Hearth Mortar Questions... Newbie Forum Discuss Hearth Mortar Questions... in the Good Background Information forums; I am planning on building a brick pizza oven with a hearth constructed from firebrick with the typical arched top. ...

[mtom]

I am planning on building a brick pizza oven with a hearth constructed from firebrick with the typical arched top. From my research, i have gathered that i want to use fireclay or refectory cement for the mortar while assembling the hearth, but i have not been able to find much information on brick/mortar spacing.

I know typical birck laying uses a 3/8 inch mortar spacing, is this the same mortar spacing i should use in my firebrick hearth?

Should i use a thinner spcing on the firebrick cooking surface?

Any guidance on this matter is greatly appreciated.

Thanks in advance,

Mike

[dmun]

Welcome. Oven floors are typically laid without mortar. This allows easy replacement of any brick damaged through use. If you use a modern refractory insulation board, you can set your bricks down directly on the insulation surface. If you insulate with vermiculite concrete, you can lay a layer of dry fireclay/sand mixture under your floor as a leveling medium.

Firebricks are typically laid with minimal mortar joints, most refractory mortars recommend a maximum 3/16" spacing and less than 1/8 is common. In building domes, where the bricks are tilted, you end up with wider spaces on the outside, but this is not a problem with either the refractory mortar, or the home brew substitute.

[mtom]

Thanks for the feedback!

regarding the arch (cieling) i was thinking that the lowest part of the arch (by the walls) should be at least 12 inches tall, is there any rule of thumb for how tall to make the cieling?

also, i am thinking of modifying the "typical" air in the doorway exhaust out the chimney approach by installing a door to close the front doorway, and installing slots in the brick cooking surface where my fire/coals will be burning so that air is drawn in from below the fire and then exhaust through a chimney on the opposite side of the oven. the theory appears to be sound, any thoughts on this idea?

[DrakeRemoray]

Have you downloaded the free plans on this site yet?

[wlively]

Yes download the plans, alot explained there.

You don't want holes in your cooking surface, no need and waste of heat.
The incoming air is cold, it will enter the doorway on the bottom, circulate inside the oven and emerge as hot air out the top of the entryway.

[dmun]

i was thinking that the lowest part of the arch (by the walls) should be at least 12 inches tall

There is a tradition of making a half brick (4") soldier course at the bottom of your dome. I don't know what this is for, except to weaken the dome. I don't see any reason not to start your rings from the oven floor, the better to achieve the desired hemisphere shape.

exhaust through a chimney on the opposite side of the oven.

The air path in from the bottom of the entry, and out through the top of the entry is actually quite sophisticated, creating a circular path of combustion air that heats the dome properly. Every attempt to put a vent in the top or the back of the dome has been a failure, wasting heat and fuel. A subsidiary air intake in the oven will also cool things down, as well as blow ash around onto your pizza.

the theory appears to be sound

The Romans got this one right.

[mtom]

hehehe, thats what i kept saying to myself...there has got to be a reason why this open door design is being used after hundreds of years of ovens being built. I think the ash in the pizza remark is what put the nail in the coffin for me.

Thanks for all your help, looks like i'm going to change my design, and just in time too as i will be forming my foundation this weekend (weather permitting).

Should i use an ash trap ? or just plan on scooping the ashes out with a shovel of sorts? also, how do i clean the brick cooking surface?

I really appreciate all your feedback!

[dmun]

The ash dump question revolves around the primary use of your oven. If you are doing mostly retained heat baking, and will be shoveling out a lot of live fires, an ash dump could be worth the considerable effort to build it. If it's mostly a pizza oven, it's easy to shovel out a few cold ashes the next day.

Your brick oven floor cleans its self. Every fire burns off oven crud from the last bake. A quick brush with a long handled brass brush moves away any stray ash before you slide your pizza in.