Yes I read the Fake Fire Brick post...but.

Hello fellow RMH folks, so I have not built one yet and am still reading, reading...reading. I have gotten into the weeds of fire brick and now I am confused.  Without all the details, charts and bantering...can I use this brick for my J-Box and riser?

https://www.lowes.com/pd/Oldcastle-Red-Cored-Clay-Brick/3433312

I am not comfortable casting the J-box, it seems that building from bricks and using fire clay as mortar is the best way for me.

Thanks
Zeek

Not sure of this exact brick and how it will hold up but I do remember reading in Ianto's book about the bricks he used being soft. Hard fired bricks will spall and crack a lot more quickly. I remember on my first stove using patio pavers. They only lasted for about a season. I moved to the heavy halved firebricks and am quite happy for now with them. Its easy to get lost in too much information which can stall you at the starting gate, so I would recommend being that this is your first build, get your hands on some cheap or free bricks and experiment outside. Its really the best way to get your own experience to match (or not) what you've been reading and what works for you.

Any cored bricks are most likely hard modern building bricks, and will fairly quickly crack and fall apart in the extreme heat stresses of a J-tube firebox. They may work fine for testing, but you need to get more durable materials for the real system. Soft insulating firebrick at all but the exposed edges would be the most efficient, but they are expensive and not widely available outside ceramic/pottery supply houses. Common hard firebrick are the most durable, but will reduce efficiency somewhat because their mass takes a lot of energy to heat up. "Splits" (half-thickness firebrick, usually 1 1/4") are the best readily available compromise. They may be sold in hardware stores as replacement liners for woodstoves.

Any bricks could work well for such things as the outer shell/skin of the thermal mass.

With the above mentioned cautions in mind . .. .

Build your rocket stove with whatever you want to build it with.
In my experience, once you get the first one built, you will realize your mistakes, want to improve the stove in some way, or just rip it apart and build it bigger !

Go ahead and have fun, just keep in mind, that after a year or so, the bricks in the fire chamber may start to fall apart on ya - as per the above warnings....