Mortar for fire Brick - Latest/Best recommendations?

It's been quite some time since I was on here ( like 7ish years ?). Anyways, I got the Batch Box RMH bug several years ago , and slapped one together for my 15' x 25'  shop. It has worked out marvelously, and held up well for several years.

I may be rebuilding a new version sooner than later, & thus my question. At the time of my original build, there wasn't a lot of info on fire brick mortar, and a search here, now isn't giving me much better results.

Has anyone come up with a better, or best at the moment (11/22) mortar mix that is more/most compatible with the expansion/contraction of the fire brick in the burn box & riser? I've been weighing the pro's & cons of "normal" vs refractory mortars, and & just now even more uncertain which way to go.


TIA!

Hi Arlyn;  Welcome back!
The newest skinny-on mortar in a batch box core is to use no mortar at all!
Matt Walker suggested that using any mortar at all in a core was doomed to fail.
That's right as you know refractory cracks out and is a bitch to clean off of bricks.
Clay and sand mortar will also crack out in a core.  
It is easy to remove and can be reapplied (sort off) in place.

I have three batch boxes, and after trying both mortars I became a believer in using none at all.
My 6" and my 7" use angle iron and all thread thru the door plate to gently snug the fire bricks.
My Walker riserless is captive and needs no snugging up.

Here is the thread where I rebuild the 7" shop batch with all thread and no mortar.
https://permies.com/t/181421/Batch-box-rebuild
And here is my 6" rebuild with all thread and no mortar.
https://permies.com/t/187667/Studio-Dragon-Batchbox-Rebuild

The one place using mortar in the core would be helpful is when your using bricks that don't have perfect faces/edges on them; salvaged bricks for example. The mortar can then be used to even/level out these imperfections and also seal air leaks that may affect burning performance.
Mortar may at first seem like its acting as a glue, but after many expansion/contraction cycles, this bond often breaks and your just left with a dry stack anyways.

Thank you.

I searched ewe tube for hours, seeking a solution, to no avail. Much like with the original build. The next time I sat down to a tube session, I came across "Rocket Mass Heater Surgery, pt 2", which describes a (more recently used) clay sand mortar mix and I thought " Well, I wasn't TOO far off..

I built my current combustion chamber using just pottery clay and lap joints, so it will be interesting to see what it looks like after several years of use. I followed "plans" from an old Peter Van Den Berg post. I used pottery clay mortar throughout. I even used the same for a outer shell, mimicking  the regular masonry shell I imagine most of you have seen him use in most or all of his ever improved test versions.

The nice thing I've found with pottery clay is how easy it is to remove from the fire brick. The outer shell has had a very few cracks, which are easily re pointed with pottery clay. Roughly 99% of the clay mortared exterior shell (fire) brick have had no issues ( I know, kind of a "waste" of fire brick, but I had a bunch, and it can easily be reclaimed). On the flip side, many of the bricks in the fire box have come loose - most likely from being hit by chunks of wood I suspect.

Thank you for the links, Thomas, it all looks so familiar. I appreciate the input & will let you know what I find in the tear it  down, and go from there..

Thanks, again...

A