Flint wall finished

I was cursing the idea of using the flints after three days of hard work, but now I'm really pleased with it. It's a very cheap and appropriate way to finish the oven. These days it's an expensive way to build (due to the labour costs), but around here it's a method that's been used for at least the last couple of hundred years, as the building material was free.

I got a bit nervy after the first foot or so, as I was concerned about the mortar bonding tightly to the thermalite blocks. To help the bond I screwed 2" screws into the blocks every 6", leaving about 1" sticking out that I embedded into the mortar. All finished on the masonry work now - just need to leave this to cure, then clean everything up.
Looks good in the summerhouse, especially now I've finished the front and side boarding off. Just need to do the back now, then lay the floor.

Using found flints

I needed to find a cheap way to cover my patchwork walls on the front of the oven. I had considered rendering, but the walls were too uneven. I also considered using bricks, but I was running low on them and didn't want to buy any more. Then I remembered that many of the cottages round here have flint block walls, as the chalky soil round here is full of loads of flints.

Two hours later I had this large pile of flints, just from my own garden. I cleaned them all up the lazy way by putting them in my cement mixer with a bucket of gravel, filling it with water, and running it for 10 minutes. It also cleaned the mixer brilliantly, and I'm sure the neighbours will have enjoyed the din it made. I then spent a happy hour bashing them all in half with a hammer. Goggles are a must for this, along with thick gloves.


I fitted two switches to the front of the oven, one for a light and the other for the thermometer power switch. I'll wire these up later. Then it was time for the flint jigsaw - this took a long time, and was very fiddly. This was about three days work, using a lime and sharp sand mortar. Here I am about half a day into it. The radio was essential to keep me entertained.

I like this next picture as it shows what a con artist I am on this build. The front half is all nice rockery stone and flints, the back is waste concrete and render. No point making the back look nice as nobody will see it!

Finishing the insulation

The next few posts will be work that I've done over the last week - I've been so busy building I've not had time to post anything on the blog. I've also added a box on the right hand side of the blog with all the costs in, after a request to know how much this had all cost. I've only described the cost of the oven - we were going to build the summerhouse anyway, so I only have to justify the cost of the oven to the family...

The first job was to finish the walls on the front using the thermalite blocks. The front is a real patchwork quilt, as I was using damaged and broken blocks. I tipped all my vermiculite into this hollow cylinder. I used 6 bags, so the oven has 6" of vermiculite at the bottom, and about 18" of vermiculite around it at the top. Plus the 4" of insulating blocks. The top is open at the moment, to help with drying it (it's covered by the roof anyhow).

The back of the oven won't ever be seen, so I decided to have a go at rendering it. I was really dreading this, as I've tried plastering before and made a dreadful job of it. This time I used a 4 parts grit sand, 1 part cement, 1 part hydrated lime mix (first time I've ever used lime, and got a lovely cement burn on my hands because my rubber gloves leaked). It went on like peanut butter, and stuck beautifully. The lime makes an enormous difference to the 'stickiness' of the mix - it also helped that I'd coated the blocks with a dilute PVA solution too. I was so pleased with this that I considered using it on the front of the oven too. I still can't believe I did this - it was much easier than I had had dreaded.

Insulation, insulation, insulation...

I've decided to build the rest of the oven as a cylinder - it's the easiest option for me, and means I can get lots of insulation in. I'm using more of the Thermalite blocks - many of these I've bought for cheap because they were damaged. They are fragile, but helpfully very easy to cut to shape. The top part of the oven will look a bit like a patchwork quilt though until I finish the surface off.

This will give me a nice insulating outer surface (4" thick), and I'll fill the space with loose fill vermiculite. This is a cheap and easy insulating option - you can see the bags of vermiculite stacked up on the left hand side below, ready to go in.

This gives me a good 6" gap between the dome and the thermalite blocks at the bottom, and much more above, so this should keep the heat in well.

I'm tipping the loose vermiculite into this gap - this will eventually fill up all this space. I've filled all the cracks in the dome with powdered fireclay, and have coated the whole dome with ~1/4" of dry fireclay. You can just see this in the picture below. The idea is that this will sift into any cracks that form in future and fill them up - a good tip from Alan Scotts book, if I remember right.

First fire!

