Identifying Fire Brick

I'm currently scrounging up materials for a small rocket stove on craigslist. Being new to the world of bricks, I don't really know what it is I'm looking at.....Is there a sure-fire (pun intended) way to tell fire brick apart from other kinds of brick? Other than subjecting them to intense heat and waiting for them to crack, I mean.....

Also, if the rocket stove is for an occasional (several times per week) cooking application, is it even necessary to use fire brick?

Thanks!

Alex; You do not have to use firebrick if you can't find any ,especially if this is an occasional use rocket stove. Regular clay brick will ultimately crack from the heat but this could take quite a while. Most firebrick you will find are the dense heavy ones ,noticeably heavier than a standard brick. Most but not all will have a name or sometimes a number cast into it. They generally are a white /yellow / color. sometimes tinted red from heat .clay brick that is old will draw a line on the sidewalk just like a piece of chalk , newer clay brick won't do that. Old clay bricks sometimes are solid and sometimes they have 3 large holes thru them to help hold them together when cementing. Fireclay is something you should be looking for while scrounging. I've included 2 photos, the bricks in the sled are all fire brick and the ones stacked by the wall are old clay bricks.

Firebricks are generally 4 1/2" x 9" x 2 1/2" or 3" (I saw some new cheap ones recently that were a bit smaller), while common bricks are generally about 4" x 8" x 2 1/2". Firebricks are commonly whitish or pale yellow, though there may be other colors. They will be very exact in dimensions and edges, though used ones may have chips and ragged edges.
You don't want any bricks that have a series of holes through them; that is an indication (not the only one) of newer hard common bricks. If you can find very old (like 80-100 years or more) red bricks that are soft enough to make a red streak like sidewalk chalk, those will work fine for your application. Hard red bricks are brittle and will not stand up to the heating/cooling cycles a rocket stove gives. Soft red bricks are even capable of being used in a RMH, though they will not last quite as long as firebrick.

By the way, what kind of area do you live in? If you have a back yard and there is clay soil, you can build a perfectly functional rocket stove entirely from that (plus some sand and straw).

Thank you both; that's very helpful.

Glenn, I live in a rural/residential zone here in Illinois (45 min west of Chicago). Since contractors love to scrape off all of that topsoil and sell it, my soil definitely runs on the clay-ish side of things Don't know if it's any good for brick or cob making though....what makes for good clay?

If you can take a damp lump of the clay and knead it smooth, then roll it out into a sausage that can be bent some without breaking, you have adequate clay for cob. The longer and thinner you can make it without it breaking, the better it is.

I'm finding quite a few of these Chicago Commons bricks in my area from multiple sellers. They look like the ones in your picture.

https://chicago.craigslist.org/chc/mat/5054530776.html

Alex; yes, those look like old clay bricks, won't last as long as firebrick in the hot part of your core , but should work just fine. .35 per brick by the pallet seems pricy to me , .10 would be better, H.D. sells new and used clay brick at .40 by the brick but they are not the old baked clay brick. Look for the best deal then try haggling the price down. Maybe you could find some to demo yourself ?

Well, yeah, that would be better Unfortunately, bricks seem to be somewhat pricey here in the Chicago area....most I've seen are going for at least .50 per brick, especially if they're sold in smaller amounts.

Alex,   I remembered this post from a while back on various types of bricks and their descriptions. In particular Erica Wisner's post:

https://permies.com/t/30551/Fake-fire-brick

1. Look at the proportion of impurities in refractory bricks.Generally speaking, impurities will be produced during the sintering process of refractory bricks strictly according to the proportion of ingredients, which is no problem. However, some manufacturers often do not produce according to the proportion of materials used, so that the sintered refractory bricks do not have a hard structure, so that the quality of the sintered bricks is not good enough, and then deceive consumers with high prices. Then when you buy refractory bricks, you can check it through its surface. Generally, the surface of refractory bricks with high impurity content will be very rough, while those refractory bricks that meet the quality standards have uniform overall color and smooth and flat surface, which we can intuitively distinguish from the surface.
2. It is the sintering experiment of refractory bricks. This method is effective. Generally, when we buy refractory bricks, we will go to the manufacturer to check. Especially before we decide to buy, we must ask the manufacturer to conduct a high-temperature test to check whether the sintering index of the refractory brick is the same as the standard index. As long as the deviation of the experimental results is not large, the quality of the brick body is still acceptable, which is also a good method. If you have doubts about this method, you can use the second method for detection.
For more information on identifying refractory bricks visit
https://www.zj-refractory.com/article/d_6441eac0e3670209c71546fc.html

How can i send a pic in this thread to help me determine if these are antique refractory bricks?