Tuesday, 25 March 2008

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.

Saturday, 22 March 2008

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...

Friday, 21 March 2008

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.

Monday, 3 March 2008

Drying the dome

I left the clay dome overnight, then scooped out all the sand and broken bricks the next day. Although the broken bricks were only there because I'd run out of sand, it made the job quicker as they were easy to remove. I put my 100W light bulb in the oven, and left it for 5 days to dry. The bulb kept the temperature above freezing, but the clay never really got warm. The newspaper soon dried though, and I peeled it off after a few days. The next few pictures were all taken after 5 days drying with the 100W bulb - about 5-8 C temperature outside

You can see the lumps of clay that I used to build the dome on the inside face - it was impossible to get this smooth, because I can't reach in that far and the oven entrance is only 13"...

Notice that as the dome has dried it's pulled the arch and cracked it. Around the base of the oven dome I could see the whole dome was shrinking inwards, but staying as a single intact piece. The whole dome could contract over the flat firebrick base as it dried. The cracks between the base of the dome and bricks show about 1/4" movement after 5 days.
The cracking was worst around the outisde of the brick arch. Stupid of me - because the arch couldn't shrink as the clay dried, it cracked instead. Oh well, I'll fill these in later.

Not known for my patience, after 5 days drying I decided to substitute the 100W bulb for a 500W halogen work lamp. After another 5 days the dome was very dry, and felt warm to the touch all the way round (even at the base, away from the light). The clay was a very different, almost white colour now, with no darker regions (although the photos don't show this well). The arch had cracked a lot now with the extra drying, although all the bricks were still very firmly fixed, and I couldn't extract any with a lot of wiggling about.

The cracks around the arch (there were three) had opened up to about 1/2" on the outside, although hardly showed at all on the inside. A couple of small surface cracks had opened up near them - but no sign of these on the inside at all.

The base had probably moved about 1/2" inwards all round - so that's about 1" shrinkage in total across the dome. The dome was intact apart from the cracks around the arch - no other cracks anywhere else. I'm pretty pleased with this.

I filled all the cracks with the spare clay/grog mix that I'd bagged up from when I made the dome - a very quick and easy repair that soon left things looking good again. I wet the dome around the cracks before doing the repairs to make sure they all stick. The repair's obvious right now - you can see the darker fresh clay.

I'm pleased with that - far less cracking than I had dreaded. The 500W lamp has gone back in, and I'll add some insulation to fill the chimney gaps in the next few days.