Elemental dilemma.

Barb has decided she wants a fireplace in the Guesthouse, which brings up a new set of problems in the construction process.

I've put in a 38" X 32" opening in the south wall for the firebox and gotten ahold of a load of bricks that the brick maker in Sussidel said are baked at high temperature, the picture shows a few of them.

Different angle of the same thing.

My dilemma comes from my hard headedness, I cause my own problems. I could just build the firebox the way they do here and everything would probably be fine but this is not a deorative fire place, it will be burnt every night. Here they use a standard mortar mix and throw in some raw sugar to try and keep the mortar joints from cracking, which is fine if you only burn a fire once in a while and aren't concerned with an air tight firebox. Fire places are the most ineffecient ways to heat a space, so I would like to have a venting system that will draw some heat from the firebox using natural convection - which will require an air tight firebox.

There is no high temperature mortar here, which is fine with me because that stuff cracks anyway, you have to use a mortar that is softer than the material you are trying to bond together. One advantage of being old is knowing a thing or two. A lime mortar will do the trick, softer than brick and self healing. Calcium carbonate is the only lime available here, agricultural lime, I use it in my mortar mix for laying blocks and making stucco becuase it makes the mortar stickier and retains moisture better but for use in a firebox it is not refined enough - too much carbon and oxygen. Carbonation of hydrated lime is what makes a lime mortar self healing.

Here is my assumption - if I use agricultural lime for a lime mortar, 1:3 with sand, the temperature of the fire will burn off the excess carbon and leave me with hydrated lime in the mortar. Since 80% of the population is smarter than I am, I will throw this out there and see if someone will straighten me out but be forewarned, by the time you tell me I am full of crap the fire box will have already been built. How is that for hard headedness ?