What is best mortar for brick J tube construction?

Hi all,
In my researches, I'm finding 'conflicting' opinions (yes, I know, every construction is different and each has their own preferences ;o) as to what's best to use as mortar in the high temp area. I have a feeling that in setting this information out, It'll help me find answers for myself, but I would really appreciate the wisdom of those more experienced if you can help me make sense of this.

The floor of the rocket is in lower temp dense firebrick, (set on 4-6” clay stabilised 'Efiperl' French product perlite/vermiculite mix) the next layer (bricks laid flat) in very old (probably 1800's) solid red house brick, sides of burn chamber in lower quality dense firebrick, roof of chamber in higher temp firebrick, riser in lower quality dense firebrick. ok, having written that, I'm now going to remove the layer of red house bricks and use the same lower temp firebrick. One less expansion worry is worth the extra cost.

i have in my 'Arsenal' the following:

#1 Refractory Mortar
French product, just add water, resistant to 1200°C (2192°F)
This is extremely hard when set, difficult to break apart if changes wanted, takes the bricks with it when you chip it off.
Q1: I have read that it can cause spalling of the adjacent bricks?
Q2:...but also read that it's the best to use for the riser?
Q3:It's also difficult to get thin mortar joints as it's heavily grogged. i think I need to make it sloppier and dip the edge of the brick, rather than trowel it on?

#2 Refractory Mastic
French product in cartridge or tub. Resistant to 1000°C (1832°F)
Easy to use, can get very thin joins. Looks pretty permanent but haven't tried to prise apart the bricks? Yet!
I have used this to glue together the six higher temp refractory bricks (at the mo they are in two sets of three, so I can grind in the peterberg 'trip wire')
I thought of using this to glue in expansion joints of ceramic cord (stuff used to seal doors of woodburners) but on reflection, it's not high enough temp
Q4: Is it better to use a higher temp tolerant clay to glue in the cord, as the mastic will surely fail in temps higher than 1000°C? I'll test this around the barrel, later. And report back!

#2 Stoneware Clay body
Smooth for throwing so v fine grog. Firing range 1240-1290°C (2264°F-2354)

#3 Craft Crank clay body
Coarser, all purpose clay. Firing range 1100-1300°C (2012°F-2370)

#4 Various earthenware clay bodies
Earthenware is usually (but not always) shades of terracotta red, with firing range 1020-1160°C (1860-2120°F) so I'm not using this in high temp areas, or in the cob mass (cost and anyway I have free garden clay supply), but may use it in final plaster coat for the colour. (I have red shades and a white in powder form)

#5 'Found' Raw clay
This was obtained from a clay works in an area of france that traditionally supplied Limoges (famous for porcelain, a high fired clay) It's pale grey so I'd guess it's stoneware type near to top of firing range 1240-1290. Can't test as my kiln is not wired in.
I am using this to make the clay slip for stabilising the perlite (without sand) and plan to use this (with sand added) to coat the flue pipes
Q5: Am I right to add sand to the slip for the flues for greater thermal stability?
Q6: clay still at 20-30%??

#6 Our garden clay
Removed topsoil and tested layer underneath. 40-50% clay so mixing 1 part of this to one part sand, to give around 25% clay content (have I calculated that right?!) we are now 18” down so need to re-test.
It is drying to buff colour, plus 10km away is an ancient pottery site, that produced stoneware ceramics, so I'd hazard we've stoneware clay body rather than earthenware. i know, I should have tested this years ago!
i am using this for the mass around the burn area, and also the bench mass, but not for mortar as I have stuff that's more consistent. i guess it could be used for mortar if finely sieved.

ok. I am now inclined to use the raw clay as a thickish slip. So I can easily dismantle the structure if I need too. I have laid four layers and am already taking down two to make changes. The refractory mortar is damaging the bricks. As a newbie, I can't get this right the first time, I need the ability to make changes. The clay has the high temp tolerance of the refractory mortar, but not the grog (non clay material)
Q7: is this a problem? The grog allows expansion and contraction with less damage.
Q8: if it did contain sand and it's slip consistency surely, does it settle out? In which case what,s the point of adding it

I plan to dip the edges of the dry bricks. They aren't completely moisture free as I had them sat outside. ( I may well dry them in the oven!)
Q9: I think I could spray them or add water to the slip if the mortar layer they 'suck up' is too thick.

I had been soaking the bricks, but read in the forum that high density fire bricks can explode as the water vapour expands. I already know this; I wouldn't dream of firing a pot in a kiln that contained any water, and the initial firing starts very slow to evaporate residual moisture, and burn off non clay material - and the wall of a ceramic pot is only about 1/4". So why did I soak my bricks??? because I read that the mortar sticks better! Yes, it does, but I have a feeling that the danger outweighs the advantage, plus Ianto Evans says to keep the mortar thin and that it is to stop the bricks wobbling, not to stick them together!
Q10: Is my conclusion to forego the refractory mortar for the raw clay, valid?

Any advice would be greatly appreciated.

Regards
Lesley

Hi Lesley; Here in the states we would use (lincoln 60) fireclay to mortar the fire brick. I do not know what would be available in france. I make a slightly thicker mix and use my hand to trowel it on, some people make a thinner mix and dip each brick in as they set them.

Hi Thomas,

Many thanks for your reply, I have some Raku clay, that has good thermal shock properties! I'll try your hand application and also dipping, see which works best for me.

Lesley

Hi Lesley, what mortar did you use in the end? Thanks for sharing, Marco

Hi Marco,

In the end, we used potters stoneware clay (I have loads) let down with water. It did the job and if we ever need to take her apart it will be easy. Taking apart the first design where we'd used refractory mortar showed how hard it is to get the bricks apart without damage. There are probably different types of refractory mortar, we definitely used the wrong one LOL.

Lesley

"Best" is variable depending on the conditions... In the US, building code requires refractory cement for fireplaces and chimneys (and possibly for masonry heaters, I don't recall offhand.) Fireclay or clay mortar is specifically forbidden. Not to say that I would not recommend clay mortar, but if you have codes to deal with you may need to take that into account.