Is this brick suitable?

[wood11]

Hi. Do you think the fire bricks shown in the photos are suitable?
The thickness is 50mm.
The bricks are more than 10 years old and have been sitting in the weather. Location is Perth, Australia.
I would say half the bricks show cracks as seen in the photos and the rest are just waiting to crack I think. They spall quite easily and if you run your hand on the spalled portion it feels like hardened sand which is easy enough to degrade. Is this common with fire bricks? These seem so much softer compared to clay pavers.
The person mentioned his oven takes 3-4 hours to heat up properly with the same type of brick (built over 10 years ago).

Comments appreciated on any of the 3 issues (thickness, strength, heating time).

[Toomulla]

why would you consider a brick that is cracking and spalling before use

[UtahBeehiver]

I have to second Toom's post. Looks for another supply, I understand the pain of what firebricks cost in the Southern Hemi but there have been several builders able to find some fire bricks that don't quite break the bank.

[wood11]

I just wanted to know if this is how fire bricks were supposed to be constituted.
Thanks for the advice.

[Toomulla]

a good fire brick is hard smooth with sharp edges and a good ring when struck with another brick

[UtahBeehiver]

I was given some FB in similar condition to what you have shown in your pics. I cut a few to see to try and incorporate in my build but ended up chucking them in the trash, they were too crumbly and cracked easily. There is an old computer saying GIGO (garbage in garbage out). Casting of an oven could be an option for you if suitable fire bricks are an issue.

[wood11]

Would they be OK for the chimney and base then?

[david s]

Strength, thermal shock resistance, conductivity and thermal mass are all different characteristics of fire bricks or other refractory materials. Suitability for application will vary. Eg. To reduce excessive fuel consumption a kiln or furnace design may specify insulating firebricks which have excellent thermal shock characteristics, partly because of their open body structure, although they have poor thermal mass and strength. For a WFO most folk want good thermal mass and conductivity for fast heat up and heat storage, as well as reasonable thermal shock characteristics. Whilst our requirements don’t need a material that will withstand high temperature (>1200 C) this demands a dense material able to withstand sudden temperature rise at lower temperatures. This then narrows it down to a medium duty fire brick. Although it’s not the only solution, probably the best compromise. A higher duty firebrick will have more strength which is better for abrasion resistance, they are more expensive and much harder to cut.
Buggered firebricks can be crushed and used as an aggregate for a castable mix. It’s heavy time consuming work but if you have the bricks free an option worth consideration.

[wood11]

david s
This is the first time I have heard of castable mix with firebricks.
Is there a set of instructions that I can follow.

Also does anyone have an opinion on the thickness required.

[david s]

The crushed firebrick can be used as aggregate to make a castable. A mix of (by volume), 3 parts crushed firebrick, 1 lime, 0.5 fireclay, 1 Portland cement will work ok. But even better 3 parts crushed firebrick, 0.5 fireclay, 1 calcium aluminate cement. When crushing the brick take it down to nothing bigger than 7 mm (approx)
The thickness of the casting is usually 50 mm which provides sufficient strength, convenient heat up time and thermal mass for retained heat cooking.

[wood11]

Thanks very much. I'm guessing the water content will be critical, please let me know of any ratio.