First post, first clay oven build, first of many questions!

[jonnygunit]

Hi everyone,

So this is my first post, I love pizza and have decided to build my very own wfo in my garden here in Wales. Originally I set out to build a small standard cob oven using only materials from my garden, with my only real expense being some good quality firebrick for the oven floor. The plan was to use the wine bottle and straw cob methods to insulate the clay dome. However, trawling through the wealth of info on this great forum it is scary how easy it becomes to justify always going that one extra step (Oh go on then just a few more inches internal dome size! Vermicrete instead of the wine bottles you say? Why not!). Expense is my main concern and the one place I will be drawing the line is with using clay over bricks for my domes thermal mass layer. I hope to one day to build a grand all brick Pompeii, but it wont be this summer I'm afraid.

So, with this in mind my first question is this (1) Is it still worth the extra expense of vermicrete insulation if I have already opted to 'bottleneck' the performance of my finished oven by using clay instead of firebricks for the dome? (I'm assuming expensive bricks must do a better job than mud from my garden)

I'm hoping my oven will see action about once a week, mostly for pizza but probably some light bread baking and roasts etc aswell. Im thinking of an internal floor diameter of 26" which I figure will easily cope with a couple pizzas at once and be sufficient for my needs. (2) Based on experience does anyone feel I'm likely to live to regret going so small?

From what I understand I can never have too much insulation, but too much/little thermal mass can be a problem. The firebricks I have sourced are from an old storage heater and, at 45mm thick, I think they are a bit thinner than the bricks people would normally use. (3) Will these bricks provide a sufficient thermal mass layer for the oven needs I have described? (4) How thick should I make my dome's clay thermal mass layer? (I believe this is just clay and sand with no fibres added, please correct me if I'm wrong).

Im really excited to get started with this. Building on my natural stone plinth will begin next time there is a break in the rain!

Thanks for reading and any help would be greatly appreciated,

Jon

[okn]

Re: First post, first clay oven build, first of many questions!

Jon,

Welcome aboard!

My 2 cents on your questions:

1) Depends on your hard line budget. I opted for good insulation and I am quite happy with the performance. I have 0 experience with wine bottles as insulation or vermiculite/perlite. I suspect that it would be in your best interest to go with the vermiculite/perlite
2) I would say 26" is too small for 2 pizzas at a time. However, I don't see a need for more than one at a time, unless you're a caterer or restaurant. I wouldn't go smaller than 26" for sure.
3) I think you will be OK with 45mm thick bricks
4) Clay isn't my forte' anybody else?

The biggest starting question is what's your budget? I know you're concerned about the dome, but that really is only one of many expenses. My dome bricks, were about 15% of the total project (mine's not a pompeii). If you have a HARD budget, let us know so we aren't recommending gold plated smoke stacks.

Kevin

[david s]

Re: First post, first clay oven build, first of many questions!

If you are making a cob oven then you may as well insulate it for free too, by mixing a little of the cob mixture with lots of straw as the insulation layer. It works very well, but you need to use only enough clay to hold the straw together. Water the clay down until it is slip (liquid)
My oven is only 21" and performs adequately cooking one pizza at a time, but as they take only 90 secs to cook you will have trouble keeping up with the prep anyway. At around 30 pizzas per hr how many people are you planning to feed?

[cobblerdave]

Re: First post, first clay oven build, first of many questions!

Gudday
One thing I would buget on is some overhead cover for that oven otherwise the wheather will ruin your good works. From what I have read its not possible to cement render over cob either as the expansion rates are different and the render will crack and fall off.
You might consider taking into account the amount of time and labour into your budget there's about the same for cob or brick and brick will last longer and be more robust. It doesn't have to be all firebrick either 2nd hand pressed clay commons or pavers are a great option and you can produce a good oven without a bricksaw a brick bulster will do.
Main thing is you enjoy yourself regardless of what type you build.

Regards dave

[jonnygunit]

Re: First post, first clay oven build, first of many questions!

Hi all,

Thanks for the replies. I wouldnt say I have hard budget as such I just thought as a first build I would go as cheap as reasonable for a decent performing oven whilst I'm learning. Enjoying myself in the process of course!

