East Lothian Build

[MikeyC]

Rendering the Dome:

Once the vermicrete had set I applied a scratch coat of cement render on top, 6-8mm thick, at 4:1 mix ratio (4 parts sand, 1 cement), with a water repellent sealer mixed through. A lime cement render was then applied on top once set, again 6-8mm thick, no water repellent sealer this time to allow the top coat to 'breath'. The top coat, once set, was painted white using a textured brilliant white masonry paint - I applied three coats.

[downunderdave]

Hi Mikey,

You don’t provide a time line or mention any drying fires, but if you applied a 50 mm layer of vermicrete over the blanket then you have added approx a third of its volume in water. Less than half of this water will be used up in the hydration process as the cement cures, leaving a large quantity of free water behind. If you have then added a layer of render containing a waterproofing agent over the vermicrete without allowing it to dry and maintaining drying fires to eliminate the water, you have effectively locked in the water in the vermicrete layer. I have learned from bitter experience (from two ovens I built early on) that failure to eliminate the vermicrete layer water can result in cracking the outer shell as the steam expands the vermicrete layer. I find it far better to do the drying fires after the vermicrete has set and dried (around a week) then when all the water has been eliminated, proceed to the outer shell. A vent from the insulation layer to the atmosphere is also useful to relieve this steam pressure rather like the hole in a saucepan lid.

[MikeyC]

Nov 18, 2017 4:13:02 GMT downunderdave said:

Hi Mikey,

You don’t provide a time line or mention any drying fires, but if you applied a 50 mm layer of vermicrete over the blanket then you have added approx a third of its volume in water. Less than half of this water will be used up in the hydration process as the cement cures, leaving a large quantity of free water behind. If you have then added a layer of render containing a waterproofing agent over the vermicrete without allowing it to dry and maintaining drying fires to eliminate the water, you have effectively locked in the water in the vermicrete layer. I have learned from bitter experience (from two ovens I built early on) that failure to eliminate the vermicrete layer water can result in cracking the outer shell as the steam expands the vermicrete layer. I find it far better to do the drying fires after the vermicrete has set and dried (around a week) then when all the water has been eliminated, proceed to the outer shell. A vent from the insulation layer to the atmosphere is also useful to relieve this steam pressure rather like the hole in a saucepan lid.

Hi Dave,

Apologies, I haven't documented the drying fires which took place after various stages. The first of a series of small fires (roughly 3 fires, one every 2nd day) was started after the dome capping was completed. I gradually built up each fire temperature from 75-100 degrees, 200-250 degrees, and then the last fire getting to 350-400 degrees in temp, enough to cook my first pizza! This was after allowing the brick dome a week (5 days) to set. We also held a pizza party for my son's 3rd birthday on the following week after the series of first fires, which was a real success!

I built up another fire post the vermicrete stage; again gradually building the oven's temperature up over the course of the day to full temp 400 degrees. The fire was started two weekends after completing the vermicrete. I tended to work at weekends on the oven inbetween work. The vermicrete had been given approx 12 days to cure under cover (gazebo roof). The weather during this period in August was relatively warm, with spells of sunny weather which assisted the drying process, and allowed; like you say, any excess water within the vermicrete to evaporate off before moving on to applying the waterproof render. You'll notice in the images above the blackening around the top of oven entrance archway caused by firing the oven up. 

I'll do a series of fire photos in the next post.

[downunderdave]

Ok, you should be ok then. If there is still moisture in the insulation layer then the insulation won’t work too well because water is conductive. This means that if you’ve had the fire going for an hour the outside feels hot. When the insulation is dry the outside should feel no hotter than cosy warm.

[MikeyC]

Cooking up!:

As touched on above, and well pointed out DownunderDave I had to do a series of drying fires at various stages; post brick dome completion, and post vermicrete completion. The last of the drying fires on each occasion, when oven temperatures were at there hottest allowed the oppertunity of doing some Cooking! Below are a series of photos. My oven's first Pizza! Also, cooking sticky marinated roasted ribs, accompanied by my first loaf of home made bread, a dish of roasted sweet potatoes with rosemary, and some garden grown roasted Kale. The Scottish weather even treated us to a spell of lovely sunny weather - fantastic!

[nky84]

Amazing build! Proper job, puts my cobbled together build to shame!

[MikeyC]

Base Plinth Finish:

It took me quite a bit of time to decide how to finish the oven base and dome. The original intention was to render both and paint the base white; both inside and out and use pigmented render on the dome; a khaki/olive green colour. The coloured render system was proving difficult and expensive to source (I was looking at Sto render products and I wasn't really sure how much quantitues I needed, and exactly which products I required). It proved far easier to mix my own render and there were readily available exterior masonry paints - white the most economic!

