Rumford Fireplace Part VII

Each day we plod forward on the fireplace project. I can get really fired up in the morning and think that I'm going to get a lot done that day, but it never seems to turn out like that. So, I just chip away every day and progress happens slowly. I'm excited to be done with 5 of the 6 stone arches that will be contained within this structure. I think I'm getting better at it, but it still takes about one day to design and cut an arch, regardless of the size of the arch or the number of stones. This first picture shows (from left to right) "the pizza oven arch," the "great room wood storage arch," and the "Rumford fireplace arch."

In this picture you can see that we've reached an elevation of 5 feet 2 inches, and poured the interior volume of the stonework with concrete again. It took 36 bags of quickcrete and two bags of portland cement, plus some sand and stone chips (I have plenty of stone chips!). All totaled, I think we poured about a cubic yard in this last lift. I am somewhat amazed that the stone walls don't blow out when we pour the wet concrete inside of them, but we are careful to keep the mixture thick and do not use a vibrator.

The pizza oven is going to get very hot and expand when fired to 700+ degrees (mmmm... 3 minute pizzas!), so there is a danger that the oven could push and weaken the concrete and sandstone around it if room for expansion is not built in. For a while, I was puzzled at how to maintain a compressible layer of fireproof insulation around the pizza oven. I considered using fiberglass, since it is not flammable, but I read on the internet that it contains some kind of binder that can give off gas when heated. Perlite mixed with cement is not as compressible as I first thought when I started this project, so I didn't trust it for this application. Loose perlite is very compressible and has a much higher insulation value than perlite mixed with cement, but how to get loose perlite to stay in place while pouring concrete around it? The stonemason had an idea of parging a wire-mesh armature surrounding the pizza oven, being careful to leave a gap into which we could pour perlite. I liked his idea and ran down to the farm store to get some wire mesh. When I got back, we came up with a better idea - line the wire mesh with aluminum foil and pour the perlite into that without parging anything. It worked great. Where ever the perlite leaked through the aluminum foil, I poured water on it and it stopped flowing out the leak. As we poured concrete against the perlite insulation, I ran my hand down into the insulation and made sure it was at least an inch thick everywhere. By the way, use a dust mask if you work with loose perlite. Unlike vermiculite, it does not have trace amounts of asbestos so up to this point I had considered it somewhat inert, but it is very irritating, and possibly harmful, to breath the dust that inevitably comes from working with it. Put the mask on before you start coughing!

The rumford fireplace kit came with a cast iron damper and a clay smoke chamber to place on top of the fireplace throat. The two-piece smoke chamber appears to be two normal 16x20 flue liners (if a 16x20 flue liner can be called normal!) that have been cut so that when placed together they form a smoke chamber which tapers from 36x16 to 20x16. (the height is about 19") The factory cuts aren't perfect, and could be made more perfect with a grinder or chop saw, but the stone mason and I said "heck with it." We assembled them as tight as we could which left a gap that varied from zero to 3/8ths of an inch. (3 years ago, this would have drove me nuts!) We will be pouring concrete around the smoke chamber, so even if the fireclay mortar cracks and falls out of the seam, no smoke should ever be able to escape.

I took this last picture from above the fireplace. It shows the cast iron damper that the mason mortared into place before we placed the smoke chamber above it. The 8.5" x 8.5" flue at bottom left is for the wood cook stove and the flue at the bottom right goes to the basement in case I ever put a furnace there. Ideally, there would be a little more separation between these flue liners (and there will be further up in the chimney), but it wasn't possible to separate them at this point.