Looking for advice on a temp fix, or rebuild

[Digitalsucks]

Hi all,
First post. Been creepin’ the forum for a while, and been gettinbg alot of useful information, figured it’d be about time to sign-up.

I’ll get to my question, but just a bit of backstory. I built my first pizza/bread oven last year. With a budget of about $0. Used clay from my yard, straw from the field, sand from the beach, old bricks for a floor, and got away with a decent oven for the summer. It was small, but got up to great temperatures. Could cook a pizza in about 2 mins, and held heat long enough without fire, to do some bread baking. However .. it started to deteriorate over the winter (i believe the clay mix was way on the sandy side). So, i decided to dismantle, relocate my oven this summer, and still spend about $0.

I built a brick dome, instead of clay, using old chimney bricks, solid brick floor over a sand/bottle cavity (which worked well the year before). Covered my brick layer with vermiculite and cement mix. Approx 2”. And I have a very thin layer of clay on the outside, mostly to keep the vermiculite from falling away.

My issue. Oven is not getting hot enough, not holding heat, and not nearly as even as my previous clay oven. Granted, I know its differnent materials, etc.
Other Issue. My second firing got pretty hot, and some bricks on the inside started to flake and fire out the oven door like a pistol! I later learned, these chimney bricks are not meant for such high heat.

#1 do I need more insulation on the outside? Will it help, even if I can fire the oven too hot on account of the bricks breaking.
#2 is there something I can put on the inside (clay, mortar, cement?) to hold heat better, and deter the interior bricks from exploding?

Knowing that I will likely rebuild a third time at some point, if i’m not happy with it, is there a way to make this setup work better for a season, or do I tear it down, and go back to clay interior?

[UtahBeehiver]

Hello to home of "Anne of Green Gables", By description of your ovens, your budget is limited (nothing wrong with that) but knowing this, I will limit my answers to less costly mitigation measures.

First, normal house bricks cannot handle the heat required for a pizza oven, they start to spall, as you experienced. Unfortunately, you either look for some refractory fire brick or you deal with spalling brick, The forum has discussed with other builders of parging the inside of an oven with a refractory mortar or such but I am not aware of anyone (long term) who has been successful.

The oven not heating up can be attributed to two things, the sand/bottle mix under the floor is not very effecient and worse if the sand is wet at all (like a big heat sink), you did not say what ratio the vcrete was on the outside of the oven (2" is bare minimum at 8 -10 to 1, the lower the ratio the lower the insulation value, typically should be abt 8-10 to one on the dome and 5 to 1 under the floor.

It is possible to cast a oven using a home brew refractory material, 3 sand 1 lime 1 fireclay 1 portland which can be formed over a sand mold. Peruse the forum for "other type of ovens"

[Digitalsucks]

Thanks Russel. Appreciate the informative (budget friendly) response. So, speaking in short term, is it possible to use “refractory mortar” to line the inside?

If I rebuild, and do a clay inside, is this the “3 sand, 1 lime, 1 fireclay 1 portland” mixture you mean? I could then do a vermiculite/concrete layer, and then a finish layer.

Then oven floor is the same I used last year. Dry sand. Dozen or more interlaced wine bottles.

The outter insulation layer. I will admit, I was aiming for 10-1. But after I found the mix could not hold together in the breeze, I was skeptical and added more concrete/water. I’m guessing I ended up with 2” of 6-1 — so not enough by what I’ve read on here.

[UtahBeehiver]

The ratio for home brew is a heat resistant mortar formula that is used quite often with members of this forum. Ideally, a dense (not to get confused with insulating mix) cast refractory proprietary mix is preferred for casting an oven. Review threads and post from David S. he is our resident casting expert. As far as you clay mix, we do not see cob ovens very often on this site so can't really help you there. As mentioned before, I do not know of any successful lining of an oven with a veneer of refractory material.

Attached is a chart (from David S) showing the K values (thermal coeff) of various ratios of vcrete, 10 to 1 gets a little hard to handle. I do think you would be better off (if you rebuild) with floor a 5 to 1 vcrete floor insulation than bottles and sand.

[Digitalsucks]

Thanks. I’ll do some more reading on that, and hopefully report back with a plan of action. Is it out of the question to remove/save what I have as a my current dome insulation layer, and crumble it into the floor cavity instead of the sand/bottles?