Homebrew castable

[BradNZ]

Hey guys, another homebrew castable question.

I live in New Zealand and would like to make a castable oven using the gym ball method.

I have found a supplier for all the homebrew castable ingredients except for the S/S needles and polypro fibres.

Has anyone here built an oven without these ingredients? Would chicken wire be suitable as a reinforcement to sub for the needles?

I've seen a youtube clip where a guy uses pummis stone in his mix but it doesn't look like it would hold heat very well. Any other kiwis on here that can chime in?



Cheers

[UtahBeehiver]

Our resident casting expert is David S so search his post. But I am pretty sure he will say no to substituting chicken wire of SS needles. Polypro fiber should be available from any concrete supplier. Where are you looking at using pumice, you don't want it in you main casting but possibly be used in an insulation coating, Some European builds have used Leca balls in their insulation layers.

[david s]

Chicken wire is galvanised mild steel, so as zinc’’s melting point is 419 C and the oven dome working temp is north of that, I suspect there to be some degradation of the chicken wire if placed in the centre of the dome casting walls. I used chicken wire on the first cast dome I sold back in 2007. The oven is still in operation and functioning well, but I have no idea of the condition of the chicken wire in the casting, unless I demolished the oven. As most castable refractory in industry sees temperatures way north of those that we use for our ovens, the recommended reinforcing is stainless needles or more properly called melt extract fibres. If you want to go down the chickenwire route remember that it needs to be embedded in the middle of the walls of the casting. Because of the compound curve of the dome it is easiest to cut the chicken wire into smaller pieces and embed and overlap them as you go.
The polypropylene fibres need to be the really fine ones, finer than human hair. Do not try to use fibres from polypropylene rope because they are too coarse and won’t melt at the lower temperature required.. The fine ones can be obtained by buying a pack of concrete reinforcing fibres that usually come in a two pack form of thicker 50 mm plastic fibres and the finer 12 mm polypropylene ones. The brand I use is Novamesh 950, but there are many others.
As Russell said you don’t want pumice in your inner casting. Keep the inner dome dense for high thermal mass and strength. Use the pumice, or whatever insulating aggregate you choose, over the top of the dense inner casting.

[BradNZ]

Cheers for the advice fellas.
I found a supplier for the polypropylene fibres so will put them in the mix.
Dave, you mentioned that you used chicken wire on your first castable, did you also have SS needles or was the chicken wire the only reinforcing.

I stumbled across an old post https://community.fornobravo.com/for...hoestring-oven which looks like it held up well. Maybe that is he way to go for the reinforcing.

In regard to a home brew castable recipe, I've seen a few out there. When sand is mentioned would sand from the river bed be ok?

cheers
Brad

[david s]

Cheers for the advice fellas.
I found a supplier for the polypropylene fibres so will put them in the mix.
Dave, you mentioned that you used chicken wire on your first castable, did you also have SS needles or was the chicken wire the only reinforcing.

I stumbled across an old post https://community.fornobravo.com/for...hoestring-oven which looks like it held up well. Maybe that is he way to go for the reinforcing.

In regard to a home brew castable recipe, I've seen a few out there. When sand is mentioned would sand from the river bed be ok?

cheers
Brad

Brad,

No, I didn’t start using the needles until about oven 10. They do make placement more difficult. There’s a reason they’re called needles, however they do increase strength and would also make the resulting casting more conductive and marginally more dense.
Regarding the sand, I don’t think the type matters too much although grains with sharp surfaces probably create stronger bonds than those with rounded surfaces, but it should have a variety of grain sizes, therefore your river sand should do well. Regarding the poly polypropylene fibres, you don’t need much (around half a handful for every 15 L of dry castable) but you do need to make sure they are well dispersed, so mix for twice as long as you would think.

Dave

[BradNZ]

Thanks Dave,
I don't think I'll go with the S/S needles... Will definitely use polypro fibres though. I'm still sorting materials/planning but will no doubt have further questions come construction time.

