The Dragonfly Den in Retrospect, June 2009-May 2012

[SableSprings]

Although the final touches on my Wood Fired Oven (WFO) were completed in May of 2012, I was convinced by several of my Forno Bravo forum friends that documenting the Dragonfly Den build would still be of value...especially if I added comments regarding what's worked well...or could have used improvement.

As I started my documentation project, I soon realized that I wasn't creating a "clean & clear" illustration of the build that was appropriate for Forno Bravo forum users--my build comments had simply become too extensive for this format. My solution was to split the more detailed build descriptions, rationales, and insights for an improved build next time into a Google blog (below my signature line) and then develop a simple build thread for the forum.

I especially want to express my thanks to Susan (fabulous cook, my best friend, and wife since 1971). Without her support, suggestions, and enthusiasm for this retirement project...I don't think I would have ever been able to bake bread or a pizza in our very own WFO.

Along the way in this build document, my intent is to create pdf versions of sidebars like Recommended WFO Tool List, Suggestions for Large Pizza Party Prep, Baking Bread for the Neighborhood, etc. and make them available here and/or the blog series.

I hope you find the blog version of The Dragonfly Den build entertaining as well as informative (even though it obviously will contain lots of my biased thoughts and opinions). For those of you that just want the basic descriptions and pictures, here goes...

[SableSprings]

19 June 2009

After deciding to match & join the concrete driveway, the proposed den area was excavated, crushed rock laid down, packed, and concrete foundation slab was poured. The corner where the oven would be built had extra rebar and a six inch layer of concrete. The rest of the foundation slab had standard rebar and the concrete was only four inches deep.

Back to studying the Forno Bravo free plans, The Bread Builders book (Wing & Scott, 1999) and browsing the Forum while the slab cures. Kept the concrete wet for almost two weeks–--cool & cloudy weather, so I didn’t cover it during the cure time.

[SableSprings]

I'm not very good at visualizing, so if possible, I usually try to make a full size mock-up out of cardboard or layout outlines so I can see how things will fit together. I stacked the blocks and set where I planned to have the oven hearth relative to the oven stand/base. When moving the side blocks around, we came up with a corner build plan for the oven stand that would accommodate two carts in the base bays. At this point I also placed a major support under the approximate center of the oven placement--looks like a Y going into the back corner of the oven stand. I'm not sure if I would have thought of this without my mock-up.

I never liked the idea of having wood storage in a deep, dark hole under the oven , so by having movable carts, I gave myself the option of having substantial storage and working space (all well lit ).

I had decided to include an ash slot and ash storage system so I wouldn't have to be moving ash & coals around during or just prior to a party. The last picture shows the support block in the center and the void box up front. I used braced plywood underneath the slab forms and wrapped the plywood with visqueen (heavy plastic sheeting). The negative ash slot form (void box) was simply two joined 2x4s also wrapped in visqueen.

Looking back at my oven stand slab form...it was really pathetic. I had no idea how heavy the concrete would be or how much outward force it would generate during the pour. I tried to make due with scrap pieces of wood instead of actually building a sturdy form structure on top of the oven stand...lesson learned! Actually one of my only concerns before the pour was if the front span would work (and not sag).

[SableSprings]

I used 3/8" rebar for the main oven stand top slab area and ran some ˝" rebar on either side of the ash slot figuring it could use a little extra strength. I also ran a piece of angle iron along the front span, again as a hedge against sagging. I bent and inserted a piece of rebar into every other core void of the wall blocks and flattened it to run parallel to the slab base. Shoved crumpled newspaper & cardboard into alternate block voids. The goal was to set the rebar into the oven stand base blocks with concrete, join the side blocks to the top slab, and save on the total amount of material needed. I regret that I didn’t get a good picture of my rebar layout before the slab was poured (What I have are some frames taken from a video my wife took during the pour).

I borrowed a cement mixer from a neighbor and mixed up my first ever batches of concrete. It was a nightmare! I mixed and transferred two batches (4 bags of pre-mix cement) by bucket to just fill some of the block voids before admitting to myself that I did not have enough time remaining in the day (or energy) to complete my task. I called a local concrete ready-mix business and they said a truck was available and would head right out...what a relief!

