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  • Forno Clamshell - Another Charcoal Grill Conversion...

    First, I would like to state that, yes, I will be putting together a Pompeii oven. In fact, I have already bought the 36" Pompeii kit from FB. But due to certain time-gobbling responsibilities, I probably will not be able to get to it soon. So in the back yard the pallet sits . But I want pizza, so...Forno Clamshell!

    I went into this project with a few requirements:
    • It will only take a few weekends
    • Most, if not all, of the parts should be available locally
    • It can be able to reach "pizza temperatures" - 600 degrees-plus
    • Portable
    • Inexpensive

    Basically, this ends up resulting in a few design considerations. Thanks to this forum, and building on the experience of others, the final design is really just a refinement of the experience here, and are built into the design. The design includes:
    • Propane gas for fuel - easier to regulate temperature than wood or charcoal
    • Decent sized burner
    • Insulate, insulate, insulate
    • Refractory mass on bottom and top for radiant heat (not just rely on convection heat)
    • Create a double-walled shell out of two different sized charcoal grills - I was considering using two weber grills, but because of the expense, I decided to use a couple of weber-clones. Besides, I was able to get a grill for the outside shell that had the top and bottom hinged

    After a failed "learning experience" with a small grill, pearlite and refractory cement, and an oversized burner, I redesigned the project, ultimately ending up using the following parts:
    • 18-1/2" charcoal grill (inside shell)
    • 22-1/2" charcoal grill (outside shell)
    • 2 15" pizza stones
    • 1 stand from the 15" pizza stone
    • 1 12-1/2" pizza stone
    • Stand from 12-1/2" pizza stone
    • 30,000 BTU Banjo propane burner (got off of Ebay for $30 plus shipping)
    • 3" double-walled vent adapter (a "3 inch Bucket Connector Assembly")
    • 3" double-walled vent elbow (adjustable)
    • 3 linear feet FB Blanket (technically, this was available locally, since Forno Bravo has set up shop in Marina!)
    • 3 cups FB Mortar (stole from my Pompeii kit, though I would have used homebrew if I didn't have the FB Mortar)
    • Bolts, nuts, washers, lockwashers, plus small u-bolts, metal strapping, etc.
    • Wood stove rope gasket

    The end result is shown in the posted pictures. The black on the pizza stone is where my first pizza kind of fell apart. The good news is that the pizza came out great, and I was able to get the stones up to almost 600 degrees, without pushing it (and the outside shell was just barely warm). I will describe how I assembled it in future posts.
    Last edited by EADavis; 12-04-2009, 02:19 PM.

  • #2
    Re: Forno Clamshell - Another Charcoal Grill Conversion...

    Assembling the Bottom of Forno Clamshell - Part 1
    First, the outside (22-1/2") shell needs to be modified, to allow the burner controls to poke thru. Note the big hole in the bottom of the outer shell. This is very important for two reasons. One, this is where the burner will draw its air from. And two, this is a safety feature - since I am using propane for fuel, and propane is heavier than air, there must be no place for the propane to pool, should the flame go out.
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    Now take the burner, and remove the outer ring/stand.
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    Drill 4 holes in the bottom of the outer shell to mount the burner. Mount the burner.
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    Cut a hole out of the bottom of the inner (18-1/2") shell, and a cutout, to clear the burner.
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    Align the inner and outer shells so that the tops are flush, and have equal clearance on all sides. Drill 4 holes in the outer shell, to align with the 4 holes in the inner shell that would originally been used for the legs. Using 1/4" bolts nuts and washers, test fit the shells.
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    • #3
      Re: Forno Clamshell - Another Charcoal Grill Conversion...

      Assembling the Bottom of Forno Clamshell - Part 2

      Now a small grill rack needs to be mounted in the inner shell. This is used to support some kind of heat plate (I used a perforated grill pan). This is necessary so that the flame from the burner is defused - the pizza stone does not like direct flames. If firebricks are used for the cooking surface, this will probably not be necessary. I used 4 small mirror wall mounts as brackets to hold the lower rack. Drill 4 holes in the inner shell, then install the brackets.
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      Using stove rope gasket and refractory cement, glue the rope to outline the hole in the bottom of the inner shell.
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      Unfortunately, I do not have a lot of photos for the next few steps, so this one will have to do...
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      Cut a ring out of sheet metal to cover the gap between the inner and outer shells. Bend a lip on the outer part of the ring (no sharp edges exposed here).

