cast dome with ks-4v plus with ss needles problems (and solutions?)

[johnedm]

about 1 year ago, we finished my castable 36" pizza oven... we have used it almost every week since (pizza one night, something slow cooked in the residual heat the next)... it is working _exactly_ how I want it to... takes about 1.5 hours to get up to temp, holds heat so that it is about 250F the next day... cooks pizza's perfectly... i'm super happy with it...

but there is a kinda ticking time bomb... the ss needles seem to be causing pieces of the cement to chip off... i'm not too worried about the pieces of cement, but I definitely don't want ss needles popping off into our pizza or shawarma or whatever we're cooking.... I have a couple options that i'd like opinions on:

1. aggressively brush the inside of the dome with a wire brush hoping to get the exposed needles to fall... do this periodically to keep loose ones out of the food.
2. build a ss sheet metal dome inside the oven and glue it to the dome inside with high temp mortar or hold it in place by cold riveting the panels together
3. tear the thing down and start over without ss needles...

I am not sure on #1.. #2 seems like it would mess up the transfer of heat to the thermal mass and mess up both my heat up times and my residual heat cooking the next day... #3 would just totally suck cause the outside is decorated with a stucco pattern that we'd never be able to replicate so it would likely look like crap afterwards... plus we'd have to totally re-think our weekly meal plan...

I would like #3 if I could re-build it with bricks instead, but the way we use it, I don't think I could deal with the 2 to 3 hour heat up times. if there were a way to use bricks but have a 2" to 2.5" wall, I would be interested in hearing about that.

thoughts?

thanks,

john

[david s]

I could be wrong, but it sounds like your problem is that the needles have rusted, therefore expanded and forcing some of the casting at the surface to break away. Rusting is accelerated in the presence of heat, moisture and oxygen. Untreated steel rebar should be well embedded in any concrete layer for this reason.
I've not seen or heard of this problem with any other ovens, but I've seen this very description in standard concrete paths, back in the 90's, where steel reinforcing fibres (not stainless) have been used. Those close to the surface are exposed to moisture and oxygen, rust easily and pop a section around the size of a large coin, off the surface of the path. In discussion with fibre suppliers they say that the steel fibres must be embedded properly, which means a topping mix without fibres should be laid straight over the top. This vastly increases work time on the job and the steel fibres have been largely replaced by the large polypropylene fibres which can remain on the surface without problems. Nickel added to steel in sufficient quantity will prevent it from rusting, but is very expensive. This has led to the development of different grades of stainless. eg 304 has less nickel than marine grade 316. Different grades are used for different applications. Just as alkaline resistant AR fibres are coated in expensive zirconium, there are different proportions of zirconium coatings and prices accordingly. If AR fibres or stainless needles are purchased on line cheaply, it is a fair assumption that their expensive corrosion protection is at a minimum. It is difficult to find out from these cheap suppliers just how much corrosion protection their products have and they are reluctant to disclose it because they want to make the sale, so buyer beware. I always purchase both stainless needles (melt extract fibres) and AR fibres from reputable suppliers rather than buying cheaper products on line. This could be an explanation of your problem, but fixing it is another. Parging the inner surface with a thin layer on to the fired interior might work, but will be difficult. Creating a stainless liner on to a compound curve sounds nigh on impossible. Some builders have attempted a thin walled 2-2.5" dome, but without success because it results in very narrow mortar joints. For the same reason you never see brick walls with the bricks laid on edge, its just not strong enough because the mortar joints are too narrow.

[johnedm]

thanks so much for the response... that is _exactly_ what happened...I was unable to find any source for the steel needles other than ebay... so i'll bet they were just not the right kind...

I think I can get a steel dome in there with 22 gauge 310S stainless steel (if I can find it) ... the plan would be to cut 8 wedges the width of my opening... then use blind rivets to attach them together inside the oven... then make a separate piece for the doorway lip and attach that with tabs and rivets to the two pieces nearest the doorway. (the gallery is made out of firebricks, so i'm good there)...

I think I need the 310S because I have a propane burner in there as well, that blasts the wall/ceiling when it is on full (though I very rarely use it I much prefer live fire and kinda regret the burner decision)... I could just use 304 ss and then build a separate shield to place over the burner when using it (kinda like a large 1/2 pipe with a 45 degree elbow or something...