The concrete delivery driver was terrific and helped me finish the top slab (again, since I had no experience in concrete work this was a tremendous boon). I’m sure he went back to the plant with a wide grin and all his buddies had a good laugh hearing about my form work and the total look of failure and relief on my face when he first rounded the corner with his cement truck.

[SableSprings]

While keeping the oven stand slab moist as it cured, again I hit the books and the Forno Bravo site and confirmed that although I'd really love to build a white oven, the black oven was going to be much more realistic for my limited brick skills. I referred back to my downloaded FB plans and started figuring out my materials list. I was able to find quite a few salvaged firebricks in the area. One fellow had a bunch of firebricks that had been used in a prune dryer in the area (we have famous prunes in Douglas County, Oregon) and another had a nice load of firebricks surplussed from some building sites. I did invest in new firebricks for the hearth since those were my actual food "contact" surfaces.

Pour was on 10 June 2009 and I removed my flimsy form the next day to reveal...it's a miracle, I now have a top slab on my oven stand. The two good things about the flimsy form I’d constructed were 1) it actually didn’t collapse (I'm sure there was a miracle in there somewhere) and 2) I hadn’t put spacer knockouts under the legs that held the plywood pieces...I don’t think I would’ve been able to remove the forms as easily as I did without “the flimsy part”...second miracle.

I realized how lucky I’'d been with the form removal when I went to pull/push out the void form for the ash drop. Although I had wrapped the two 2x4s in plastic, water got in and the wood swelled. I worked for about an hour trying to get those pieces of wood out...never saw that coming! Next oven stand with a ash slot I did, we used a Styrofoam block for the void form...much easier to remove (So, I guess you can teach an old dog new tricks. )

I was pleased how the cart bays in the stand looked and the front span didn’t sag at all. The plastic visqueen that lined the inside bottom of the forms gave the concrete a very smooth texture. The top “scars” left from my 2x2 cross braces looked ugly, but were only superficial blemishes in the slab.

I talked to the welding instructor at Umpqua Community College where we worked and he said that building the bay carts would be a great project for one of his second year students. All I needed to do was to get him some projected measurements for the carts and he'd put 'em in the project queue.

[UtahBeehiver]

Mike,

Very interesting to see how you did your build. I am sure the new builders will be able to use your experiences. Throw in a picture of your baking here and there.....

[Gulf]

And, I have always liked your idea for the carts under the oven. That really maxes out the "usabilty" and "accessibility" of a corner build . I'm glad that this build will finally be preserved on this forum for future bulders .

[SableSprings]

17 July 2009

Now with my top slab cleaned off, I was ready to lay the insulating pad for the hearth. Based on forum posts and materials availability, I chose to mix perlite and cement in a 5:1 ratio for a perlcrete insulation base. I simply used a bucket to add 1 part cement into a wheelbarrow containing 5 parts perlite, mixed the dry ingredients together with a hand trowel and then slowly added water. I then used a shovel to gently fold and mix everything evenly. When I got just enough moisture mixed in that I could make a handful of the perlcrete stick together, I transferred the mix by bucket into the form on top of the slab. I used a 2x4 to scree (not compact) the perlcrete to the top of the 2x4 form.

As everyone has noted, mixing either perlite or vermiculite with cement is very strange and almost impossible to believe the cured mixture will provide any sort of compression strength to support a hearth. I let my perlcrete set for several days but it seemed too crumbly to support the oven's weight. When my brother-in-law stopped by about this time, he thought I'd left my mix a little dry. With the forms still on, he dug out a couple inches around the inside perimeter and filled back in with a slightly wetter version than I'd used before...it hardened up much better and I felt like the insulating base was now going to be stable throughout the remaining build. You can see this outer ring of slightly “different looking” perlcrete in the last picture where the forms have been removed.

As usual, I placed bricks on and around the insulating base pad as a visual aid/model to make sure my oven opening would line up properly with the ash drop slot. I also wanted to see that I’d have adequate space for the bullnose brick front landing.