      Take everything but the burner off. Assemble the original legs from the 22-1/2" grill to the shell, install the handle, hinge hardware, and anything else that goes on the bottom half from the original kit.

      Line the inside of the outer shell with FB Blanket insulation. I used aluminum foil to help contain it while putting it together. I was originally going to use perlite or perlcrete, to keep with the "Buy Local" philosophy, but since FB is local, I bought a box of FB Blanket. I used about 3' of the blanket, and will use the rest when I finally get to building my Pompeii oven.

      Assemble the rest of the lower assembly. Insert the four 1/4" bolts thru the outer shell inward. I used the mica washers in the original grill kit as insulators on the bolts. Slip the ring onto the inner shell, then assemble the inner shell to the outer shell. Align the tops of the inner and outer shell. Use stove ribbon gasket to insulate the outer shell from the ring, and stove rope gasket to insulate the inner shell from the ring.

      Insert lower grill rack, the defuser, and the upper grill rack (use the original 18-1/2" grill rack).

      Last, I used two 15" pizza stones mortared together (bottoms facing each other) using FB Mortar (or homebrew will probably work). This gives me almost 1" of mass. I cured the mortar in my kitchen oven, using the same philosophy as curing a new WFO. Fire brick can also be used here (I may try that later, in case the cheap pizza stones I used cracks).
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      Coming up next, the upper assembly...
      Last edited by EADavis; 12-03-2009, 08:13 AM.


      • #4
        Re: Forno Clamshell - Another Charcoal Grill Conversion...

        Assembling the Top of Forno Clamshell - Part 1

        The top assembly is similar to the bottom, an inner shell and outer shell sandwiching FB Blanket insulation. The inner shell is the lid from an 18-1/2" charcoal grill, and the outer shell is the lid from a 22-1/2" charcoal grill. A side vent is built into the top assembly, and a single 12-1/2" pizza stone is mounted to the top of the inside shell - this acts as refractory mass, to radiate heat down onto the top of the pizza being cooked. With the lid shut, the top pizza stone hovers about 4" above the pizza.

        In the case of the charcoal grills I bought for this, the outer shell had the vent at the top, where the inner shell had it on the side, at about a 45 degree angle. By using the side vent, this design allows more flow of heat on both sides of the upper pizza stone, and allows a bit more convection heating of the pizza.

        Take the outer shell, and seal up the top vent hole.
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        Open up the vent hole on the inner shell to accommodate the 3" vent flue adapter. Cut a hole in the outer shell that aligns with the inner shell vent hole, sizing it for the vent flue adapter (since I used a double-walled adapter, the outer shell hole is bigger). Drill two holes for the mounting tabs of the adapter, and mount the adapter to the outer shell.
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        Take one of the large (15") pizza stone racks that came with the stone, and mount it to the inner shell using 4 small U-bolts (holes will need to be drilled into the inner shell).
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        Slip the 12-1/2" pizza stone behind to mounted rack (the mounted rack may need to be loosened first). Slip the small (12-1/2") pizza stone rack behind the pizza stone, and pull the racks together, so as to snugly hold the stone. Use some metal strap and bolts/nuts to hold the two racks together on both sides - this is probably easier to see in the pictures below.
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        By using this method of mounting and suspending the top pizza stone, it allows for heating of the stone from both sides.

        More to come in the next post...
        Last edited by EADavis; 12-03-2009, 08:16 AM.


        • #5
          Re: Forno Clamshell - Another Charcoal Grill Conversion...

          Assembling the Top of Forno Clamshell - Part 2

          Now that we have the top stone mounted, it is time to get the top assembled. These steps are similar to those done for the bottom assembly.

          Drill 4 holes in each of the top inner and outer shells, lining up each set of the holes so that the rims of the two shells are flush (use existing holes, if possible).

          Attach the outer shell handle and hinge brackets from the 22-1/2" grill kit.

          Line the inside of the outer shell with aluminum foil, leaving a couple of inches overhang at the rim (this will help contain the fibers that may come loose from the FB Blanket).

          Line the inside of the outside shell with one layer of FB Blanket insulation. Fold over the overhanging aluminum foil around the edge of the insulation

          Place the inner shell inside the outer shell. Use a short length of stove ribbon gasket to insulate and fill any gaps between the inner shell, and the vent flue adapter. NOTE: The inside portion of the vent flue adapter may need to be trimmed, so that in protrudes into the inside of the grill by a maximum of about 1/4". Attach the shells together with four 1/4" bolts, flat washers, lock washers and nuts. Use 4 of the mica washers that came with one of the grill kits to insualte the bolts from the outside shell.
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          Run a 6 foot length of 1-1/4" stove rope gasket between the rims of the inner and outer shells.
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          Attach the 3" double-walled vent 45 degree elbow to the adapter (this is our chimney).
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          Tomorrow, we put it together and make pizza!