I'd planned this for the weekend, but since the weather was so windy and wet I saved it until today. I started a small fire using a couple of handfulls of wood offcuts and paper, and kept it going in the dome for about an hour and a half before leaving the embers to cool. Thanks to my new thermometer I could see that the temperature got up to about 220C at the inner surface of the dome, and about 180C an inch into the dome. The hearth brick never got much above 80C - the thermocouple in the hearth insulation never changed from its starting temperature.

The unsurprising part is that the dome cracked a little (early on, after about 30 min), but only hairline cracks that run all the way around the dome, and down the back of the dome. You can't see them from the inside, and they didn't leak any smoke. I don't think they'll be a problem.

This was a big step forwards - it works! I'm particularly pleased with the chimney - no smoke out the front at all - all went straight up. One happy builder.

Fitting the oven thermometer

Before I start burning stuff, I want to know what temperature my oven is. A pyrometer was way too expensive for me, so my cheap fix for this was a PID temperature controller - the sort of thing you'd usually use to control the temperature of a kiln. The one I bought was from www.auberins.com and cost me less than £20. I bought my thermocouples from there too - pretty good for under £4 each! I bought the ones rated to 1000C - hopefully should be plenty. They've been buried in a variety of parts of the oven - fortunately I took extensive notes:


Another reason to sort this out before the fires - I'll end up accidently burning the notes... I also picked up a rotary thermocouple selector off ebay for £5. I've no idea where you'd get one of these from apart from ebay, but it allows you to select 1 of 12 different thermocouples.

I mounted them both in a bit of scrap aluminium (offcuts from chequer plating that I took off a Landrover several years ago). After a bit of work with sandpaper and wire wool it looked presentable.


The rotary dial lets me select which thermocouple I look at - the power supply to the unit is fed in through a cable I buried in the masonry a long time ago. I can look at the inside of the oven (thermocouple number 1) ...

Only at 42C at the moment - the halogen light was off for most of the day when I was fitting all this. Or I can look at the outside of the oven (thermocouple number 6) ...

11C in the base of the oven walls - I guess this will be an average of the outside temperature over the last few weeks as there's a fair few tons of concrete there to heat up and cool down. This closes off the lower front brick hole, and will remove the irriation of people asking me how the pizzas fit in that tiny hole...

Filling the gaps

Thanks to being busy at work, and some pretty foul weather I've not done much on the oven recently. This meant that it's had even longer to dry out with the 500W halogen light - another 3 weeks almost. Two days ago I mixed up some vermiculite concrete (1 part Portland cement to 5 parts vermiculite), mixing it up well when dry and then slowly adding water until it was wet, but not runny. I packed all this around the dome and the chimney supports. I didn't fill the gaps around the entry arch - that would come later.


You can see how the bit of the dome that's had the halogen light aimed directly at it has turned almost white (the patch in the middle). This bit is too hot to touch right now. I'm glad to say that there haven't been any new cracks over the last three weeks, so I'm hoping this means it's all dry.

I left this to dry for a couple of days, then made up a mix to fill the gaps between the oven entry and the chimney support. These are uneven, anything between 1/2" and 4" depending on the area, and I wanted to fill them with an insulating mix to stop heat from leaking out of the dome. The mix I settled on was 5 parts vermiculite, 2 parts dry fireclay, 1/2 part Portland cement and 1 part sodium silicate solution (all by volume). The addition of the sodium silicate and fireclay made a mix that was very sticky and could be packed together even when fairly dry, and could be moulded very easily. The addition of the cement meant that it started to set after about an hour, and hopefully shouldn't shrink and crack. It was fairly hard after 2 hours.

I used this mix to pack into the gaps, smoothed it all off, then washed it with a wet sponge to get rid of the excess.

This worked better than I'd hoped, and ended up being quite a neat job. Notice that I've filled the gaps between the firebricks and red bricks with the same mix too.




I filled all the sharp corners, and made things as smooth as possible to help the smoke go upwards. The gap between the two black bits of angle iron is the chimney. In the chimney itself I used the insulation mix to mould a nice smooth transition between the rectangular brick chimney and the metal chimney pipe:


This was a very easy way of filling the gaps, and an easy way to get a smooth transition between the brick chimney and the inner clay dome. It really smartens up the oven entrance - just need to wait for this to dry now.