Heat Break Materials

[MarkJerling]

All metals are pretty conductive which generally makes them unsuitable as a material for an insulating door. Reducing the thickness will reduce the amount of heat they’ll conduct. Copper and aluminium are easy to work but they’re both very conductive. Thin mild steel is subject to corrosion (never use galvanised, it is toxic if heated high), stainless has lower conductivity, but warps badly, from heat especially if it’s thin, which creates problems where it will seal against the oven mouth rebate. The only way is to make one and try it out, but if I were selecting stainless, I’d go for around 20 gauge.

Regarding toxicity of galvanised steel: Zinc has a melting point of 419degC (786F) and a boiling point of 907degC (1664F). As such, our ovens simply don't get hot enough for galvanised steel to be a problem when used with a removable door that is fitted, usually, once the oven starts to cool down. Having said that, if you see yellow or green discolouration of the galvanised steel, then the heat is too high and toxic fumes will be emitted. (zinc oxide fumes) I can't see that being a problem at oven temperatures but care should be taken when welding galvanised steel as welding temperatures will cause off gassing of zinc oxide fumes and welders sometimes get poisoned this way.

Regarding stainless steel: I used 316 grade stainless for my oven door's inner skin and have not had any issues with warping. This is thin steel, around 1mm thick. For the face away from the oven, I've used galvanised steel, but the insulation inside the steel box means that the galvanised parts do not see much heat. On the outside, I have a 30mm plywood skin and while that has charred a little at the very top edge, it's worked well.

[david s]

Holding moisture in anything made with Portland cement will enhance its strength. Critical construction use 28 days, but at least a week is advisable. Calcium aluminate cements achieve full strength in 24 hrs, but I hold the moisture in for at least 48.. The following chart for Portland cement products is useful.
https://theconstructor.org/concrete/...me/5933/?amp=1
of course in your case you will need to balance or compromise the need to keep the water in with the danger of it freezing.

[UtahBeehiver]

You can start with halogen lights right away(maybe get you 125 F), then charcoal briquettes (gets you about 200 F) several times and you can do a dutch oven while you are at it. But again, do NOT do any live wood fires until the dome is insulated then you go really slow, one extra log will spike the temp in the upper dome and crack it even in an insulated oven.

PS you need to keep any more water from getting into the CaSi.

[paulkjrobbins]

Russel, you are experiencing the same weather that I am just a little bit further North. Do you have any advice for how long to wait, after I mortar the outer-arch, before I start curing?

[UtahBeehiver]

I would not do any curing live fires until the dome is insulated. The thermal differential between the inside of the dome and the uninsulated side of the dome has more potential to crack. An insulated dome tempers the temp differential. Yeah, CaSi is very water absorbent and get mushy but it does dry out. With wet insulation and vcrete it is going to take a lot of curing cycles to drive the moisture out. This is where we see a lot of builders get impatient, cure too soon, too hot, too quick and they damage their ovens so caveat emptor.

[paulkjrobbins]

I have yet another question for the forum. I have read a lot of posts about curing the oven. I'm not worried as much about the dome as it will have significant time before I start the curing process, but the outer arch is a concern to me. It has started to freeze here and I am not sure what time to give the outer arch before I start curing it. My thoughts right now are to put a halogen light or small (700W) space heater under it to help the mortar set for a week and then start the curing fires. My floor is also another concern. My FB is thoroughly wet on the edges and I don't know what it is like as it gets closer to the center of the dome. I am thinking it is going to take considerable time to dry out with curing fires. I am wondering if this is even a concern though since the floor is not set in mortar, does it even matter how hot or cold it is during the curing process, the point seems to be to me that it just needs time to dry out.

I did not have the foresight to leave a hole in the hearth slab for draining purposes so I am planning on drilling a 3/8" hole near the center of the dome that goes up through the vermicrete to just below the fiberboard to help drain water. After curing I plan on caulking lead flashing to the base around the oven on the outside of the vermicrete render to wick moisture away. I had originally planned on enclosing my oven in a house, but I am now leaning towards rendering it in a rapid set concrete with fiber reinforcement added and then finishing it with weatherproof paint. I might add a tile or cobble finish next spring. This is just to get it working while the temperatures are going so low.

I know that is a lot. I am unsure about a lot of this. Does anyone have any advice? I am hoping the best option is not to just cover it up and wait till spring!