          • #6
            Re: Forno Clamshell - Another Charcoal Grill Conversion...

            Ciao Ed,

            If you can make a great pizza (and not blow up), do you want a display at Forno Bravo Expo?

            I can't wait to hear how this comes out.
            Pizza Ovens
            Outdoor Fireplaces


            • #7
              Re: Forno Clamshell - Another Charcoal Grill Conversion...

              I plan on fully testing out the grill this weekend, to see what kind of sustained temperatures it can hold, how it works after several consecutive pizzas are cooked, see how cooking a pizza with just retained heat works (turning off the gas before cooking), etc. If the grill survives this workout, then I would be happy to bring it to the Expo.

              I do want to make a couple of modifications to it in the future. For one, I want to install one of the FB Door Thermometers into the lid, as soon as you have them back in stock (Marina said she would call me when they came in). Also, to make it truly portable, I want to design a way to easily remove and replace the legs.

              Thanks for asking,


              • #8
                Re: Forno Clamshell - Another Charcoal Grill Conversion...

                Final Assembly of Forno Clamshell

                Finally, we are almost done. Attach the top assembly to the bottom assembly using the pins for the hinge.
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                Make pizza. Be happy.


                • #9
                  Re: Forno Clamshell - Another Charcoal Grill Conversion...

                  Forno Clamshell - Postscript

                  After cranking out a number of pizzas, and putting this Franken-grill through it's paces, I now have a feel for how to use it, and what to recommend to anyone who wants to try this for themselves...

                  First and foremost, this is literally a blast furnace. The insulation works well, and the burner can generate a lot of heat. I had the lower stone up to 803 degrees and the upper stone at 782 after 20 minutes of warm-up, using a mid-low burner setting. In fact, in the area between the the burner and the stone, it got so hot the ceramic coating on the inner shell started to bubble around the mounting screws, and the grill grate that held the defuser warped. The defuser itself (a stainless steel perforated pan) glowed red, it got so hot! And the outside shell was only warm to the touch.

                  Pizzas take 1-1/2 minutes to cook with the gas on while cooking, and 2 minutes if you rely on just the retained heat in the stones. The upper stone needs to be recharged (reheated) after two pizzas if cooking without the gas on.

                  As expected, there is a cool spot on the stones near the vent, so the pizza does need to be rotated 1/2 way around half way thru cooking.

                  A metal peel probably works better than a wooden one - yes, I made a mess with a problem sliding one of the pizza off the wooden peel.

                  In order to help with the heat, I made up insulating caps to go around the lower mounting screws made of stove rope gasket and refractory cement. Also, I put a disposable layer of heavy duty aluminum foil on the inside of the lower inner shell. This helps reflect some of the heat back into the grill (as opposed to being absorbed into the black inner shell), and catches pieces of food falling and burning.

                  Next time, I would use a smaller burner. Even on a lower setting, it is still generating way more heat than needed.

                  Realistically, the grill will probably be overheated after 10-12 pizzas, so I would not use it for a big party. But for tailgating, small parties, or a quick pizza for dinner, it will work well.

                  And I did not blow myself up! Now to go start my Pompeii oven...


                  • #10
                    Re: Forno Clamshell - Another Charcoal Grill Conversion...

                    Glad to hear you are safe and sound. Experiments are a great thing, or as the VP of Engineering at one of our high tech companies used to say -- you need a little R with your D. The R is the fun part.

                    Pizza Ovens
                    Outdoor Fireplaces


                    • #11
                      Re: Forno Clamshell - Another Charcoal Grill Conversion...

                      Thanks - no mushroom clould over Monterey Bay!

                      There was a quote from one of the Star Trek shows that I can relate to, where Scotty came back 75 years later. In it, he said "Oh, I may be captain by rank. But I never wanted to be anything else but an engineer." That is where the fun is...


                      • #12
                        Re: Forno Clamshell - Another Charcoal Grill Conversion...

                        Just a final post, to close out this thread. I added a FB Door Themometer and a chimney cap to my "Frankenweber". The thermometer gives me a better idea where my air temperature is at (less opening of the lid to measure stone surface temperatures), and the chimney cap does help hold a bit of the heat in, without impeding air flow (important when using gas!).

                        Pizza every weekend now!