Lessons from Big Bill

Big Bill
I love Big Bill the clay oven. However, being an engineer, pretty much as soon as Big Bill was built, I had a list of things I’d do differently if we were ever to build another clay oven. (Read how we built him here.)

I thought I’d share the lessons I learned with you regarding the design and building aspects. If you are reading this, I hope you understand my terminologies. If you have any questions at the end, please do leave a comment 🙂

Oven base

If I were to built another clay oven, I’d have a solid base filled with landfill and dirt, rather than Big Bill’s fancy welded metal support with a concrete slab on top. (The bricks you can see in the top photo is completely aesthetic and serves no structural purpose. You can see the welded structure here.) It IS nice having the room for wood storage underneath Big Bill, but on hind sight I don’t think it’s really worth the extra difficulty in building a base. Plus we learnt that concrete is not a good insulator, and what can beat a solid base in terms of insulation?! (We tacted industrial grade foam insulation to the underside of our concrete slab after finding out too much heat is being lost through the base. Luckily the foam insulation is very cheap.)

Chimney

chimney

Me putting a chimney on Big Bill. I really should have put on a chute without a top at this stage, THEN put on a outer chute WITH a top (to stop the rain from getting in) after the roof is installed.

The right way to build a chimney

I do think it is better to have a chimney for a clay oven. It really does help with getting a happy healthy fire. Building Big Bill involved building a chimney for the first time for us. When we installed the chimney, a friend had already riveted the top on for me to stop rain from getting in. It wasn’t until we had to put sealant between the chimney and the roof that we learned a normal chimney has double layers. Both the chimney top and the sealant should be applied to the outer chute because the inner chute gets too hot and would burn the sealant away. We had to stuff aluminium foil in the joint between the chimney and the roof to act as our sealant. We did also apply some heat proof sealant but it eventually got crispy and crumbled away… luckily the foil seems to do the trick…

(Update: The original chimney top eventually crumbled away from the heat, and you can see the new outer chimney and top here.)

Chimney shutter

Chimney shutter being used while pizzas cook

A chimney needs a shutter. A chimney is great for letting the smoke out while the fire is going. However when pizzas are cooking with just kindlings and embers around the the sides of the oven, an open chimney draws too much heat away. We didn’t realise that until our first mini pizza party. We improvised a chimney shutter by bending a piece of aluminium sheet into the curve of Big Bill’s doorway so we could fit it into the doorway and therefore shutting the chimney. It’s a simple solution and it works.

Roof

Don’t put your roof beams too close to your chimney… I almost burnt down Big Bill’s roof… The radiating heat from the chimney scorched the frame. We put some foil around the frame since then and it’s fine…

Structure for the roof

It was also the first time we ever built s roof when we built Big Bill. I’m very glad it has stood through strong gust, pelting rain, and Christchurch earthquakes. There are 2 lessons I learnt. The first one is not to put any post too close the the chimney. The second one is to put on the sloping beams first, followed by the horizontal beams because those are the ones you nail the corrugated roof to.

I hope if you are building a clay oven, you can learn from our silly but thankfully harmless mistakes… 🙂 Have fun!

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5 thoughts on “Lessons from Big Bill

  1. It is all very exciting, isn’t it! Of course, since I built my oven, it has rained & rained and I have only been able to fire it up twice. That’s England: We don’t have summers.
    I am impressed with your thoroughness regarding insulation. Mine is very rudimentary (as a first attempt. I shall rebuild it when it falls apart, using some of the lessons you recount here). I particularly like the idea of vermiculite in the clay as insulation. I shall do that on my next attempt. Just one question though: What mix of sand & clay did you use for the first layer? I used 1:1 which i think was wrong and should have been 2 sand to one clay.
    A marvellous, satisfying project all round. More people should do it!

    • Hi Pete 🙂 It’s good to see you here. I checked out your blog in more details, and first of all, I want to wish you a very speedy recovery!
      I had a friend who built his oven about 2 years before mine so I had time to observe. Then he helped me planed mine also. (He’s moved away since.) I knew I wanted to be able to cook for hours in one firing, that’s why I made sure it’s got good insulation.
      I think your one should last a good while anyway (provided good shelter from rain). Given the size and thickness of the wall, you may not be able to cook for hours in one go, but I also think your one would not go through as much firewood as Big Bill does in one setting.
      I think when you fired yours the first time, it was most definitely still drying out, and that’s why it took hours to get properly hot. Next time you’ll find it won’t take that long at all. I think if the outside of your oven is hot to touch, you should be good to go with pizzas. Big Bill’s outside never gets hot though, just lightly warm because the wall is so thick (10”).
      The amount of sand you need to add to clay depends on the actual clay content of what you dug out. One good way to test is making a firm ball out of the final mix, drop (not throw) it from elbow height while standing on the ground. If it stays more or less a complete ball, you’re good to go. If it flattens out and flop, it’s got too much clay. If it shatters and cracks, it’s got too much sand.
      Finally, a good way to make sure your pizzas stay whole while getting in and out of the oven is to make sure you have lots of flour on the paddle before putting the base on. Then use a light thrusting motion to see if the base can slide on the paddle or not. If yes, start putting the toppings on (not too much though!). If not, take it off and put more flour on. I meant to write an article about it but haven’t yet got around to… Shall do soon!

  2. Hi! Me again! 🙂 My oven withstood the rigours of the winter but I am going to rebuild it for various reasons (on Sunday if the rain holds off). It worked ok and probably still will but i’s too small, on the wrong base, needs to be higher, better insulated etc etc. I have some questions about construction. Firstly, it looks like you put each layer on before the last was dry. Is that the case? Secondly, how thick was the perlite/clay mix? I like this idea and am going to borrow it.
    How has Big Bill endured? Do you still get cracks in it?
    I am quite excited and have brewed a special barrel of beer for any helpers that turn up on Sunday (I would invite you but its a bit of a long way 🙂

    • Hi Pete, how exciting! Yes the layers were put on before the last one dries – you want it not to be dry so the layers will bond well. It’s good to roughen the surface before another layer gets put on to let them bond even better.
      Big Bill is nearly 4 years old now and still going strong. Initially there were a few cracks but nothing worrying. As long as the innermost layer is solid, and you shelter your oven from rain, it’ll be fine. Hairline cracks will open up and look much bigger when the oven is hot, but those should not be a problem. The best way to minimise cracking is making sure your clay mix is stiff and not at all sloppy, also make sure to compact well when you build. The mix for the first layer should be like a stiff playdough. You should be able to roll all the mix into a log on a tarp, stand on it and not sink. The next layers can be mixed to be just stiffer then your earlobe. The stiffer the dough, the less shrinkage you’ll get in the end, hence less cracking.
      Good luck and I look forward to hear how it all goes 🙂

  3. Ok, Thanks for the advice. The first attempt yesterday seemed to be going well when suddenly, after the sand was removed, the roof caved in. Our little team of friends were most disheartened. But it was evident why it collapsed (too wide and not high enough) and today, we rebuilt it to a higher, more arched shape. After a few hours of drying out it is pretty cracked though this may be just on the outer straw/clay layer and may not be structural. Anyway, I shall write it all up when I get time. Thanks again for the help.

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