Uncommon firebrick size - or is it firebrick?

Hi all. I acquired some firebricks for my RMH before a thorough research into building it, so I have some questions about these firebricks. They measure 7 5/8" x 3 5/8" x 3 5/8" - I don't have a scale, but they're extremely heavy. The color is white with the barest hint of pink and there are sparsely scattered specks of what looks like silica. I can't find any reference to this size brick online, even going to mfr sites for every type of firebrick I've come across.

One concern has to do with insulating the heat riser. With these clunky bricks inside a 30-gal drum having an inside diameter of 18 1/4" I'm afraid the gap between the heat riser and drum will be too small if I wrap insulation all the way around. If I knew the rating of these firebricks...

The other issue is I think I want a 6" system, but the closest I can get to that is 6" x 7 5/8" with these bricks. Is a rectangular system doable, or should I make it 7 5/8" square? Which leads to yet another question. I'm going to make a very small RMH with a footprint of about 20" x 48" (there's wiggle room). I think I glommed somewhere that the length of the ducting through the thermal mass is dictated by the system size and vice versa. Does anyone have any thoughts on this?

Thanks from Fayetteville, NC!

That's not a standard size for firebrick. Then again, firebrick can be made in any size.
The fractures suggest these brick are A) used or B ) have been left exposed to the weather for a prolonged period.
The brick on the right looks to have a wedge shape. Is this a photographic effect? Are these RKB (rotary kiln brick)? If RKB, the heat rating would likely be 2k degrees at least.
The color is about right for a higher temp brick, but clay bricks come in every color of the rainbow.

Space
Would it make sense to install the brick on the outside of the drum, rather than the inside to get the space right? You'd get the insulating properties, but the drum steel would take a pounding.

Thanks for the input, Ken.

The bricks were outside for over a year, according to the guy I bought them from - and unused. He said he got them from either Lowe's or Home Depot (can't remember) for .60 each, which is what he sold them to me for. The wedge look is because of the angle the photo was taken.

I was planning to make the entire 'J' with this brick, so it would have to go inside the drum. From what I understand, the heat riser can't get too hot on the outside of it or there won't be enough of a temperature differential between it and the drum to cause draft.

I'm thinking maybe either refractory mortar or a perlite/clay slip mortar to fill the gaps and angles (as opposed to a full wrap) might be enough insulation, without increasing the outer dimensions of the heat riser.

I'm also thinking maybe a 4" system would be more appropriate for the size RMH I want to make.

Something is not adding up. Firebrick carried by Lowes or HD will be a standard size 9x4 and 1" thick.
60¢ was his price? Perhaps $6 each for 1", but these are 3"+
Having been left outside for a year, I fear they will crumble after just a few firings.

I would be hesitant to use these bricks. I think you are asking for some big headaches.

Thanks very much for the info, Ken. Would it be worthwhile to do some heavy duty burn tests on the bricks, or should I just buy new ones from a storefront?

By all means-Test them.
It would be prudent to dry them completely with low heat for a while.

If they don't crumble, you won' have to shell out more cash.

I'll do it. Nothing ventured, nothing gained, right? If they're not good for the system, I can use them as the floor of the bench. I really appreciate you.

BTW, I just found this AWESOME thread: https://permies.com/t/30551/rocket-stoves/Fake-fire-brick

Jacquie, test theses in a campfire, embers are hot enough for that. And if they crumble? Make chamote with theses for your clay. So you would have refractory clay

That's a great idea, Satamax. I inspected the rest of the bricks (I have 44) and found that only a couple of them have cracks. However, I now want to test the bricks without cracks also.

Would you, or anyone, happen to know if my system size/CSA would be limited by having about 8' of ducting through the thermal mass plus about 7' of vertical exhaust?

Jacquie, if you don't have the lengh, use bells instead of flues. Better heat transfer. Matthewwalker's half barrel system is a cheap one to make bells.

With two half barrels you get about 1.5m² of internal surface area, contained in about two feet wide, by six feet long.

For a 6 inch system, with a single bell, you can use about 2.5m² to 3m² two barrels would do the trick. You could build a 6'x6' bed with thoses easily. It doesn't take too much space nor height. Or if width is at a premoum and you are not too bothered about seating on the bench, you could do two half barrel bells above each other.

If as you say the bricks are heavy, they are not good for most of the high heat areas regardless of their thermal capacity, as they will conduct heat readily away from the burn tunnel and riser, lowering burn efficiency. They would be good for the floor and feed tunnel where abrasion resistance is important.
You do not want to try a 4" system unless you have a lot of experience tuning larger systems, as the smaller the system, the trickier it is, and the less efficient it will probably be. Surface area versus interior combustion volume means too much heat gets drawn away from the combustion zone.

How big is the space you want to heat, and what are its characteristics? What is your climate like?

The stacked bells would likely be the best option, Satamax. I was going to have a very short bench, but that could very easily be converted to height.

Glenn, thanks for the warning about 4" systems. I'll leave that alone. This will be my first build. Maybe I can use leftover firebricks for mass. Heated area will be 420 sq. ft., which is my kitchen and living room (sparsely furnished - I'm somewhat of a minimalist). The house was built in 1953, stick frame, 8' ceilings, oak tongue & groove flooring, crawl space beneath. The climate is erratic here in the Piedmont area of NC. Last winter we had shirt-sleeve weather followed by sudden snow and ice storms a couple of times. This summer has been more like spring so people are saying that means a cold winter. Who knows? Generally, this is a mild climate with an average of 20-25 degree difference between day and night temps, so a massive heater isn't warranted. Moving off grid is motivated by economics and principle.

Edit: No insulation under floorboards, therefore drafty.