What kind of bricks are these?

I got these from an outdoor fireplace demolition.  

They look like same/similar material as cinderblocks.

How are the heat-shock handling properties of these bricks?

Can I used them as a base under firebricks, to save on firebricks?


Pictures attached.    

Those look like firebricks to me, but I'm certainly not an expert. The ones we have are yellowish-tan and have a groove on one side and a corresponding ridge on the other side so that they can be fitted together in the same way you would fit tongue and groove boards. Do yours have a ridge on the other side? Also, firebricks tend to be pretty heavy compared to regular clay bricks.

Deb Stephens wrote:Those look like firebricks to me, but I'm certainly not an expert. The ones we have are yellowish-tan and have a groove on one side and a corresponding ridge on the other side so that they can be fitted together in the same way you would fit tongue and groove boards. Do yours have a ridge on the other side? Also, firebricks tend to be pretty heavy compared to regular clay bricks.

My firebricks are yellow-tan as well.  These are cinderblock-grey.    Flat on other side.

I just had a thought. We dismantled an old glass kiln once and it had huge slabs of a material that was sort of like massive firebricks in it but they weren't the usual color. We figured they must have had some sort of insulative application. Could those be a mixture of clay with something fire-resistant and yet lightweight and insulative--like vermiculite or glass beads, etc.? Have you tried just googling firebrick images to see if you can find something similar?

Deb Stephens wrote:I just had a thought. We dismantled an old glass kiln once and it had huge slabs of a material that was sort of like massive firebricks in it but they weren't the usual color. We figured they must have had some sort of insulative application. Could those be a mixture of clay with something fire-resistant and yet lightweight and insulative--like vermiculite or glass beads, etc.? Have you tried just googling firebrick images to see if you can find something similar?

Yes.   Everything was variations of yellow-tan bricks.

I think what I've got is cinderblock... More fire tolerant than clay, but not as much as firebrick.      I'll used it where a second layer, not directly fire contacting, is required.

How much do they weigh?

An important diagnostic factor, as mentioned above, is how heavy the bricks are. Do they feel like dense firebrick? Do they feel almost feather-light?
If there are any cracked corners or surfaces, what does the raw surface look like?


If what you have is cinderblock, or cement block, it is far less fire-tolerant than clay brick. It would still be fine for a backup layer that will not be exposed to direct heating.

And the dimensions, knowing the density would go a long way to id'ing the material.  Were the bricks between the house structure and the back of the fireplace or flue?