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.