumina (AL) ingredient is looked at which ranges ordinarily between 18% to 40% of alumina in modern product’s body. The percentage range is important for choosing the right product for the right temperature or 

Alumina influences bulk density a lot and therefore also porosity, or if you like the weight of fire bricks. No need to apply above 26% in wood fired oven temperatures range but you can in case a low grade isn’t available to buy. Absolutely safely 18% AL amount firebricks can be used in wood ovens (you can melt and cast color metals in it too.) Furnace’s chamber build out of 18% will perform and last the same way as 30% alumina product.

Apart higher cost, additionally, higher Alumina content grades make these bricks harder and brittle (more glossy if you like) making them absorb less steam e.g. from under pizza dough bases being cooked or bread dough. However one can get used to cooking in such oven fast.

Insulating fire bricks

Watch out for their details as the insulating fire bricks, the soft and lightweight refractory bricks, are NOT the hard dense and heavy firebricks. Due to those many empty air spaces in the brick body, insulating firebricks will not absorb nor conduct heat nearly at all, thus they have a very specific application purpose.

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.

How to use light weight fire bricks insulation

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.

IMPORTANT:

Do not confuse lightweight insulating fire bricks with heavy dense firebricks. Insulating bricks are refractory too and of coursewithstand 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.

Ref:
http://www.traditionaloven.com/articles/84/firebricks-heavy-dense-fire-clay-bricks
http://www.traditionaloven.com/articles/81/insulating-fire-bricks