I already have plenty of sand, cement, rocks for the plinth, the firebricks for the floor. Other than some straw I think I could make a functioning oven without any expense, although I accept I'm bound to have overlooked something! Firebricks are expensive here at around ?1.50-?2.00 each before delivery so I imagine the bricks for the dome alone would cost in excess of ?200.

I thought if I did a 'proper job' on the plinth and insulation under the floor I would always have the option to knock down the clay and retrofit a more serious brick contraption at a later date.

Is it true to say I could use vermicrete as dome insulation and benefit from improved insulation (as opposed cob) and also have something I could render over to avoid the need for a roof? Anybody with any experience with vermiculite any opinions on far it will go? Would, say, a 100L bag be enough for a few inches insulation beneath the floor as well as the dome? (when mixed with the correct ratio of cement, 6:1 I believe)

Cheers

Jon

[cobblerdave]

Re: First post, first clay oven build, first of many questions!

Gudday
100 lt of pearlite is .1mtr2
Good idea to use firebrick as you hearth so insulating it well is a good thing. Certainly leaves you more options to change in the future.
I think that pearlite and cement is not a good idea over clay dome. It's more related to the nature of the clay dome. It excepts moisture out of the air so it really needs to breath also it expands more than a more rigid cement product would . Pearlite/ cement is a rigid product still when dry.
But if you do go ahead anyway because you looking at the clay dome as experiment it would great if you would post the results for future builders.
Regards dave

[cobblerdave]

Re: First post, first clay oven build, first of many questions!

Gudday again
Had a bit more if a think about the pearlite cement over the clay dome.
If it was me doing it I would mix up my pealite cement 10/8 to one. Mix by hand adding the water carefully to it clumps together. Pick up handfuls and place at the bottom of the dome working your way all the way around layer by layer. When it just starts to set get a steel float or trowel and tap any high spots down then trowel the surface smooth. With the addition of a breathable water resistant paint made for brick and render it should work.
Regards dave

[Gulf]

Re: First post, first clay oven build, first of many questions!

I just want to add this to CobblerDave's advice. If you really want to "marry" a cob with the FB recommended techniques, think about sourcing a locally found clay and mixing it with Type N Mortar mix. At least you would be making a refractory type dome that would better stand up to the elements. Mortar mix is relatively cheap . You may not be able top find it in your garden (unless you dig deep enough), but there is some gulley, excavation site, roadway etc, near most locations where it can be found.

just sayin'

[jonnygunit]

Re: First post, first clay oven build, first of many questions!

So I've been pretty busy over the weekend and the weather hasn't been cooperating but I'm happy to at least be making progress with the planning. I'm confident with how I'll be going about building/insulating my plinth and laying my oven floor!

I must admit talk of breathability, and types or mortar has me a little bit confused! Gulf, are you saying add mortar to the clay to make a more hardy thermal mass layer? Or when added would it become a material for insulating?

The main question I am left with is how to insulate my dome? I know I could mix straw with my clay to form cob and use that. Is there any reason (big difference in performance, weather resistance etc) to not simply go down that route? I liked the idea of a more modern insulation (ie. vermicrete) especially it it gave me the option to render and do away with a roof, but I'm not too keen on the idea of it being experimental!

Cheers as always

Jon

[jonnygunit]

Re: First post, first clay oven build, first of many questions!

I also found a local listing for 'clay soil' on freecycle today. I have zero experience in clay and the lady has kindly allowed me to go check its what I need before I cart it all off her drive. What should I be looking for to be confident it's 'clayey' enough?

Thanks

Jon

[cobblerdave]

Re: First post, first clay oven build, first of many questions!

So I've been pretty busy over the weekend and the weather hasn't been cooperating but I'm happy to at least be making progress with the planning. I'm confident with how I'll be going about building/insulating my plinth and laying my oven floor!

I must admit talk of breathability, and types or mortar has me a little bit confused! Gulf, are you saying add mortar to the clay to make a more hardy thermal mass layer? Or when added would it become a material for insulating?

The main question I am left with is how to insulate my dome? I know I could mix straw with my clay to form cob and use that. Is there any reason (big difference in performance, weather resistance etc) to not simply go down that route? I liked the idea of a more modern insulation (ie. vermicrete) especially it it gave me the option to render and do away with a roof, but I'm not too keen on the idea of it being experimental!