[SableSprings]

My brother-in-law Mark has serious masonry skills and experience and after his help with my dry perlcrete mix problem, we offered him the “opportunity to stay and help” with the building of the oven. (Sounds a little bit like a scene out of a Mark Twain novel ...but it worked, he stayed and really taught me a lot about working with bricks and mortar– as well as doing a lot of the important base work.)

First picture below shows Mark as he laid & leveled wet builders sand on top of the insulating layer. He then set the hearth bricks firm and level on top of the sand. I wish I’d have seen the newer forum advice of starting to lay the hearth bricks from the front to the back to eliminate the small brick triangles that needed to be cut...but it worked out just fine.

I wanted the hearth outline to be in a slightly shortened beavertail shape. We laid out the first chain on the hearth bricks to see that I had enough room and the insulation base would be used to its full potential. My primary reason for this beavertail shape was so I could sweep/pull/brush the coals or ashes smoothly into the ash slot. I also felt that loading and unloading the oven would be easier with this design.

[SableSprings]

With the shape, size, and layout design of the first chain set by the dry placement, the actual build began. I chose not to taper the firebricks for the dome...again, I really have difficulty visualizing things spatially and figured I'd just fill in the outside gaps. My brother-in-law Mark started the build base--with him tutoring and showing me how to lay the bricks properly as we went. The first course/chain was lightly mortared to the hearth. This thin layer would keep it in place but would easily shear away (hopefully) from the hearth during the curing process. I had purchased one 5 gallon bucket of ‘Sairset (pre-mix high heat mortar) for the job--I didn’t find the make-your-own version on the forum until long after I had finished the dome. We finished the fifth course along with the third bucket of 'Sairset --–takes a lot more when you don’t taper the bricks.

Note in the pictures below, that we had used some mortar to hold the sand in place between outer hearth brick edges and the perlcrete base.

Set the oven opening as a rectangle (per the FB plan set) and used a piece of 2"x2"x 3/16" angle iron across the span as the top. Mark knew it was important that this angle iron was simply laying across the side bricks and had room left for expansion on the ends. The steel expands/contracts as the oven cycles heat and the metal needs to “slide” freely to accommodate these volume changes. We put in an offset of about 1.5" for the front entry--top angle iron flush with brick sides--to provide a good door seal area. (p.s. At the time, we simply used the width of a pizza peel to determine the required door opening width...I was very happy later that my sheet pans all fit easily through the same width as the pizza peel.)

Mark had to return home, but I figured I could manage to finish the dome based on what I'd learned so far. Before he left, we talked about how the draft/chimney/flue pipe needed to be placed and I was confident I’d be able to do that as well.

[SableSprings]

Because of the profile & outline of the beavertail dome, doing the transition/connection cuts were fairly traumatic for me (being spatially challenged ). I had a lot of incorrectly cut pieces, but I finally got a relatively level chain over the entry section as evening approached...at least it appeared that way in the dark. Note that the brick that's canted toward the front (left side in photo) over the entry is actually keeping the angle iron over the oven opening from being bound up or constricted in any way. As I continued to cut bricks and work my way up on the dome, I finally reached the point where I did not trust the bricks to stay put with the wet mortar.

I had not seen the Indispensable Tool in the forum yet, but I'd read about using a large inflatable beach ball as an internal form. That didn't seem a good option with the teardrop shape. Neither did I want to fill the dome space with sand as Kiko Denzer did with his cobb ovens, so I took a piece of scrap Styrofoam and shaped it on the edges so it would act like a coarse plug for the open dome. Next I cut a plywood piece that would support the Styrofoam and simply used bricks to support the plywood from below. I then started placing plastic bags full of sand on top of the resulting platform. With the plastic bags pressed into the approximate inner dome roof line, I then covered the bags with damp builders sand. Once the sand had been smoothed and shaped, I started to cut, place, and number bricks for the next chain.

[SableSprings]

I found out that when I put bricks in place to determine how they needed to be cut, I was disturbing the smooth sand form way too much. It also bothered me that the mortar might pick up sand as I laid each chain in place, so I covered the sand form with plastic to keep the sand out of the mortar. (Yes, for those of you that noted the plastic bag logo, we’ve been to the SPAM museum...)

