Good "lite mortar" sealer

I'm experimenting with small brick RMH designs which could be disassembled and re-assembled fairly easily. (That's both for reasons of experimentation and portability). To do that, I need something that acts as a sealer (or "lite mortar"), but is not permanent. In other words, it seals and sticks a bit, but is easy to scrape off. For horizontal and very thin vertical gaps, I've learned from this forum that slip (aka "watery clay goo") can work. However, for any larger gaps, slip is just too liquid to do the job. Think, for example, of the gaps around a circular metal flue as it joins with a square hole in a brick mass. Even if the flue is snug in the square, there are substantial gaps between it and the bricks in the four corners of the square.

I thought of using clay for such larger gaps, but I think it's pretty certain that it would shrink and crack pretty soon. I've been using chunks of rockwool, and it certainly works a bit, but it is not a good seal.

Any suggestions?

Hi Kit,   Welcome to the RMH hotline where many operators are standing by to help a fellow rocketeer out!

Just yesterday I tore apart some of my stove and built up the manifold area with adobe bricks and clay slip. To fill in the larger areas, clay slip can magically turn into its much thicker cousin "cob" when you add some sand (which helps the clay from cracking) and possibly some form of fiber (depending upon how hot it will get and how much tensile strength is needed).

Removing the old slip and cob was a breeze. A little water and some scraping with a trowel and I was reusing the old material no problemo.
Various sized rocks can also be used to fill in larger gaps to help reduce the amount of cob needed and also helps with initial setup strength so you can build more height in one day with less chances of slumping.  Hope this helps....

Thanks Gerry--It would need to "take the heat" in places, so I'm guessing some "perlite" mixed in for a high-heat fiber? Any other fiber that could work? I'll prob try it with just the sand first, though, as you suggest.

How about rockwool and clay together, high tech cob?

William--Now, why didn't I think of that? (Don't answer that question!)
Great idea. I can try to put those nasty bits of fluff to better use.

Kit Collins wrote:Thanks Gerry--It would need to "take the heat" in places, so I'm guessing some "perlite" mixed in for a high-heat fiber? Any other fiber that could work? I'll prob try it with just the sand first, though, as you suggest.

Perlite is round, not long and fibrous so it wouldn't work very well to increase strength. I like Williams suggestion of using rock wool. Tease some into the cob mix.
Some people have used straw and know that some of it is just going to burn out.