vermiculite hearth question

[jeffscogson]

hi there all you fellow brick oven nuts just a quickish question
i just had my vermiculite delivered and am looking forward to getting the hearth laid .

do i need to leave a gap between the vermiculite hearth and outer course of bricks ?? as at the min i got one layer of normal balast and cement bout 3 inch and then a course of all my old broken bricks so now was going to put vermiculite on top of that about 2 inch then bricks on there side then clay tiles . any help would be fantastic cant belive how many times i said vermiculite lol
thanx jeff
ps also just thought should i use rebar in the vermiculite ?

[lwalper]

Jeff,

If you use the 6:1 vermiculite mixture it seems to me that it should not move much with the heat. Sure, everything expands a little when it gets hot, but I don't think you'd need to leave any gap here.

The hearth slab is a different story. It does get hot and moves. You'll need to leave a little gap there or you may have problems when that slab expands.

[jeffscogson]

hi thanx for replying i have just put in my vermiculite mix was hard work lol as did not mix enough so was frantic trying to mix more but got there in the end just waiting for it to dry as only did it today . what did you meen by hearth slab ? did you meen the layer of bricks i am using on top of the vermiculite? my plan was to put a layer of clay bricks down then maybe on top of that some fire bricks or clay tiles not sure really so if you could advise that would be great i have been reading all i can about it but seems to be a lot of different opinions but am sort of going with the alan scot way up 2 now but my oven is not as big as the usual 32 internal mine is 27 inch so again thanx and any more advice and help would be fantastic
jeff

[lwalper]

Yep, I'm doing a modified AS design too. The "hearth slab" that I mentioned is actually poured on top of the vermiculite insulating layer to increase thermal mass within the insulated space -- and it will get hot. Just yesterday I was re-reading that section in his book and it looks like he leaves a bit of space (whatever the form is made of) around that slab between the slab and the cement block walls. The slab is actually hung from rebar supported on the frame of surrounding block.

There were pictures posted somewhere of someone's vermiculite falling out from under the oven, so I thought I'd pour a support slab similar to what is usually done on the dome ovens featured on this site. Then, the insulating vermiculite (or ceramic foam board) and brick will go on top of that.

The main difference between the barrel ovens and the pompei style dome ovens is the ultimate finished purpose. The dome ovens are generally designed for rapid heat-up, then a small fire is maintained throughout the baking cycle, thought to be more of a "pizza" oven. The barrel oven is generally considered to be a heat retention design including more mass and insulation in the construction, and requiring a longer firing time to heat all that mass. The fire is allowed to burn out, be swept out, and the oven is expected to retain and release heat for 24-48 (or more) hours without additional fire allowing the baking of a variety of breads based on the temperature of the oven at the time.

Either design is great, but depending on what you have in mind for your baking needs, they each have their advantages and disadvantages. I'm planning on putting 3 inches of masonry below my firebrick, as well as 3-4 inches for the oven cladding.

[dmun]

Just yesterday I was re-reading that section in his book and it looks like he leaves a bit of space (whatever the form is made of) around that slab between the slab and the cement block walls. The slab is actually hung from rebar supported on the frame of surrounding block.

With all respect due to Alan Scott, I'd really think carefully about hanging the entire massive weight of your oven from a few flimsy lengths of horizontal rebar. Good masonry practice holds that the rebar should be totally encased in concrete, and that no piece of it should be closer than two inches from any surface. This prevents corrosion related failure, as ferrous metals expand when they rust.

We can argue round/rectangle and the optimal amount of thermal mass all day and derive considerable amusement from it, but I think supporting the whole mass of your oven on exposed rebar is plain wrong.

[lwalper]

That's why I've opted for the support slab and building on top of that. It will be a barrel design, but not one suspended on rebar. For some reason, that just didn't look like a good idea to me.

[gdest]

I'm going to have to look this up in Alan Scott's book again. But I agree that having exposed rebar is not a good idea. Not only will the exposure to the O2 in the air cause rusting but the added heat will also promote the rusting.

[Tscarborough]

Alan Scott, whoever he is, needs to send out personal messages to everyone that has bought his book, Ala Steve Martin in The Jerk, explaining that he is wrong.

[david s]

I've seen steel reinforcing fibres close to the surface of concrete pathways that have corroded and expanded which has forced the concrete around them to crack away, leaving lots of little divots in the concrete surface. They now seem to use plastic fibers, presumably to overcome this fault. Heat simply accelarates the rust problem as evidenced by any steel, apart from stainless, in any kilns I've seen. Ovens would be the same.

[jeffscogson]

hi again my vermiculite has set was a bit worried but all seems to be good but the layer is a little shorter than i wanted was thinking of using some kind of refactory cement mix as in maybe fire clay sand and portland on top of vermiculite before i lay my sand fire clay bed so i got a nice level hearth what you think??
jeff