Can anyone offer advice on the type of mortar I should be using?

I've been using Heat stop 50 for a rocket stove build core every time I fire the first time it pulls a part at the mortar seams heres a video of the aftermath    I gladly accept all critics I am willing to follow advice. The ceiling held up good until I placed weight on it. I'm sharing a failure to save others from the same issues. I have been advised by a local contractor to build a dome made of firebrick over the top wrap with metal mesh and cover with heat stop but I really want to keep it sort of light weight for use in mobile application. I have thick skin please don't hold back.

Hi Brian, without a lot more information it is impossible to give advice or even criticism. For example: 1) did you follow the water to dry mix ratio suggestion. 2) Was the heat up schedule followed to the letter. (usually it is something like increments of 100F per hour from 300 to 900F) 3) Were the bricks cleaned properly prior to laying them. 4) did you allow the mortar to properly air dry before heating it.

Most of these refractory products have very narrow success windows and anything outside of the design spec is pretty much a guaranteed failure. Also, if I understood your comment correctly, you poured the top cover out of heat stop 50 as well. I don't think it is spec'd for that. It is a mortar, not a castable. Very different product. Mortar is like a glue to fix bricks in place; castables are designed to make components.

I hope this helps.

Yes I followed the instructions on the bag for the water mixture. I was unaware of the heat up process. Are you thinking too hot too fast? I cleaned the edges with a grinder blade the second time. The entire thing cured for about a week. I called the manufacturer on a Sunday at dinner... well at least that's what it sounded like I was asking him about adding perlite and casting. He said I don't know you could try it. It feels to me like there's too much motion in the expansion from the heat. Would covering it with metal and more mortar only yield the same? The ceiling had crushed fire bricks in the mixture.

I haven't played around with refractory mortar. I have used castables, though. My thoughts, after seeing the cracking in the first part of the video, are around the difference in expansion between the firebricks and mortar, and how that movement appears to be overcoming the adhesion. As Duane mentioned, these products have precise mixing and application instructions along with initial firing tolerances. They're really pretty touchy. The temperature profile experienced in firing the batch box probably ramps up too fast, and most likely has hot and cool spots instead of an even gradient pattern.

You might want to try a simple fireclay slip mortar with sodium silicate. This will be a lot more plastic than the refractory product and may be more forgiving of the erratic heating of a wood fire. If you get it nice and hot, the clay and silicate will flow a bit. Also, you might have to accept some shifting around as the nature of the beast and count on the wrapping to keep everything in place. I'd try laying bricks across the top, or if that simply won't work get a bag of castable, make a top using a form and then mortar it in place.

I agree the two materials plus hot and cold spots are the issue. That was my first thought. Can you talk a bit more about clay slip and sodium silicate. A plastic type mortar material is a great solution. I live in Virginia where the soil does not perk there's tons of clay out here for the taking. When you say clay slip does that mean are we talking about the stuff outside watered down or is there special clay? Is the sodium silicate a liquid in a bottle that I'm seeing online labelled as water glass? I'm close to charlottesville Va and will be heading towards Arkansas any good places for materials along that route? Also Ive read the thread about ceramic risers does anyone know a current supply for those?

Clay slip (in this case) is just with enough water to make it a thick liquid. I'd recommend using actual fireclay since you are dealing with high temperatures. Sodium silicate (aka waterglass) is used in blending slip to lower viscosity and allow a higher ratio of solids to water in the mixture. It has the added benefit of acting like a low-temperature glaze when fired, flowing and wicking into the body. This property may help the mechanical bonding of the joints, especially during the first few firing cycles when the materials are still settling.

I guess the best thing for it is experimentation, and it looks like you're well down that path with this project. You probably have nothing to lose by trying the clay slip and if it fails as well, you should be able to grind it off as you did with the mortar.

Can you recommend a brand name and location you buy fire clay? I searched high and low. How is it sold dry or moist? Bag or bucket? average price?

It's pretty much a commodity item and cheapest dry by the bag. I found a mid-Atlantic pottery supplier that has a few types: http://www.clayworkssupplies.com/loadItems.cgi?c_id=0&sc_id=&keyword=fireclay

$16.50 for a 50-lb bag seems about right.

Any of the water glass / sodium silicate products will work, or you can make your own from lye and kitty litter crystals. Fire clay is available from pottery suppliers online or from the same places where you buy fire brick. Another choice is fire place mortar, usually available from companies that sell and install fireplaces; it comes as both powder and in 5 pound tubs, premixed.