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Post by lovepizza on Mar 25, 2016 14:57:43 GMT
Ok, I have built my plinths ( in a very limited space). I wanted go for a traditional "river cottage type" build but now think I have settled with this. Perhaps someone could advise if I am going down the right track with dimensions, shape, size and materials ( just about everything then) I will be building a 2:1 sand/ clay inner oven, approx 6 cm thick. The shape off the dome will be an elongated dome (front to back) The inner dimensions will be 53cm wide, 70 cm long with a opening height of 17 cm (63% of inner height). Trad build with sand former etc.... My base is going to be well insulated with a 125cm this base of vermicrete ( sitting on solid brick), then a 25mm vermiculite board on top. The Hearth is going to be 64mm fire brick. I said that I had limited space, this is 89cm width But 120 cm length. As I have loads of vermiculite' I wanted to use this up for the insulation layer 9 cm (10:1 vermicukite: cement) rather than splashing out on a ceramic blanket (hence the restricted hearth width), then finish of with a thin crust of cement as weather proofing, taking me to the 89 cm width. I appreciate that this is only big enough for a 9" pizza but I have not seen anyone do a build this small on here. Will it work? Should I put a chimney on a oven of this size or not? is the shape ok ( not a perfect dome, but domes at each end). Will the 9 cm of vermicrete be sufficient insulation for roasting (after pizza). Is it on to put vermicrete ok on top of clay and should I prepare/ key the clay in some way for this? Is any of this making sense.......sorry, bit of a brain dump! Any advise would be truly appreciated. Many thanks
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Post by oblertone on Mar 26, 2016 8:45:59 GMT
I'll try and answer some of your queries, and am sure someone else will chip in, probably with an alternative.
The under oven insulation will be plenty; many ovens (including mine) have been built with less. The concern I do have is regarding the internal width which might make it awkward to maintain a rolling fire whilst cooking pizza, although the the fire could be pushed to the back of the dome.
The hemispherical dome is an optimum shape, but not the only shape and I think your elongated dome will be fine if you maintain the door/oven height ratio you've already mentioned; this will allow the heat from the combustion to soak into the dome before exiting.
As to wether you need a flue, again it's an optimum solution; I used my bare dome for a year perfectly well before I got around to building an entry arch and flue, but having a flue means your not working in hot gases quite so much and improves the experience.
Vermiculite is a wonderful insulation material, and even better when it is used 'dry'. With your space limitations I'd be tempted to build an enclosure (often called a dog-kennel) around my dome and backfill the void with vermiculite.
Hope this helps, but please come back with any further questions.
ps - Don't forget to factor in an insulated door of some sort as most of your retained heat will be leaving via the entry !
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Post by cobblerdave on Mar 26, 2016 21:12:47 GMT
G'day Make the oven a 1/2 pear shape with the entrance at the smaller end will help to encourage the hot gases to stay in your oven. A lot of cob ovens use this shape. Think of making your oven entrance a bit higher than the 63 per cent "rule" cause it's not a hard and fast rule as such. An WFO can still operate well at 70 or 55 per cent . Having a small entrance is hard on your back to bend to see in, consider a comfortable hieght for your oven. Hope something helps Regards dave
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Post by lovepizza on Mar 27, 2016 6:09:06 GMT
Oblertone & Cobbledave, Thank you for you responses, much appreciated. I should have mentioned that my plinths are made of railway sleepers with inner brickwork so maybe a "Dog kennel" would not be possible as I couldn't make it from brick..? For the main, I will be using this for pizza, however it is a crime to not use the residual heat for other cooking, hence my question on the insulation. Also, I don't want to be supplying central heating for back gardens for the whole of my neighbourhood via a badly insulated pizza oven! I think I will go for a flue. Built into the brick arch or just behind?...or does it matter? My other question was, is it OK practice to follow on from a 2:1 Sand: clay inner, with 10:1 ratio vermicrete? Would there be potential bonding/cracking issues? Also, on the final layer, is Portland cement with good plasterciser ok and will I avoid cracking or should I go fo a more specialist cement? I really appreciate your involvement Best regards P.s image of base attached (I think). Council slabs to go on, then vermicrete base, vermiculite board then 64mm firebrick. As you can see, will suit the pear shape cob oven.
