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wlively 03-14-2007 11:17 AM

Casting vent
Hello all

I finally got back to my oven and will be casting the vent transition soon. When I bought my insualtion I also bought a few pounds of stainless steel needles to reinforce the casting.

I would like to ask what materials are giving the best results? I have been considering insulcast or refrax mixed with crumbled brick. Thoughts/experiences??

Thank you,
Wade Lively

redbricknick 03-14-2007 12:13 PM

Re: Casting vent
I used refrax to put my opening arch together and slow cured it with wet linen for a few days.. It's the hardest substance I've ever come across. Well, i encountered some grape nuts stuck to a cereal bowl once which were harder, but I'm unsure of their refractory qualities.. I was grinding my arch to fit some bricks yesterday, and the cured refrax (Refmix?) was extremely difficult to grind. Bloody hard stuff. Of all the refractories i've experimented with.. 1,3,11, Heat Stop, Selleys fireplace mortar, everything but La Farge Fondue, Refrax is by far the hardest. I have one and a half bags left which I'll be casting my vent with. I also have a ton of firebrick offcuts and a ton of dust left which i don't know what I'll be doing with yet. I was thinking of cutting them into small pieces, and mosaicing a refractory layer over my dome..

johnrbek 03-14-2007 01:21 PM

Re: Casting vent
Last time I spoke with james, he said he was out of refrax.. In any case, though, my understanding is that is more of a mortar which is designed for joints.. However, I will say I am not a mason, so who knows.. it may be a good application.

On the other hand, there are refractory products designed specifically for casting. The folks who sold you the needles should be able to provide you with some. I went with KS-4... the data sheet is here: I'd check to see whatever is available locally that will provide similar qualities... Just MHO.


johnrbek 03-14-2007 01:31 PM

Re: Casting vent
By the way.. you want a refractory castable, not an insulating one.. you want your vent hot to promote a better draw... In addition, insulating castables are not as strong structurally as refractory castables... Insulcast, from it's name sounds like it might be an insulating castable...


james 03-14-2007 01:57 PM

Re: Casting vent
I think JB has it right. You want the vent to get hot, and create the draw for the chimney. The standard vents with the FB ovens are made from the same refractory as the oven chamber itself -- not an insulating material.

I like the idea of creating a custom refractory with ground up fire brick piece in it. Refmix, plus additional refractory aggregate and a structural steel, wire or rebar could work well.

What did the folks who have cast their own vents use?

How does the cast vent section of the Pompeii Oven plans look to everyone? Is there enough detail?

james 03-14-2007 01:59 PM

Re: Casting vent
One more thing. We have re-named Refrax to Refmix (yes, I know it's a terrible name) for various reasons. It will be back in stock in NorCal in 3-4 weeks.

fdn1 03-17-2007 07:14 AM

Re: Reinforcing a vent casting
This question may be a bit off the current topic but it is related to the general subject of casting a vent. Hopefully someone with considerably more knowledge about the structural properties of reinforced concrete will be able to provide an answer.

Is it recommended that the vent casting be reinforced with steel to increase its ability to withstand large bending stresses when it must support loading from heavy masonry flue structures?

One concern in this regard previously raised on this forum by Christo (6/17/06) related to the high temperatures the vent structure would experience. He inquired about the advisability of including thick rebar when casting a vent because the significant difference in its thermal coefficient of expansion compared to refractory concrete coupled with its thickness might lead to a strength failure with repeated heating cycles. (I could not find a response in the archives.)

Another concern might be related to the lack of thickness of the concrete structure. Does the concrete have to have a minimum thickness before the addition of steel increases the bending stress it can withstand without breaking?

To summarize: Does the addition of steel (rebar or wire mesh) actually help the vent to support heavy flue structures? If so what thickness is recommended for inclusion?

johnrbek 03-17-2007 08:26 AM

Re: Casting vent
Rather than experimenting with rebar and similar materials, consider using stainless fibers.. They are cheap, $3-4/lb, used in ratios of 2-5% by weight, and general available on the web... Here's one manufacturers product description: D & C Supply Co., Inc. -- Stainless Steel Fibers

From what I understand, this is the material that is more commonly used when strengthening castable refractory. The detail on the link gives a decent explanation of how they work.

Not sure about the thickness question, but I've seen cast pieces as little as 1" thick used in similar applications. I would imagine the thicker, the better.

Good luck.


james 03-17-2007 08:41 AM

Re: Casting vent

Good idea. I added zirconia fibers (like glass but they don't melt at high heat) to the mortar around the dome of a precast oven a couple of times to see how it worked, and it came out fine. It felt like you were adding straw to waddle in the good old days.

One other idea. I have also broken into precast oven domes, and there is a wire net (thin), set in the middle of the refractory.


wlively 03-17-2007 06:37 PM

Re: Casting vent
You definitely don't want rebar.

Steel and refractory have very diff coefficients of expansion and the large piece will move alot causing large cracks. Also, the mortar is corrosive and high heat accelerates oxidation (rust). That is why they use stainless steel fibers or needles. Very thin pieces don't cause much expansion pressure, yet grab enough to reinforce and the stainless resists oxidation.

Also, thanks to all and JB I will be buying some KS-4.

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