Cut, numbered, and mortared in place, the bricks of the 9th chain are almost vertical. The neck of the teardrop has been relatively easy here since I could use nearly full length bricks to close up. In an earlier chain at the front over the opening, I actually did use a full brick and although it sticks up above the surrounding bricks, I figured it would just be covered up with perlcrete later. The sand form seemed to be settling so I used small slices of bricks here and there on the 10th chain to keep the bricks even with the previous chain’'s inner surface.

Lots of angles, minor corrections, and grinding to fit the 11th chain in place. The keystone will need to be made in two pieces. A friend dropped over with an angle tool and we figured out a way to cut the two pieces so they'd fit reasonably well. Mortared the keystone pieces and tapped them in place with a rubber mallet.

From bare, top slab to setting the keystone (16 July 2009 to 11 Aug 2009).

[SableSprings]

Although I was confident the dome would not collapse when I pulled out the brick/plywood/sand supports & dome form, I went slowly and kept my head out from underneath the dome . The bagging of sand in the platform really helped clearing the oven cavity in a short amount of time. The smooth curve of the short beavertail toward the front also helped in the sweeping out of the loose sand. I was amazed how humid it was inside the dome. Even though there was a fair amount of mortar visible on the inside surface it was, by and large, quite smooth and I didn’t see any reason to try to remove it. There were a couple larger knobs of mortar that descended below the dome surface (just before I put a layer of plastic between the sand and bricks), but they were easily knocked off with a trowel. I found one gap between bricks in about the seventh course that bothered me enough to fix with a pointing tool and ‘Sairset.

Because of the extreme humidity in the cleared dome, I planned to let the chamber air dry in our hot summer weather while I worked on the landing and chimney.

[SableSprings]

In order to bring the outer landing (beyond the ash slot) up to hearth level, I used regular mortar to set in some 4"x8"x16" cement blocks as a foundation and lift/level. (With the added blocks it was easier to get inside the oven and check for any more big mortar gaps or gobs. Everything looked good enough, so back to work on the landing and chimney arch.)

On top of these cement blocks, I set bullnose bricks as the landing forward of the ash slot and across the entire front of the oven. In the second picture below you can see several of the bullnose bricks just resting on the base blocks. Picked up a two pieces of 2"x3"x3/16" angle iron for the front & back support (and leveling) of the cut chimney flue pipe section. Also set some bricks in place for the front arch side supports so I could experiment to see how everything was going to fit together. I knew that I wanted my chimney to have bricks on the outside (for looks) and my front arch was going to need some serious side bracing to accommodate the resulting downward & outward thrust/weight forces. The front side column bricks had holes in them (standard hollow bricks) and I put some pieces of rebar along with mortar through the stacks to make them pretty damn solid.

I used my curved piece of scrap wood and cut & mortared into place a back and front arch that would support the double flue tile inner cores and the outer brick surface. The gap between the arches was set at about 2" less than the length of my firebricks and enough so that an 8" flue tile would rest evenly between arches. The tops of the two arches were set parallel to each other, again so that my first flue tile would be level for the chimney base.

[SableSprings]

To fill the gap between arches, I simply cut a notch from each end of a firebrick and set it into the gap. The long end of the firebricks wedged firmly into place so I only needed to use a small amount of mortar to seal the opening–keeping smoke & creosote out. Based on a flue design I’d seen on the forum, I cut two 8" flue pipes at an angle and then mortared them together. This creates a wider collection area that reduces down to the original opening size of the flue pipe. The bond is obviously weak, so I kept a string wrapped around it until it was supported by the outer chimney structure.

Once the 8" cut & joined trapezoid flue tile was set in place over the arch top center, I filled in the gap between arches with the notched firebricks. I continued to build up the front brick work so it tied in both side support (buttresses) columns and created a nice looking entry area. Since we were still in fire season, I put a chimney cap with cinder screen on top of the flue pipe and decided it was time to start the curing fire process.