Clay slip vs backyard clay - quantity question

Hi all. I have a goodly sized pile of clay in my backyard that I dug out of a pond I built this past summer, so I think I have enough of that to build a mass (and presumably to mix with the perlite underneath the heater). However, my understanding is that I want some pretty well processed clay from a pottery supply store in order to put the firebricks together in the heater itself. I'm planning to make a 6 inch rocket heater and I have an opportunity to travel to the place where I can get the clay (they don't sell it in my home province), so I want to make sure I have enough for this purpose. Does anyone have any rules of thumb as to how much I should get to ensure I don't run out?

Also, is there a specific fire clay to use or avoid? I assume anything from a professional pottery supply place is going to fit the definition of 'fire clay'. Do I go with stoneware? Earthenware? Does it matter? Also, is there some advantage to porcelain over these other clays, and if so, is that make or break?

Thanks in advance!

Since no one has answered your question I am hoping these threads will help:

Fox James said, "I am assuming you want the clay to fix your bricks together, the standard grey potters clay will be more than adequate as it is normally around 25-28% alumina and fired to around 2200f in a standard kiln.
I mix mine with 50% silica sand and water to make a nice mix, the thing about clay is it doesn't really stick anything together very well at all!

https://permies.com/t/40/73645/Walker-Stoves-Brick-Rocket-Mass#1617986

Thomas said, "Locate a masonry supply and source your firebrick and fire clay there.

https://permies.com/t/190222/Fire-Brick-Alternative#1571374

If this doesn't answer your question maybe someone with more knowledge than I will respond.

Thank you Anne, I appreciate it.

For anyone else who might have hints or more info on my above question, I'll add this: did you go with straight clay slip, sand mixed mortar, a mix of the two? Or even refractory cement from the brickyard? I'm a newbie so I'm trying to find as many pros / cons as possible.

Cheers all!

I'm sensing some confusion about your question.  Mixing clay and sand is great for a mortar that is easily removed/replaced/repaired.  Clay slip is what you use for firebrick when you just dip your brick in a very liquid clay slip for a thin coating of clay that serves to eliminate air gaps.

Depending on the quality of your clay you could just  mix some up in a bucket with a paddle mixer, work all the clumps out of it and strain it through a paint strainer.  
That's why I did when I was experimenting with my backyard clay and my clay is of a quality that fires very nicely.  Haven't tried it for setting firebrick yet, but I'm pretty certain it would work just fine.

Backyard clay can be anything from fine smooth nearly pure clay to glacial till mixed with silt, sand, gravel and rocks (what I have). So we need to know the character of your own clay before saying whether it will work for mortar. If you can liquify it and screen out anything larger than fine sand, it will probably work well for you. If that is not practical, clay from a pottery supply would be fine. Stoneware would probably be best - make sure any  grog in the clay is very fine.

Andre,

If you want to purchase a pottery store clay look for fireclays that are refractory - they have low shrinkage at higher temperatures and also they do not vitrify at these temperatures - if they do then they behave like stoneware clays. After some research I concluded that the best properties may be offered by Hawthorn Bond fireclay. I would mix it with Mulcoa grog of a coarse mesh 20/50. Remember that the largest grains in the fire mortar should not be larger than half of the joint size, so if you lay bricks with 2 mm joint then the coarsest particles should not exceed 1 mm.
For bell I would use what you have available, but it still would make sense to dry the clay, break and strain through some mesh.

hello. I am starting a small heater and have fireclay and builders sand(it has some silica).  Also have some basic builders cement.

Just laid the foundation layer of firebrick with 3 to 1 of the sand to fireclay and it went well. But in working with it, it doesnt seem like it would be an ideal mortar for the rest of the build . Due to it not having much 'adhesion and leaves a larger mortar line than ideal I suspect.

So, am wondering if I can alter the mortar for the other TWO parts. I see the burn tunnel and heat riser as the Hot portion and would settle on one mix, and the later ends of the bell and bench the cooler portion with a potentially different mix

After searches and thinking the best idea I have is
for the hot portion:
1- either straight fireclay (slip or thicker variation of a slip)
2- a 1 to 1 of sand to fireclay (the thinking is the sand helps with expantion and contraction?)
3-a 1 to 1 to 1 of sand to fireclay to cement (the thinking here is a bit of 'glue'/cement would help the tall riser hold together but still allow for some dissasembly later?

for the cool portion:
1- 1 to 1 of cement to sand
2- 1 to 1 of cement to fireclay
3- 1 to 1 to 1 of cement/fireclay/sand


or should the basic 3 to1 or 2 to 1 sand to fireclay just be used throughout?


Any ideas as to which (if any of these mixes would work?  thanks much

Straight fireclay will crack every time. (I know, I tried)
I only build using 3 parts sharp silica sand to one part Lincoln 60 fireclay.
Mortar joints made with clay should be 1/4" or less, and concrete joints should be 3/8" or more.
Refractory cement will crack as fast as clay joints but is hard work to remove, clean up, and reuse the brick.
Clay mortar scrapes off and rehydrates with no effort and bricks are ready to reuse immediately.
Remember, always soak clay bricks before using mortar or it will dry too quickly with no adhesion.

thank you. The sand I have is 'construction sand with silica'. It seems course to me. So wonder if I can do a 2 to 1 ratio instead so that it is a little more mangageble and might even stick a bit compared to 3 to 1....is 2 to 1 enough to still allow for contraction expansion. If in fact that is the purpose/benefit of the sand?

Carol,

Particle size will affect the cracking.
I have coarse sand with grains up to 5 mm and local grey clay. I have to mix it using ratio of at least 1:2.8 to prevent cracking. I used it to lay the CEB blocks of my home. The same clay mixed with silica sand mesh 90 (particle size 0.17 mm) in ratio 1:1 did not crack at all in an experimental pottery piece. Assuming that regular firebricks are designed to be laid with 2 mm joint, the grain should not be larger than half the joint, so in this case 1 mm (mesh 18 or finer). Of course coarser aggregates, thicker joints will also work and will heat your house nicely, but 2 mm is a standard.  High-alumina or other specialized firebricks are laid even with smaller joints.

I would just make a few various ratios mixes with the materials that you have on hand, dry them and check their strength and cracking level.