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Post by lovepizza on Mar 27, 2016 6:26:57 GMT
Oh, and there was one other thing...., Dia of chimney. Would 4" be adequate (I was thinking about a terracotta wine cooler with the bottom cut out).
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Post by downunderdave on Mar 27, 2016 8:05:00 GMT
Because your oven is not very wide you will probably find that a fire at the rear will work best. Unfortunately this tends to make the rear of the oven hot and the front cooler. It will still work ok because all pizzas need a 180 degree turn after one min anyhow. a fire on the side does work better though. A 4" flue is not big enough you need a 5" for your oven size. Regarding over the oven insulation, as you are making the oven with cob anyhow, how about considering chopped straw with just enough clay slip added to it to make it workable. It dries out faster than vermicrete and makes excellent insulation. While on vermicrete that under floor slab will take weeks to dry out and if you cover straight over it with floor tiles you'll be trapping the water in it and it will take months of firing to eliminate. It's just like a pile of damp sand that can be sitting in the sun on dry concrete for months, yet still damp in the middle and on the bottom. This is ok because your oven will dry eventually and you'll find your oven improves with performance as the water is gradually eliminated. I find it way better to allow the sun and wind do much of the drying but that of course depends on your weather. Your oven as per plan will perform beyond your expectations and have plenty of thermal mass for cooking roasts and bread for the family. You will also find it extremely fuel efficient. In ideal drying conditions the inner cob dome will take around 10 days/inch of thickness to dry. You can use fire to hasten it, but you'll risk cracking from uneven shrinkage and at worst steam explosions if you get too eager with the fire too soon. Good luck.
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Post by chas on Mar 27, 2016 8:30:41 GMT
Oblertone & Cobbledave, Thank you for you responses, much appreciated. I should have mentioned that my plinths are made of railway sleepers with inner brickwork so maybe a "Dog kennel" would not be possible as I couldn't make it from brick..? For the main, I will be using this for pizza, however it is a crime to not use the residual heat for other cooking, hence my question on the insulation. Also, I don't want to be supplying central heating for back gardens for the whole of my neighbourhood via a badly insulated pizza oven! I think I will go for a flue. Built into the brick arch or just behind?...or does it matter? My other question was, is it OK practice to follow on from a 2:1 Sand: clay inner, with 10:1 ratio vermicrete? Would there be potential bonding/cracking issues? Also, on the final layer, is Portland cement with good plasterciser ok and will I avoid cracking or should I go fo a more specialist cement? I really appreciate your involvement Oh, and there was one other thing...., Dia of chimney. Would 4" be adequate (I was thinking about a terracotta wine cooler with the bottom cut out As touched on before, the accepted view is to get heat 'rolling' over the cooking area, so bringing the flue forward to keep it in as long as poss would seem to be the way to go, and dia is to some extent immaterial - if not quite all the heat/gases are diverted away from the door (and you), at least some will be - but you may find the wine cooler (neat idea) cracks. In itself that's no big deal, cracks and ovens seem to go together and it will probably still function. I don't think bonding the clay liner to the insulation is very likely or necessarily to be aimed for. Some independence might be good. The same applies to the final coat. Cement render may well crack, but that's not a big deal either as long as you aren't exposed to rain that could wick through. K-Rend basecoat is available with chopped strand reinforcement if you want to go that way. With plenty of insulation, the outside skin will be cool to the touch, so heat shouldn't be a shell-cracking issue, just general buggeration, if it happens. You'll probably put a cover over the oven when not in use, so penetrating rain is not an issue. One word of caution - be patient with your dome-building. There is a lot of moisture in the clay that needs to dry slowly to avoid major shrinkage. If you can rig up a gazebo over during construction to keep rain off and plenty of air circulating, then that will help. Do not plough on to the final 'waterproof' coat whether cement or K-Rend or you will trap moisture that may convert to steam when you light fires and under considerable pressure may well force its way out - by cracking the shell right through. Chas
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Post by downunderdave on Mar 27, 2016 10:30:23 GMT
Oh, and there was one other thing...., Dia of chimney. Would 4" be adequate (I was thinking about a terracotta wine cooler with the bottom cut out). You could use two terra cotta wine coolers. I've seen another oven with a double barrelled flue done this way, it looks pretty cool.If you can insulate around it and then render over the insulation you shouldn't have any problems with the terracotta cracking.
