Newbie, first time builder 🙈

[rnb1989]

Hi, I'm a first time builder I am using the exercise ball method and then the oven itself will be cast out of cement and vermiculite mix, I am struggling on what to do regarding the base/floor of the oven, I have purchased a 25kg bag of refractory cement, if anyone knows, would this be ok to make a slab out of it and use this as a base?? Thanks in advance!!

[downunderdave]

Feb 28, 2017 18:25:25 GMT rnb1989 said:

Hi, I'm a first time builder I am using the exercise ball method and then the oven itself will be cast out of cement and vermiculite mix, I am struggling on what to do regarding the base/floor of the oven, I have purchased a 25kg bag of refractory cement, if anyone knows, would this be ok to make a slab out of it and use this as a base?? Thanks in advance!!

Any large piece of refractory will crack due to uneven heating and therefore uneven expansion. That is why most ovens use unmortared floor bricks so they can expand independently.A cast floor wil work, but you need to reduce the number of sections.

[rnb1989]

Feb 28, 2017 20:28:21 GMT downunderdave said:

Feb 28, 2017 18:25:25 GMT rnb1989 said:

Hi, I'm a first time builder I am using the exercise ball method and then the oven itself will be cast out of cement and vermiculite mix, I am struggling on what to do regarding the base/floor of the oven, I have purchased a 25kg bag of refractory cement, if anyone knows, would this be ok to make a slab out of it and use this as a base?? Thanks in advance!!

Any large piece of refractory will crack due to uneven heating and therefore uneven expansion. That is why most ovens use unmortared floor bricks so they can expand independently.A cast floor wil work, but you need to reduce the number of sections.

Would it help if I mixed vermiculite in the cast?

[rnb1989]

This is my refractory cement I have bought

[downunderdave]

Mar 1, 2017 12:08:48 GMT rnb1989 said:

Feb 28, 2017 20:28:21 GMT downunderdave said:

Any large piece of refractory will crack due to uneven heating and therefore uneven expansion. That is why most ovens use unmortared floor bricks so they can expand independently.A cast floor wil work, but you need to reduce the number of sections.

Would it help if I mixed vermiculite in the cast?

There are a few problems in using vermiculite in the castable mix for the inner layer (both floor and dome). It will make the mix less dense and less conductive, as it increases the insulative qualities it makes it more difficult to get heat into it. It will also reduce the strength of the mix resulting in abrasion damage. Most builders use a dense inner layer tapprox 2" thick hat will be nice and strong, heat up faster and hold heat longer. Then that layer should be well insulated. You can use a vermiculite mix there. If you are still keen to cast the floor with dense castable, a Portland, lime, clay and sand mix won't work well, but a proprietary castable, reinforced with stainless needles and cast into at least four sections is a better option. Best option is loose laid firebricks. Not quite sure if you have bought refractory cement (calcium aluminate), refractory mortar, or refractory castable (contains fine aggregate)

[rnb1989]

Mar 1, 2017 20:22:03 GMT downunderdave said:

Mar 1, 2017 12:08:48 GMT rnb1989 said:

Would it help if I mixed vermiculite in the cast?

There are a few problems in using vermiculite in the castable mix for the inner layer. It will make the mix less dense and less conductive, as it increases the insulative qualities it makes it more difficult to get heat into it. It will also reduce the strength of the mix resulting in abrasion damage. Most builders use a dense inner layer tapprox 2" thick hat will be nice and strong, heat up faster and hold heat longer. Then that layer should be well insulated. You can use a vermiculite mix there. 

So basically I have a 25kg bag of refractory cement for the base, could I make a base out of this then sink some fire bricks where the cooking area would be?

