Some info to help me get started

[neilos83]

Hello all.
I'm looking to start building my pompeii oven this year.
I've never done any brick or block laying before, so this will be a learning curve for me!
On the plus side though I have a wall to build between us and the neighbours first so plenty of blocks for me to practice with first.

I have a few questions though, I hope they don't seem too silly, I've been trawling through the site for a few days looking for inspiration and found more than my wee head can handle, especially when i'm reading this in the early hours when my baby daughter has woken us up!

Here goes.....

I am building a 36" oven, so internal size of 900mm, I get that the height of the internal arch should be 280mm as per the 63% of the height of the dome, but what about the width of the arch? i was thinking of 450mm? would this work? How much bigger than this should the enterance arch be? Is it simply a matter of beinf half a block wider at each side of the internal arch?

Secondly, I have been given some second hand fire bricks to get me started. there are two diferent types of brick, one is the yellow style brick, and the other is black. I believe they are for very high temperatures of a furnace of some kind. IS it ok to mix the two kinds of brick in the dome? Or best stick to one type?

Thirdly, when laying my hearth on top of thermalite blocks, should I leave a gap around the perimeter (between the hearth blocks and the walls of the dome) or should the butt up? I was thinking for expansion? Also wit regard to the hearth, what is the reason for the herrinbone pattern? the bricks i have for the hearth are odd sizes and i may find it difficult to make this pattern. could i just lay them i a straight line?

Thanks, look forward to sharing my progress over the summer!
Neil

Neath
Wales.

[cobblerdave]

G'day
The 63 per cent rule comes from a study of surviving 1880 ovens in Toronto Canada in the 70s. That's average of all working ovens. The average was the magic 63 per cent. Same with the width. It's roughly 1/2 the internal width of the oven. If you bisect the dome with you oven height and a perfect round arch this is what you'll get . But what if you place it back or forward more. Or not have a perfect circle. It will turn out too be roughly the same. Mines an oblong entrance still turned out at under 1/2 the width of the internal dome.
The different colour of the bricks, you right to be a bit worried about uneven expansion. You could use one type on the hearth floor and an other type on the dome. I'd wait till local knowledge sees you right, I'd wait to find out. They could be engineering brick something I've just found out about as they don't exist in Aust.
As for the gape between dome and hearth yes leave it . But tape it it or fill with cardboard least the mortar drips from your construction don't fill it. Mines well
Filled with ash now.
But ..
You can build directly on the hearth brick, and this is the big but , don't mortar to the hearth only mortar that first layer to the sides to allow for expansion .
Hope something helps
Regards dave

[simon]

The forno bravo plans suggest an opening of 19" for an oven of your dimensions, although they also say have an opening height of 12" which seems a little high. I think your value of 280mm looks better.

The thing is the arch shape will have an effect. A more curved arch would measure higher (to be right) than a flatter arch.

Vitcas sell black fire bricks, and they may have matching expansion and conductivity values. If they are genuine fire bricks and not something like engineering bricks I reckon you'd be OK to mix, but if you don't have that many, I'd personally not use them.

With regards to the hearth, I agree with cobblerdave.

[kstronach]

theres no reason for the herringbone pattern its just aesthetically pleasing i guess! just try and get the hearth as flat as possible to avoid catching any lips with your peel.

are both sets of brick the same dimensions? could you post a picture up of them and of them split in half so we can see the middles?

keith

[cobblerdave]

G'day
The herringbone pattern looks good and works because all the bricks are on an angle to the entrance and the peel is more likely to glance off if it hits the lip of a brick. If you lay them square the peel can catch and stop dead. If you can't lay a herringbone pattern lay them at 45 degrees to the entrance. Not as fancy looking as herringbone but will do the same thing
Regards dave

[cobblerdave]

G'day
I'll add to that ... Whichever way you lay the bricks it doesn't really matter in the big scheme of things. The important thing is that enjoy building and using your oven really.
Enjoy the journey it can be as much fun as the final destination
Regards Dave

[neilos83]

Many thanks to you All for your comments and help. I think I will stick to one kind of brick in my construction. Will keep to my original size ideas too. I tried to download the fb plans but had no luck. I will keep you posted on my build through the summer. Thanks again.

[simon]

Send me your email address through the forum messaging, and I'll send you a copy of the FB plans I have if you want them. I don't think I have the latest copy, but still worth a look.

[neilos83]

Many thanks for all our replies. I'm still gathering materials for my build.
What is the best to use for my mortar? I've been offered some of the ready mixed fire clay in a tub. Is this suitable?
I was thinking I may need a lot of it for the dome. Or would I be best sticking with the powdered clay and make the home brew that I have been reading about? is the powdered stuff readily available? I'm trying to keep my costs down.
Many thanks again
NEIL

[oblertone]

The ready mix fire clay would be ok if you were building using tapered bricks individually cut to size because the gaps you'd be dealing with are <5mm; if however you're using a technique with larger gaps then homebrew (3-1-1-1 Sand/cement/clay/lime) is by far the best option as it's very forgiving to work with and cheaper because you use quite a bit.
Powdered clay can be hard to find but a bit of phoning around works wonders.