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Post by oblertone on Mar 27, 2016 11:06:42 GMT
There are lots of reports about terracotta chimneys cracking, which is not really surprising as their 'normal' position is far from the flames; that said a few cracks just add character and won't detract from the performance of your oven and I love the double barrelled look.
You mentioned that you have access to vermiculite board, have you considered using more of that and less vermicrete in your base as then there would be nothing to dry out ? I know it's an expensive option and may be dependent on your supply route.
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Post by lovepizza on Mar 27, 2016 11:13:23 GMT
Thanks Chas and downunder dave. Chas, when you say 5" flue, is that ext dia? I suspect not, but the options for clay drainage pipe seems to be 4" or 6" int dia. I cant seem to find 5". With regards to the insulation, Chas you mentioned trying straw and clay slip. For 4" insulation layer, would you consider this to be a better insulator than a 10:1 vermicrete mix?
Thank again
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Post by cannyfradock on Mar 27, 2016 11:51:33 GMT
Hello Lovepizza.....welcome to the forum. You have an interesting oven build on the go. You have many questions which have been answered well by the guys. I can't add much but here's my twopenneth worth.. You say that you are using council slabs on top of your base. If you are trying to pinch every bit of internal space you can then you could canter lever your slabs so they protrude perhaps 4" either side of your base. (Have a look at "Davedownunder's" profile pic to get an idea of what I mean) I have used a ceramic flower pot for a chimney on many of my builds and so far none of them have cracked. B & Q used to do one about 14" tall, 8" base (once the bottom is disced off and turned upside down) and 5" at the top. You can cut off a slice of the base to reduce it's diameter. Vermecrete will work better as an insulation layer over your initial clay layer but you will probably need some chicken wire between the two. You can buy vermiculite and vermecrete board from SIG. Just type in "sheffield Insulation" into your search engine to find your nearest store. Terry p.s I shall be adding a few pics (with the flower pot mentioned) to my current build ukwoodfiredovenforum.proboards.com/thread/2407/governess-cart-pizza-wagon-project as soon as I post this. It'll give you a better idea of how I cut down the flower pot. p.p.s....apologies. I just realised I haven't taken any pics of the latest stage but I have posted 1 pic of pot (not the one I used) I used the one mentioned in your post.
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Post by lovepizza on Mar 27, 2016 14:47:52 GMT
Thanks Terry,
5"....Are we talking internal or external diameter?
Stuart
P.s. Loving the cart oven
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Post by downunderdave on Mar 27, 2016 23:39:58 GMT
The inverted flower pot is a great idea. The Americans often use terra cotta flue tiles which are rectangular tapered forms and they often report them cracking if left uninsulated. If it gets really hot on the inside and the outside is cooler, particularly if there's some cool wind around, it will crack. Some overenthusiastic fire going up the inside of the flower pot is usually the main cause. The problem with the terra cotta flower pot cracking can be reduced by insulating around it with vermicrete.This prevents the uneven temperature difference. There's not particularly much height with only one flower pot, you could try stacking the next size down on the top of the lower one for more height. An angle grinder fitted with a diamond blade will cut the base off easily. Make the internal diameter no smaller than 5" (for a small oven)
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Post by cannyfradock on Mar 28, 2016 9:49:27 GMT
Thanks Terry, 5"....Are we talking internal or external diameter? Stuart P.s. Loving the cart oven Stuart We are talking internal diameter. Here are 2 photos which show the pot better. This is an oven I built for Kate Humble (I used the oven for the second half of a clay oven course). The one pic shows the oven just after completion. Other pic shows oven as part of course. If you look at the course pic you will see a piece of galv. pipe to the right of storage space under the oven. I pop this on top of pot when I light the oven for a better draw. Once there's heat in the oven I remove the pipe. Terry p.s...I shall remove pics and replace with links once seen. (don't want to impose on your thread)
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Post by lovepizza on Mar 28, 2016 19:17:46 GMT
Thanks Terry,
Picture perfect, I understand now. leave the pic on there if you want, no problem.
Downunderdave...I have managed to reduce my vermiculite bed to 75mm by putting another brick course on the inner plinth.
Concrete slab now on, will be putting the vermiculite bed on tomorrow evening......Progress!
This will now reduce the drying time. Would you say 2 weeks to dry prior to laying the board and fire bricks (weather dependent)?
Stuart
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