Cheers as always

Jon

Gudday
Type in adobe into goggle ...it's basically cement /lime stabilized earth. Very common in Mexico.
I stick by my first comments .....if I was to go down the path to the pearlcrete insulation on a clay oven I would keep it "breathable" so any moisture can them escape. I have a breathable paint over my dome and a breathable sealer on the exposed brickwork on my own oven. I live in sub tropical QLD and its inportant to allow the moisture to escape as it will crack a render coat as 1 ltr of water becomes 1600 ltr of steam.

Regards dave

[cobblerdave]

Re: First post, first clay oven build, first of many questions!

Gudday again
There's a set of plans for an adobe oven
In this section it's called a "hybrid oven brick and clay" or something similar .
On the first page there is a one of the posts has a link to a set of plans for a "Sunset outdoor oven" I'm not saying to build this type but the plans and that thread it will increase your knowledge base.
We have had a news report here that UK is due to have another wet summer so its looks like last year again for you.
On water again this the other end of you oven its important to put a waterproofing layer under the slab another under the insulation would be good as well. No use having the moisture wicking up from the ground
Regards dave

[Gulf]

Re: First post, first clay oven build, first of many questions!

Clay, Portland cement, Hydrated lime, and sand are mixed to make a refractory (thermal mass) shell. In the states Type N mortar is 50/50 Portland cement and lime. I'm a little leery of the term clay soil. In my area, that could mean just about anything. True clay can be found deep enough under ground to be free of sticks, sand, or any organic material. In my area, that can be from 4 to 16 feet deep. The depth can vary. When wet, the clay should be very similar to the consistency of modeling clay.

[jonnygunit]

Re: First post, first clay oven build, first of many questions!

Summer still hasn't quite arrived here, although it is threatening and I've had a few nice days to work on my plinth. All the footings are in and the walls are slowly creeping up. Im going to incorporate an old slate from a pool table as a (rather large) countertop. Ill try to post some pictures once theres something more interesting to see.

My search for clay continues..... I found an environmental recycling place with a huuuuge pile of 'clay subsoil' sat waiting to go to landfill. I took a couple of handfuls of the freshest, clumpiest stuff for the shake test and turns out its about 50% clay. It was brown, similar in colour to topsoil. This is option 'a'. My only concern is that, because the place is basically a dumping ground, by the time I go back the stuff I tested will be buried under another 10 different soils from 10 different places.

I thought I was onto a real winner with option 'b'. A relative of a friend of mine mentioned his farm was sat on a old coal pit....... fireclay?! Me and my spade paid him a visit at the earliest opportunity. I explained to him what I was looking for and he walked me 500 yards straight to a quite boggy area in the shade of some trees, and confidently pointed at an area of grass for me to dig. My spade pierced the ground and it was like digging through modelling clay. It had rained in the morning and this earth was like light grey plasticine. There were a few stones and roots as Id expect but the other noticeable 'impurities' were chunks of coal. Everything tells me this is the good stuff, my only issue is the results of the shake test. This time the top clay layer which separated out was only 20%-25% of the total volume. I just cant believe there could be less clay in this that the basically clumpy soil of option 'a'. My hope is that option 'b' is actually 2 different types of clay which settle at different rates in the shake test so the top 2 layers (~70%-80%) are both clay. Anybody any thoughts on this? I suspect it is jus5 me telling myself what I want to hear!

How should I decide which to use? I dont have enough for a test brick of aoption a atm but I could easily go back. Im thinking of making a test brick of pure clay from option b. What should I hope to see? Lots of cracking and shrinkage if its high % clay?

Cheers all

Jon

[cobblerdave]

Re: First post, first clay oven build, first of many questions!

Gudday
From what I understand of the shake test . The sand and heavy materials fall to form the bottom layer the next is the clay ? And the the water at the top should contain the organic material in suspension.
Could you not seperate the materials on a bigger scale and retrieve the clay?
Just a bucket for an experiment . Fill with water at least 3 times and decant the liquid off to remove the organic material. Then let it settle so you can retrieve the clay that settled off the top of the sand. Then you have a clay that you can mix back with sand in more correct proportions.
I know its crude but it might be worth an experiment?
Regards dave