[downunderdave]

Ok, what you have there is not refractory cement. It is refractory castable which contains refractory cement and high temperature aggregates and other goodies designed to make it nicely workable when mixed with water to a stiff mix as well as fine burn out fibres which assist in the water elimination during the early fires. It is also designed to be fired in a kiln or furnace up to operating temperature (in excess of 1000 C) when the material undergoes sintering to give it strength and permanency. Firing only to around 500 C will not do this and the material then only relies on the chemical bond of the calcium aluminate cement in the mix. Unfortunately the 500 -650 C range is where various chemical changes occur and where different materials in the mix have different expansion rates. It should be fired in a slow and controlled atmosphere, not possible with a WFO. This means the material is prone to fine cracks and fractures. This is ok for the dome, but not so for the floor, which is why I said you need to cast it into a number of smaller sections. The addition of stainless steel needle reinforcing would also help.Sinking firebricks into the mix will negate the bricks ability to expand and contract freely and individually. As the centre of the floor takes the most punishment you could probably set some bricks there loose and use the castable around them. I'd guess around half the total floor area should be firebrick
.
Apologies for the longwinded reply, but I like to qualify my advice, hope you can make sense of it.

[rnb1989]

Mar 2, 2017 6:30:32 GMT downunderdave said:

Ok, what you have there is not refractory cement. It is refractory castable which contains refractory cement and high temperature aggregates and other goodies designed to make it nicely workable when mixed with water to a stiff mix as well as fine burn out fibres which assist in the water elimination during the early fires. It is also designed to be fired in a kiln or furnace up to operating temperature (in excess of 1000 C) when the material undergoes sintering to give it strength and permanency. Firing only to around 500 C will not do this and the material then only relies on the chemical bond of the calcium aluminate cement in the mix. Unfortunately the 500 -650 C range is where various chemical changes occur and where different materials in the mix have different expansion rates. It should be fired in a slow and controlled atmosphere, not possible with a WFO. This means the material is prone to fine cracks and fractures. This is ok for the dome, but not so for the floor, which is why I said you need to cast it into a number of smaller sections. The addition of stainless steel needle reinforcing would also help.Sinking firebricks into the mix will negate the bricks ability to expand and contract freely and individually. As the centre of the floor takes the most punishment you could probably set some bricks there loose and use the castable around them. I'd guess around half the total floor area should be firebrick
.
Apologies for the longwinded reply, but I like to qualify my advice, hope you can make sense of it.

Thanks for your patience lol, so how could I make a base for my dome? It's to be sat on a old electrical bobbin reel??

[downunderdave]

I had one of those wooden cable bobbins years ago and after 3 years in the weather it was done. A wood fired oven should last generations and is far better built on a masonry stand. However if you have your heart set on using it you will need an insulating slab at the least, between it and the cooking floor. A mix of vermiculite and cement (5:1) is probably the cheapest way to go.

[rnb1989]

So if I had a heat sheet then a base of vermiculite and cement? Then sat the dome on that?

[dave36]

Mar 2, 2017 8:58:57 GMT downunderdave said:

I had one of those wooden cable bobbins years ago and after 3 years in the weather it was done. A wood fired oven should last generations and is far better built on a masonry stand. However if you have your heart set on using it you will need an insulating slab at the least, between it and the cooking floor. A mix of vermiculite and cement (5:1) is probably the cheapest way to go.

Agree the above. An alternative could be celcon blocks (there are types with better insulation than others) which are very easy to cut to shape. On top of which you can lay hearth (firebricks) and even better would be a layer of insulation between.

[rnb1989]

So vermiculite sheet (as the cooking floor) sat then on a vermiculite base? Would that work??

[downunderdave]

Mar 2, 2017 9:30:36 GMT rnb1989 said:

So vermiculite sheet (as the cooking floor) sat then on a vermiculite base? Would that work??

No, you need thermal mass for the cooking floor . Dense firebrick provides this.

[rnb1989]

So if I did vermiculite base, with fire brick on top? How would I retain the fire bricks on place?

[rnb1989]

Changed my mind on the bobbin base, still going to be wood though, will be a frame with exterior ply, vermiculite then a sheet then the bricks