I went for a 'brilliant white' paint finish around the dome exterior (it's quite bright. I have a slight worry that it will attract quite a few dirty marks!). I opted instead to source tiles for the base. My thinking was that a tile surface would be easier to clean and be less prone to kids 'decorating' it - wipable, hard surface. I went back and forward on whether to incorporate a more traditional stone tile (grey slate, a local sandstone or even a granite) or a more modern uniform in tone finish, and also whether to use larger sized tiles as these would be less Labour intensive. In the end I chose a white, 200 x 100mm glazed bevelled tile (typically used in old Victorian bathrooms), and laid these in a linear, non staggered pattern as I was keen to express bands of thin horizontal lines across the base. These tiles were also remarkably cheaper than the other variations mentioned above. I intend to use white grout around the tiles.

Below are some images of where I have gotten too. I've pretty much stopped work for the winter, as temperatures have dipped these past weeks. It's getting to below the 4 degree minimum temperature mark recommended for outdoor tiling!

[MikeyC]

Sealing My Oven's Terracotta Tiles:

As picked up on briefly above I applied several coats of Linseed Oil, diluted partially with white spirit (as per the instructions on the side of the can) as a sealant to the terracotta tiles located around the oven's base and the plinth slab. Due to the porous nature of the tiles, I had read that this was the best and most cost effective way to seal them from both the wet weather and from staining (cooking grease, sauces, etc). Prior to sealing, I cleaned the tiles as best I could using a cloth with white spirit (you'll notice some vermicrete and render staining on some of the tiles around the base of the oven perimeter which I cleaned up as best as I could. In hindsight I should have protected the tiles better during these stages). I picked up a litre tin can of Linseed Oil for around £5.50 online, with postage on top. I applied 3-4 coats of it, with approx half an hour or so between coats (the tiles I found soaked/dried the Linseed Oil up quickly). Below is a series of photos. Keep an eye out for the chicken and cat foot prints found inlaid into some of the tiles! (they're hand made). Additional character!

[oblertone]

Home made tiles, nice touch !!!

[albacore]

Impressively thin grout lines in your terracotta!

[MikeyC]

Dec 15, 2017 19:25:18 GMT oblertone said:

Home made tiles, nice touch !!!

Haha, sorry that should read 'handmade'! I wish I had the skills and access to a kiln to have a go at doing these myself! I bought them from a supplier online and had them delivered to me - less of a romantic tale! Thanks guys.

[MikeyC]

Last week or two has seen quite a few of the tiles fall off my oven base. I didn't quite manage to finish off the tiling work before the cold weather set in, and I unfortunately didn't manage to grout in between the tiles I did get done. I think water may have gotten inbetween the tile joints during periods of rainfall, which has then frozen during as temperatures dropped during December, prising the tiles away.  

Earlier on in my build thread I mentioned that I'd chosen not to lay a damp proof membrane below the slab, and that I would touch on this later on in thread as I expereinced using the oven over time. I am wondering now whether the lower block walls of the oven base may be prone to rising damp which might effect the tile mortar (dampness behind, which again may be open to freezing temps), causing the tiles to come away? 

Thankfully the tiles haven't broken as they have faen off. I've set these aside for now, and will look to stick these on again once the weather gets milder. For info, I used a standard cement mortar (without lime) to stick these on. I'm wondering if I should use a water proof mortar next time? 

[MikeyC]

I haven't yet fired my oven up over winter as unsure of best practice in low temperatures? I'd hoped to try today.  Temperature here in East Lothian is currently -3 degrees, rising to 1 degree around 11am/midday, it's pretty cold! Is there a risk to the oven's rendered dome by doing so - expansion and contraction through extreme temperature variations? What are other people's experience?

[albacore]

Regarding the tiles, I can think of a couple of options:

1) Use a waterproof tile adhesive, buttering the back of the tiles with adhesive as well. Don't use tub adhesive - powder only.

2) Use sand and cement again, but add some SBR and butter the backs of the tiles with SBR/cement mix. I don't think lack of lime is an issue here, incidentally.


Lance

[MikeyC]

Jan 8, 2018 9:57:46 GMT albacore said:

Regarding the tiles, I can think of a couple of options:

1) Use a waterproof tile adhesive, buttering the back of the tiles with adhesive as well. Don't use tub adhesive - powder only.

2) Use sand and cement again, but add some SBR and butter the backs of the tiles with SBR/cement mix. I don't think lack of lime is an issue here, incidentally.


Lance

Thanks Lance. Appreciate the advice. Leaning towards option 1.