Brad

[BradNZ]

I scored some firebricks for the cooking base from my grandma. They are old, my grandad used to build pottery kilns, but they are in good nick, and heavy so should be good.

With regard to the homebrew refractory mix for the dome, what type of lime do you use? is it special lime specifically formulated for concrete? Or garden lime such as this https://www.mitre10.co.nz/shop/yates...10kg/p/294842?

I don't know much about concrete at all so this is all new for me!

[wotavidone]

Not that one mate. That would be most likely just ground up limestone. ( Calcium Carbonate)

You need something like this ( calcium hydroxide)
https://www.ablazenz.com/shop/Landsc...3Fsku=443.html

[BradNZ]

Hey team, well its been a slow start but am finally under way.

Got the lime sorted, Placemakers sells 20kg bags at a reasonable price.

I've built the base, poured the support slab, and poured the 4" vermiculite concrete insulation layer (wouldn't call it poured though). I'm gonna go with a 900mm internal diameter. Yesterday i was cutting firebricks for the floor, when I started thinking about what to use for bedding them, I did some reading and I see that the usual way is a 50/50 mix of sand and fireclay mix, which leads me to my question.

Is there an alternative anyone has successfully used for the fireclay(for bedding the bricks and also in the homebrew mix)?
I have bought two 20kg bags, which cost $95 each. In my view it is a rip off, the fireclay is labeled Shiraplas FG-30.
I might just have enough fire clay for the dome, but should I run out do you think it would be ok to just skip it from the homebrew mix? I'm picking that it helps plasticise the mix and makes it workable without slumping too much, by the time I have run out of it then I should be near the top and it will be less likely to slump, hence my thinking it should be ok to leave out if I have too??


If it is absolutely necessary I guess I will just have to bite the bullet and buy another bag for $95.


Also I plan on reinforcing the dome using either 10mm rebar, or smaller gauge reinforcing mesh cut down, any objections to that? I've read the brickless oven on a shoestring budget build along and that oven seemed to work well. I will be skipping the stainless needles also.

[david s]

$95/25 kg bag of clay is exorbitant. Can you get Cement Australia's bricklayers clay? Alternatively try pottery suppliers and ask for powered ball clay. Maybe you can get your money back on the Shiraplas. The clay in the brew helps make the mix more workable, but more importantly is very refractory so it's pretty important. Avoid Bentonite as its extremely small clay particles lead to excessive shrinkage. I think any clay would suffice and you can get this for free under the layer of topsoil in your own garden.
Regarding the 10mm rebar, i think you will regret it. When heating the oven the heat will jump to the more conductive steel which will expand more than the material that surrounds it. This will create minute cracks which will gradually get worse the more the oven is used. Also because it's mild steel the heat will accelerate any corrosion. The reason stainless needles are the recommended reinforcing is that firstly their stainless so won't rust and secondly being small and thin they can disperse their heat to the surrounding refractory thereby overcoming the expansion problem. You can try chicken wire reinforcing but be aware that zinc melts at around 420 C and parts of your oven will exceed this.

[wotavidone]

I suppose you could always strip the zinc off the galvanised mesh with a quick bath in hydrochloric acid from the pool shop. Make sure you rinse it in several changes of fresh water straightaway.

Some interesting points:

https://www.engineeringtoolbox.com/l...ents-d_95.html
300 series stainless steels expand more than steel, iron, mortar, and concrete.

https://www.engineeringtoolbox.com/t...ity-d_429.html
Yes the stainless steels have lower thermal conductivity than iron and steel, but my belief is that, since the reinforcing is encased in the castable, it is the conductivity of the castable that counts. The energy can't "jump" the castable, it must be conducted by the castable to the reinforcing.

The main reason to use stainless steel rather than ordinary steel is because, as the lime becomes carbonated, the pH of the castable reduces. pH needs to be quite high to protect ordinary steel from corrosion.

