How do you identify/differentiate firebrick?

Seattle Pottery Supply will sell me a firebrick for 15 dollars. Several people on Craigslist/FB market want to sell me hundreds of "firebricks" for ~$2 a piece. The big box hardware store wants to sell me a pallet of 138 firebricks for 400 dollars. The stuff inside of electric kilns seems to be a completely different material (very low density). These all seem to be different products. Some of them are made of refractory cement? I think? and are probably good enough for a low temperature fireplace. I have dreams of a 2400ºF wood fired kiln, and reading here it doesn't sound like that's a very high number for what the bricks in a RMH need to withstand. How do you know that the stuff the lady on craigslist that she said she pulled out of a chimney is good enough for what you need?

Shahar,

For wood fired kiln it makes sense to build the firebox from dense firebricks and the rest from insulating bricks.
If you want to experiment with salt glazing you will need high alumina bricks (at least 70% Al2O3) to resist alkali attack.

Big box stores usually have inferior masonry products.
Quality super duty 42% alumina Alsey brick, made in USA can be purchased for $5.
$15 per brick is outrageous - for lower price you can get USA made 80% alumina and ship them to your location.
Insulating US made firebrick 2600 F can be purchased for $5-6.

If buying used you will have to do your own research. Please look for light yellow bricks with good edges and no heat distorted surfaces. The faces should be smooth and feel dense, not crumbling.
$2 per used brick may turn out to be quite expensive if you factor in your time needed to remove old mortar. If the refractory cement was used you will need to use diamond grinding wheels to remove it and for such bricks I would not pay more than $1 a piece.. Please wear a mask.
Do you know the brand of the used bricks? Old ones frequently had names stamped on the face.

There are so many types of fire brick that identifying them without labels is nearly impossible

At the top of the heap is white light weight soft firebrick porous like open cell styrofoam.  It is used for kilns.  Very easily damaged.  Mostly alumina materials with added other high temp stuff.  Poor chemical resistance as well as being soft.  Depending on material mix max working temperature can run over nearly a 2200 thru 2600 degrees F.  The white color or really near white color and the light foam type make this on fairly readily identifiable

Next layer is broken into 2 types.  Both high temperature:  A medium weight fairly durable, usually light tan with a mottled color and a really heavy, very durable, usually dark brick: both not quite as heat resistance as tier 1 but close. The dark heavy one while durable at high heat is a bad choice for rmh use because of its high mass dense nature.

Next tier is often called fire brick but only intended for fireplace type use and not real high temperature.  When used at high heat typically decays to powder fairly quickly.  This is what common fire clays make.

And there is no way to tell how good the bricks are just looking at them.

If you can find out the temperature rating, that should give a good indicator towards the price difference.

Not sure where it falls in the information provided here by people more knowledgeable than me, but soap stone plays in this world. High-end stones use it because it takes more heat than many fire bricks, AND transfers it.

If a person had access to a lot of talc, and could pick up some waterglass (sodium silicate) like could be found in the 55 gallon drum after the Obo car thing (they used it to destroy otherwise good engines), the two mixed and baked will produce soapstone.

Thank you, Cristobal for the great information! I was pretty much uninformed when it comes to fire brick. I figured it was all jut about the same. Thank you for sharing some very valuable knowledge!

I have access to several firebricks that were used in a bread baking oven. They’re in Mexico and they have
6331324003 stamped in them. Any idea what that number indicates? Thanks

Jeff,

Please post a picture(s) of the brick so maybe we will help.