Best,
Paul

[paulkjrobbins]

Dave and JR,
That is really great advice. I think will get a stainless in a 16-20 gauge range for the door. I am going to forget the stainless mesh idea as well. I think instead of the L-brick I am going to taper the bricks so that they only touch the inner arch at the low point. I am considering using carboard to make a 1/8" gap where the arch bricks are closest - at the bottom of the arch. After the arch is done I would stuff the taper with fiberglass rope and use rigidizer on it o make sure it doesn't flake off and fall onto the floor. I am thinking this gap would also be sealed off over time with soot from the smoke, maybe? Any advice? Thanks for all the info you've given already.

Best, Paul

[JRPizza]

I had one question. What is the benefit of the L brick? I realize that it allows the gallery to sit further back making a smaller footprint, is there another benefit?

I had to go back to my build to see why I went with the L shaped bricks - the start of the discussion is in the link below. I can't take credit for the idea - I got it from a fellow builder DavidApp. In summary, the shape worked well with the desire to have the vent arch bricks higher than the oven arch to provide a reveal, the extra length of the bricks made for a more stable arch, and the horizontal gap surface made trapping and retaining my gasket rope very easy and I did not need to fix it in place or have any contact with my oven arch to keep pieces of gasket from falling into the cooking floor. I think using a piece of stainless in contact with both vent and oven arch will serve as a conductive "couple" and you would be better off with an air gap.

https://community.fornobravo.com/for...600#post382600

[david s]

All metals are pretty conductive which generally makes them unsuitable as a material for an insulating door. Reducing the thickness will reduce the amount of heat they’ll conduct. Copper and aluminium are easy to work but they’re both very conductive. Thin mild steel is subject to corrosion (never use galvanised, it is toxic if heated high), stainless has lower conductivity, but warps badly, from heat especially if it’s thin, which creates problems where it will seal against the oven mouth rebate. The only way is to make one and try it out, but if I were selecting stainless, I’d go for around 20 gauge.

[paulkjrobbins]

Hi Russell,
Thank you for the advice. I'll be using SS. I know a guy here in Provo where I can get 32 gauge pretty cheap.

[UtahBeehiver]

Riveting is the better option if you cannot weld the door vs using HT silicon. I cannot remember if you are using SS or carbon steel (don't use Al or Cu, really high K(s). Carbon steel is substantially higher in K value than SS so it is better to use SS. Old SS panels can be found on old refrig. or BBQs, or washer/dryers, just snoop around for someone throwing away a broken unit.

[paulkjrobbins]

I had one question. What is the benefit of the L brick? I realize that it allows the gallery to sit further back making a smaller footprint, is there another benefit? Because of my redesign, my gallery will have plenty of room in front of it so that is not an issue. I am considering doing more of a tapered brick instead of an L brick. Here is a drawing of what I am thinking along with a picture from Russel's build of what i mean by the tapered brick approach to the heat break. Thank you for any advice you have. This forum is amazing, below is a picture of what my pizza oven would look like if without your advice!

[paulkjrobbins]

Thank you again for all the advice. After looking into it I realized that riveting is not the most intense thing to pick up so I am going to rivet the door together and caulk with the gasket cement to seal the FB inside.

[paulkjrobbins]

Hi Mongo and Fox, those are some really good points that you bring up. I don't have a rivet gun and don't want to have to deal with bending tabs. I think I will try to use the cement I mentioned to assemble it, but put some SS tubing on the inside maintaining the thickness with SS bolts going through them. Thank you for responding. That confirmation that I'm not missing some logic that is really obvious helps to go forward.

[mongota]

I was thinking of something like Rutlands Stove Gasket Cement). It bonds well to metal . It is commonly used on the forum to adhere fiberglass rope around the inner arch for a heat break. Some have used it to cement a cast-iron door to their gallery. I'm not sure how those doors worked out though. When it is fired it cures hard and is supposed to withstand up to 2000 degrees. Does anyone have any advice on using this to assemble a stainless steel door around fiberboard? I suppose if no one has done this before or can see a reason why it would be a bad idea I can try it out and be a ginny pig...

You're right, it does state it can bond metal to metal, but it does state something about being used to make 'minor' metal to metal repairs.
I'd consider adding a few pop rivets. The rivets would provide holding power and the sealer would hold and seal.