My plan was to have a path for moisture from the perlcrete to escape out the chimney/flue system. Once the chimney rose above the smoke collection chamber, I intended to nest my primary clay chimney (8" diameter) inside a 12" clay chimney flue up to the rain cap and cinder screen on top. (I know in many English speaking, European countries this is called a chimney pot...but as colorful as the term is, it sadly just doesn't seem appropriate here in Oregon with our new Marijuana laws.) I had built a smoke collection chamber (two sections of 8" clay pipe cut and mortared together to form a single piece that was wider at the bottom and tapered down at the top to match a standard 8" flue pipe). The picture below shows the 2D concept of the cuts on the flue tiles. (The red sections are the cut outs.) Done properly, the top cut on each tile should leave the outline of half of a flue tile. When the two sections are brought together, an 8" clay flue tile should sit directly on top (hopefully, flush) of the smoke collection chamber construct.
I’d seen a similar illustration on the Forno Bravo forum and thought it looked simple enough to do... Remember I said in an earlier post that I was spatially challenged? Well, it took me quite a while to actually turn the illustration into the desired smoke collection chamber. The keys were realizing that a square was really helpful for setting up the required cuts, but a construction marker/pencil was not really good for marking the cut lines (score one for a black magic marker). The other problem, at least for me, was that I could mark one side and begin the cut (with a diamond blade on my skill saw) but when I started going around the “tube” it was difficult to keep on line or even figure out where the line was (you see, if you use a construction pencil, the graphite is blown right off the surface by the clay particles whizzing out of the cut)... It also didn’t help that trying to hold & turn a heavy piece of clay tile in one hand while trying to control a skill saw with the other, is a feat somewhat beyond my capabilities (or at least it was against my resolve to retain all my fingers and both of my hands throughout this project).
Eventually, I did get two tiles cut (while retaining all my digits) and was able to mortar them together. (Just so you know, I did butcher one "test" tile section beyond all hope. I guess you could say I wanted to develop some experience before doing the final work...but that would be stretching the truth quite a bit.)
Below's a picture of the final flue piece, joined with mortar, and sitting on top of the oven's entry arch smoke vault. It doesn’t look all that impressive, but the enlarged lower section does help significantly to gather/collect the smoke from the oven and let it pass smoothly into the upper chimney sections.
Some of you may notice that my construct doesn’t look exactly like my drawing...and you’re correct. I basically spent a lot of time with my saw and new grinder trying to just get the two pieces to fit together (remember, I'm spatially challenged), so I had a wider base and a top upon which to set the next tile...so, yes as usual there is the ideal (and correct way) and then there’s my way.
I know that my struggle constructing the smoke chamber was directly related to the easy time I had with the last of the perlcrete application. Some sort of cosmic/comic justice I suppose, that was imposed just to keep me really humble.
p.s. I realized later (too late), that I assumed...yes, I know 😞...that an 8" flue tile was referring to the inside measurement. FYI, an 8" flue tile provides a 6" internal diameter...perfect for a smaller oven!