[david s]

I suppose you could always strip the zinc off the galvanised mesh with a quick bath in hydrochloric acid from the pool shop. Make sure you rinse it in several changes of fresh water straightaway.

Some interesting points:

https://www.engineeringtoolbox.com/l...ents-d_95.html
300 series stainless steels expand more than steel, iron, mortar, and concrete.

https://www.engineeringtoolbox.com/t...ity-d_429.html
Yes the stainless steels have lower thermal conductivity than iron and steel, but my belief is that, since the reinforcing is encased in the castable, it is the conductivity of the castable that counts. The energy can't "jump" the castable, it must be conducted by the castable to the reinforcing.

The main reason to use stainless steel rather than ordinary steel is because, as the lime becomes carbonated, the pH of the castable reduces. pH needs to be quite high to protect ordinary steel from corrosion.

Stripping the zinc off the chicken wire then brings you back to the same corrosion problem in an environment of porous material which will allow moisture and oxygen in. Additionally we apply heat which accelerates any corrosion.

There are many types of stainless melt extract fibres. The manufacturer of the ones I use have five with different melting points, thermal conductivity and thermal expansion. The ones I use (ME 446) have a thermal expansion coefficient of 13.1@ 870 C and conductivity of 29.8. The 304 fibres 20.2 and 20 respectively.

To illustrate what I meant about avoiding thick rebar, the pic shows an 8 mm steel bar used for reinforcing in the outer decorative arch which sees way less temperature than an inner dome, yet the conductive steel has expanded more than the material surrounding it because it is way more conductive. Granted it took 8 years for this crack to show. Although it is standard practice to use steel reinforcing in heavy construction like buildings and bridges they are not subject to the rapid temperature changes that we use. Ambient temperature changes are so slow that the materials different conductivity doesn't matter much, but when heated rapidly makes a big difference.
Another example of this temperature difference is during firing if i place my hand under the oven the steel supporting the concrete supporting slab is hotter than the supporting slab itself. This is because of the steels much higher conductivity.

[wotavidone]

Yeah no argument from me regarding the thick reo. Regardless of relative conductivities, the thicker the steel, the bigger differential in relation to the concrete.
At a certain thickness and temperature the stresses from the expansion of the steel overcomes the internal strength of the concrete.
When we use thick reo in a building or bridge, we are only talking about the material moving maybe 30 degrees from the temperature on the day it was laid, hence not much differential in expansion.

Of course the steel under your slab will feel hotter - it has higher conductivity.
Until you use non contact temperature sensing that isn't influenced by conductivity, and track it from cold to temperature, the "science" of sticking your hand on there for a feel is flawed.

As for the stripping off the zinc - merely a suggestion for those who worry (I ain't one of those people) about the zinc "fluxing" the steel.
My thoughts - based on years of dealing with blast furnace slags - is that the zinc will be zinc oxide before you know it. Melting point 1975 C.

At the end of the day -
Thin is better than thick for the reinforcing
Stainless steel is better than ordinary steel
For people on limited budgets chicken wire or plain steel needles are better than nothing
Non stainless steel reinforcing will fail earlier in a pizza oven than in a building, because the lime will carbonate earlier.
The lime will give the concrete a pH around 13 which protects steel. The carbonation drops the pH to about 8.5 which ceases to protect the steel.

[david s]

“”Of course the steel under your slab will feel hotter - it has higher conductivity.
Until you use non contact temperature sensing that isn't influenced by conductivity, and track it from cold to temperature, the "science" of sticking your hand on there for a feel is flawed.””

Interesting. I shall try to remember to take some IR temperature readings on my mobile oven next time I take it to market.

[BradNZ]

All very interesting stuff. Thanks for the advice.

So I'm taking it chicken wire would be better than some 5mm reinforcing mesh cut down and formed around the dome?

I had a read on this link, and it's what i was going for, but after hearing what you guys had to say I'll probably refrain/rethink https://community.fornobravo.com/for...hoestring-oven

Cheers
Brad