Hot bits questions..

[bigandy]

Hi folks,

I'm building a wood fired oven in my back garden here in North Wales, and I've been trying to find some information regarding the "hot bits" in the oven. I've designed my oven to be made from firebricks (9"x4.5"x3") as sold by kilnlinings.co.uk, and it is in the style of an upside down half barrel.

Firstly, what is the most cost effective mortar to use when building the arches up? I'm going to have bricks touching, through to gaps of no more than 20mm. I have looked at the stuff sold by Kiln Linings, and apparently it needs to be heated to 300°C to set properly. Is there a better product, or DIY mix that can be used?

Secondly, regarding the insulation under the hearth. Again, this will be firebricks butted up against each other. I was planning on using a vermiculite concrete mix, and I understand from reading here, a mix of around 6:1 with cement is a good one to try. What I am not sure, of, is how thick a layer I should be aiming at under the hearth. I was thinking about 3" should be sufficient? The one thing I wasn't sure baout though, was the ability of the vermiculite concrete to support the weight of the roof of the oven?

Any help/advice on these questions would be gratefully received, thanks!

Andy

[cannyfradock]

Andy

In my view the best mortar to use is the "homebrew" mix of 3:1:1:1 sand, hydrated lime, Powdered fire-clay (also from Liam) and sand. If the normal sand in your area is very fine then go for 5/6:1:1:1.

The fire-cement that comes ready mixed in tubs is only good for a thickness of max 3mm. If you are buying the wedge shaped bricks from Liam (Kiln linings) then the fire-cement may be good enough....please ask Liam for advice on this.

Vermecrete...or vermiculite with cement 6:1 ratio was always the preferred method of an insulation layer beneath the fire-brick hearth. It does form a crust and will support an oven. About 2/3 years ago somebody used thermolite blocks in place of the vermecrete and found they give the same value, as vermecrete with the added bonus of being the third the price.......and half the work. The true difference between the two is perhaps not known, but I have gone down the thermolite block route with good results......I haven't built an oven using vermecrete under the fire-brick hearth so I haven't been able to make any comparisons.

I always use thermocrete over the dome, on top of the insulation blanket, but it is a bugger to work with.

Many Barrel/Vault ovens as your are building, form a housing (enclosed roof), so only a thermal blanket and then loose vermiculite is used to fill the void of the space.

Hope you get a bit more feedback from the guys (I haven't built a Vault oven....yet)

Terry

[bigandy]

Hi Terry,

Thanks for the response. My design uses standard blocks, not the tapered bricks (mainly due to cost!), so on the arches that I build the bricks will be on edge to give a wall thickness of 114mm. On the inside surface, the edges will be touching. On the outside edge there will be a maxium gap of 20mm.

You mention the mix ratio for the homebrew recipe, but in it, sand is used twice? Should the last part be cement or something else? Also, is this mix suitable for joints that will see the full heat from the fire? And, does it just set like normal cement, or need 300°C of heat to cure it properly?

I was intending for my vault to be insulated to a thickness of about 8 inches, then rendered over the top. I don't really want the expense of building a mini-house for it, and I'm also not too keen on building a dome shaped oven. So, I'm going to compromise and do a mix of both!

I like the idea of using thermolite blocks as the underhearth insulation, that should save a shedload of work. I presume that you just use the lightest class of thermolite blocks (2.8N strength ones are the lightest at my local builders merchant).

Thanks again, and apologies for the torrent of questions!

Andy

[cannyfradock]

Andy......Quite right !!!.....it should have read....3:1:1:1 Sand, Hydrated lime, fire-clay and "cement".

I don't believe that the homebrew needs to be heat cured. It acts a bit like normal mortar as in....it will start to harden in 24hrs, but won't start the curing process until after the 7th day of using. Make sure you 100% fill the whole of the void between each brick. Make yourself some folding wedges to support your arch former for ease of releasing the arch former. You can remove the former between 24 and 48 hrs after the arch has been laid. This makes any cleaning of the underside easier......just go steady and loosen the folding wedges so the arch former drops away from the underside of the brick.

Not sure which type of thermolite block is best....I've used 2 types....and both work fine.

Don't worry about asking too many questions ...if in doubt...shout out. Sometimes the forum goes quiet so if you don't get an answer in 1 to 2 days.....just reply to yourself with ...."bump"

Terry

[pete123]

I considered using these:-
www.hhcelcon.co.uk/documents/52344/7e1120df-e931-4a54-bafa-a06bd4d4f772
because they say they have a better "insulation factor" for want of a better word. I think the word they use is "conductivity".

These are the standard ones:-
www.hhcelcon.co.uk/documents/52344/45af5a2d-e673-4d60-b074-bdacd0d74b0f

In the end I used Calcium Silicate instead - Only because it was getting too high & I needed to keep it down a bit.