Insulating fire bricks

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.
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.
Insulating Fire Bricks’ Physical Properties
•    Bulk Density : 604 Kg/m³
•    Modulus of Rupture : 1.52MPa
•    Permanent Linear Change on reheating 24hrs. @ 1280°C : 1.95%
•    Cold Compressive Strength : 2.01 MPa
•    Thermal Conductivity 300°C : 0.2 W/m.°K
•    Thermal Conductivity 750°C : 0.28 W/m.°K
•    Thermal Conductivity 1000°C : 0.32 W/m.°K
Chemical Composition of Insulating Fire Bricks
•    Alumina : 37%
•    Silica : 61%
•    Ferric Oxide : 1.6%
Light weight bricks for insulation can be purchased by public in pottery supplies shops.
Firebricks – heavy dense fire clay bricks
 Nowadays they are called heavy and dense Firebricks but 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 bricks fireplace 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, and under continuous heat, 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 Orton Cone but mainly if a high temps are applied.  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 co$t, 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.
Even though you can hear other words from a few suppliers who sell not locally manufactured product, conductivity and heat absorbing capacity is not influenced much at all by higher or lower alumina content. Higher grades won’t create magic temperature difference in cooking environment and vise verse. Main reasons for this are modern imports, one high alumina range suits all business and applications, less varieties to stock, higher markup and margin, those are the reasons if 18% to 26% brick isn’t sold by a store. Look for locally manufactured product, lower grades cost less to produce and they sell them for less. Where I buy refractory 26% AL firebricks costs $1.98 per one brick.
BTW mineral Graphite does not contain Aluminium oxide (Alumina, formula Al2O3 – density: 3.95 g/cm3) and it is lighter in weight by half if compared with Alumina mineral or refractory bricks. And even so, the Graphite absorbs much more heat than firebricks do – more reading about firebricks’ thermal conductivity.
Alternative for firebricks replacement page is handy and also the excellent article containing technical data with all physical properties of soapstone – soaprock.
•    21% Alumina firebrick : 1850 tons Bulk density per Kg/m³ : Thermal Conductivity : 0.98 W/m.°K : 23-Orton 1620 degrees Celsius = 2948 degrees Fahrenheit
•    24% Alumina fire brick : 1.925 tons Bulk density per Kg/m³ : Thermal Conductivity : 1.02 W/m.°K : 27-Orton 1640°C = 2984°F
•    27% Alumina fire brick : 2 tons Bulk density per Kg/m³ : Thermal Conductivity : 1.05 W/m.°K : 30-Orton 1670°C = 3038°F
•    33% Alumina fire brick : 2.15 tons Bulk density per Kg/m³ : Thermal Conductivity : 1.07 W/m.°K : 31 ½ -Orton 1700°C = 3092°F
•    38% Alumina fire brick : 2.2 tons Bulk density per Kg/m³ : Thermal Conductivity : 1.13 W/m.°K : 32 ½ -Orton 1720°C = 3128°F
•    40% Alumina fire brick : 2.25 tons Bulk density per Kg/m³ : Thermal Conductivity : 1.13 W/m.°K : 32 ½ -Orton 1720°C = 3128°F
Physical Properties of Firebricks (23% alumina product)
•    Bulk Density : 1915 Kg/m³
•    Modulus of Rupture : 5 MPa
•    Permanent Linear Change on reheating 5 hrs. @ 1400°C : -0.35%
•    Cold Compressive Strength : 15 MPa
•    Thermal Conductivity @ 750°C : 1.01 W/m.°K
•    Apparent Porosity : 28%
Chemical Composition of Dense Firebrick – 23% Al grade
•    Alumina : 23%
•    Silica : 73%
•    Ferric Oxide : 1.4%
•    Accessory Oxides : 1.1%
•    Titania : 1%
•    Fused Frits (ceramic composition/s)
Mass densities, weights of volumes and vise-verse, for various types of heat resistant materials can be easily calculated with refractory material calculators.
Who is your fire brick or refractory supplier, do you have contact-s on them and where are you located? Have you noticed chipped or seconds fire bricks being sold for better price somewhere? Please leave your comments for others below…
Names for fire bricks may vary
Fire bricks nay be called by various definitions. It depends on who works with them, names them too. Yes various entities — one more special than the other :)
Out of my head right now: 
fire brick = fire-clay brick = refractory brick = chamotte brick = fireplace brick = heat resistant brick = chimney brick = dense/heavy kiln brick (for building kilns) = industrial ceramic brick (heavy).