New build

[simwilli]

Hi
I intend to make a wfo using gym ball mold using vermiticulite and cement (5:1 ratio). I will then render the outside of the dome with heat proof screed.

Will this be ok with regards to heat cracks and longevity and will this be ok with the british weather. Do i need to cover it etc
Any help would be appreciated
Thanks

[downunderdave]

Feb 5, 2018 20:42:37 GMT simwilli said:

Hi
I intend to make a wfo using gym ball mold using vermiticulite and cement (5:1 ratio). I will then render the outside of the dome with heat proof screed.

Will this be ok with regards to heat cracks and longevity and will this be ok with the british weather. Do i need to cover it etc
Any help would be appreciated
Thanks

5:1 vermicrete is not very strong. It will also be subject to abrasion damage. Because the material is insulating there is little thermal mass to store heat.using a gym ball as a mould also means you will have to either dig a hole so the lower half of the sphere is buried or build up formwork to halfway. It also means that once the casting has been made it has to be moved onto the oven stand. It is far easier to make a sand castle over a firebrick floor, cast the dome in situ using a dense castable then dig out the sand.
Whatever way you go the inner dome and under the floor, need to be insulated.

[simwilli]

Hi
Thanks for the reply. What you recommend I use? I noticed homebrew formula gets mentioned a lot. Can anyone go into more detail about this?

Thanks again

Simwilli

[oblertone]

Homebrew is Sand/Cement/Lime/Clay in the proportion 3-1-1-1 by volume; there are variations but this is the most common formula. It is cheap (compared to air set or kiln cement), perfect for a WFO temperature of 550c max, and relatively easy to work. Highly recommended.

[simwilli]

Thanks I'll give that a go!!

[downunderdave]

Feb 7, 2018 12:27:50 GMT simwilli said:

Hi
Thanks for the reply. What you recommend I use? I noticed homebrew formula gets mentioned a lot. Can anyone go into more detail about this?

Thanks again

Simwilli

If you use the homebrew as a castable mix you also need to add some burnout fibres which assist water removal at relatively low temperature. These fibres melt at 160 C creating a random network of tiny pipes that provide pathways for steam to escape. Without these steam spalling is more likely during the drying fires. The fibres are polypropylene, finer than human hair and are used in concrete. The ones I get are part of a two pack system called Novamesh 950, but there are many other brands. Alternatively you could chase down some dense castable refractory which is a far superior but more expensive product that uses calcium aluminate cement in its mix which is far better for higher temperature applications. This material already has the burnout fibres added in the mix.

[simwilli]

Hi
If I use homebrew, will have to render it and will it withstand the weather here I the UK. Will I need to build a shelter over it?

[oblertone]

My view is that unless you want to keep starting from scratch and drying out your oven every time you use it, then yes, you'll need to make it waterproof in some way.

[simwilli]

Hi thanks for all your advice. I'm going to make my dome with 1 layer homebrew dolled by a layer of wood chips in clayslip, wrapped in chicken wire then another layer of homebrew. I will then render it once dried out and cover with mosaic tiles.

Thanks

Simwilli

[oblertone]

Remember to post pictures of your progress.

Top tip: use pre cut mosaic tiles ! See my build thread for reason why.

[simwilli]

Thanks oblertone. Have you got a link to your build thread?

Thanks
Simwilli

[downunderdave]

Feb 9, 2018 16:45:27 GMT simwilli said:

Hi thanks for all your advice. I'm going to make my dome with 1 layer homebrew dolled by a layer of wood chips in clayslip, wrapped in chicken wire then another layer of homebrew. I will then render it once dried out and cover with mosaic tiles.

Thanks

Simwilli

Wood chips and slip will be fairly dense and not a great insulator. You need air in the mix. An improved method is wood shavings or chopped straw with just enough slip to hold the stuff together, leaving plenty of air spaces in the mix. A slightly more expensive brew, assuming you can get the straw and slip free, is to buy vermiculite (or perlite) and make a 10:1 mix of vermiculite and cement with a little powdered clay added to make it workable.Each grain of vermiculite contains lots of air.

[simwilli]

Hi could I then cover the vermiticulite with chicken wire the homebrew followed with render / mosaic tiles?

Thanks
simwilli

[downunderdave]

Feb 10, 2018 20:29:53 GMT simwilli said:

Hi could I then cover the vermiticulite with chicken wire the homebrew followed with render / mosaic tiles?

Thanks
simwilli

Yes. The chicken wire will impart some strength to the rendered outer shell, although I don’t do it that way. I found the chicken wire too time consuming to apply over a compound curve. Adding random fibres to the outer stucco layer is far easier. I make the outer shell only around 12 mm and have done many this way. Not sure why you are wanting to do another coat of homebrew rather than going straight to a cement render. Because of homebrew’s high clay content it has a tendency to suffer from shrinkage cracks. If you plan on tiling the exterior search “trencadis” a technique pioneered by Gaudí, which overcomes the problem of tiling over compound curves.
Also, make sure you dry the insulation layer before any external render, otherwise you riskexpanding the insulation with steam resulting in big cracks to the outer shell.

[simwilli]

Thanks downunderdave I'll make my inner layer thicker, then insulate, then finally finish off with render. Is there anyway to dry out without fired. The reason why is I'm on building the dome in the garage over the winter, and will build the plinth in the spring.