|
Post by gracie on Apr 25, 2016 19:29:22 GMT
Would love to help but I don't know! Followed your build with interest, then forgot it and made the same mistake with taking my sand out too soon! Doh !
|
|
|
Post by lovepizza on Apr 25, 2016 21:18:53 GMT
OMG! All I can say is, I feel your pain, the good news is, you will get to the same stage quicker next time round if you end up starting again.
I Literally got so excited the first time, I was digging the sand out within an hour or so....sooooo impatient!
I left my sand in overnight under tarp In the end but that did not dry it effectively. It was lunchtime next day before I excavated. After 6 hours uncovered in the sun it had created a hard "crust" but the time it takes is dependant on the weather I guess.
Keep the faith, you will get there.
I am getting to the end of my build and this bit was the most fun, Clay is an absolutely wonderful, natural and forgiving material.... This was the first time I have ever used it and what I learnt is that you can't rush it!
Stuart
|
|
|
Post by lovepizza on May 1, 2016 16:37:44 GMT
Ok, for those following this thread, an update. Got round to fitting the chimney. It was an old terracotta flowerpot that I had lying around in the garden. It's a very heavy one but I think it may be a little soaked so will need to go easy with the firings. I lined it with a stainless steel flue liner and insulated it with dry vermiculite and sealed the bottom with fire grog. I also got round to doing the Vermicrete insulation (10:1 ratio, 3parts water). Used cardboard as shuttering then built up over the top (thanks for the Idea Zelige). It is about 100 MM at the sides and about 150mm on top (as much to make a nice shape than for insulation, but every little helps. Below is with the shuttering still on. And below with it off!
Will now wait a week before small curing fires and maybe some tidying up of the Vermicrete.
|
|
|
Post by cobblerdave on May 5, 2016 8:09:08 GMT
G'day That's going to be interesting. The clay will dry and shrink as it losses it water content which I'm guessing could be as high as 10 per cent. The cement/ pearlite will not as its chemically "set" I'm expecting cracks. Sorry usually it's cob insulation so the 2 are comparable. The only clay oven I've seen with cement outer layer had a blanket insulation layer to account for this. Regards dave
|
|
|
Post by lovepizza on May 5, 2016 18:36:30 GMT
Hi Dave. Yes, I had considered the compatability issue. In fairness, the clay inner should be fairly dry as it had 4 full firings and crack repaired before I insulated. What exactly can I expect cracks in, the inner clay or the vermicrete insulation?
|
|
|
Post by downunderdave on May 5, 2016 19:05:50 GMT
Nearly all the shrinkage with clay occurs between the plastic and leather hard state, so your cob dome shouldn't shrink any more. Your problem is now to eliminate the excess water from the vermicrete. Although it may appear dry on the surface it will still be wet deeper in. When firing you will be able to feel the clammy moisture coming out. Try throwing some plastic sheet over the oven when firing and you'll notice water condensing against the plastic. When firing the oven, if you notice visible steam then you are going at it too hard, back off, or the vermicrete is likely to crack. It will take lots of fires to dry a 100 mm thick layer, but you can use the heat to cook stuff, just be content with roasting and baking and forget pizza until the thing is properly dry.
|
|
|
Post by lovepizza on May 5, 2016 19:31:23 GMT
Thanks for your input Dave,
When you say lots of fires, more than 10?
Stuart
|
|
|
Post by cobblerdave on May 5, 2016 22:09:29 GMT
Nearly all the shrinkage with clay occurs between the plastic and leather hard state, so your cob dome shouldn't shrink any more. Your problem is now to eliminate the excess water from the vermicrete. Although it may appear dry on the surface it will still be wet deeper in. When firing you will be able to feel the clammy moisture coming out. Try throwing some plastic sheet over the oven when firing and you'll notice water condensing against the plastic. When firing the oven, if you notice visible steam then you are going at it too hard, back off, or the vermicrete is likely to crack. It will take lots of fires to dry a 100 mm thick layer, but you can use the heat to cook stuff, just be content with roasting and baking and forget pizza until the thing is properly dry. G'day thanks dave I'm relieved to here that! Stuart....Daves a arts teacher so he is the expert on clay! He also makes his own line of cast ovens in Nth Queensland Australia. regards Dave
|
|
|
Post by lovepizza on May 6, 2016 6:16:12 GMT
Thanks Dave(s),
To be honest, I am still a little concerned about the clay expanding when it get to pizza temperatures and it having nowhere to go but hoping that a few light firings will be enough to cause a little expansion in the clay and some "give" in the vermicrete while it is not fully cured. Also, hoping that the 10:1 ratio will help to manage minor movement..... Thats the theory anyway, will let you know the outcome!
|
|
|
Post by oblertone on May 6, 2016 7:31:08 GMT
Experimentation is what drives us forward, keep on firing low and slow, despite the temptation !
|
|
|
Post by downunderdave on May 6, 2016 9:51:54 GMT
Thanks Dave(s), To be honest, I am still a little concerned about the clay expanding when it get to pizza temperatures and it having nowhere to go but hoping that a few light firings will be enough to cause a little expansion in the clay and some "give" in the vermicrete while it is not fully cured. Also, hoping that the 10:1 ratio will help to manage minor movement..... Thats the theory anyway, will let you know the outcome! The expansion of an unfired (clay that has not been sintered) cob body from ambient to 500 C is way less than the shrinkage the body undergoes from plastic to dry (typically around 10%). In addition the high proportion of sand in the body also reduces shrinkage. I doubt whether the cob would expand 0.5%, which is fairly negligible. I think you are over worrying this issue.
|
|
|
Post by downunderdave on May 6, 2016 9:58:15 GMT
Thanks for your input Dave, When you say lots of fires, more than 10? Stuart 100 mm is a long way for the water to travel. Both wind and sun also help to dry the thing. I have no idea about your weather conditions, but I'd be confident in saying that it will take you longer than you'd think to dry it out. The base of the dome will be the last to dry and will be characterised by a persistent black sooty ring around the inside of the base of the dome. This is just the time that the oven starts to fire quite efficiently and the very time that you need to take it slow or damage will result from uneven temperature and therefore uneven thermal expansion. If you have access to an infrared thermometer it will show a massive difference in temperature between the top and the base of the oven.
|
|
|
Post by lovepizza on May 6, 2016 14:53:38 GMT
Thanks Dave, that is good to know. Well, we are expecting 10 days of quite decent weather now so I can keep it uncovered . Hooray!! Gave it a decent firing today ( about 4 hours) of steady heat. I do have an infrared thermometer, the top of the oven got to about 250 c and the base of the dome about half that. The hearth got to 120 c also so I can see that there is a way to go as yet! I put the door on when I finished to see what the temp was inside the oven and it was around 150 c once the fire was out but it dropped to 120 c within half an hour. I assume that this will improve over time once the vermicrete is free of water and it is at its most efficient. Stuart p.s. Photo below of door front. Still got another piece of 12 MM ply to go on front and then screw the handles through both pieces of ply) Bolted to a thermolite block and fitted with some fire retardant rope for good seal.
|
|
|
Post by downunderdave on May 6, 2016 19:13:25 GMT
Hi Stuart, As predicted you have more than double the temp difference between the top and the bottom. Continue with the present regime of fires. You will find that as the top half of the. oven dries it will suddenly start firing quite efficiently. This is when you need to back off somewhat as the temp at the crown will increase rapidly and the soot will burn off while the base will still be cool. There will also be a persistent ring of black soot around the base of the dome, so proceed gently until it has gone.
|
|
|
Post by lovepizza on May 10, 2016 4:08:21 GMT
Another update.... Fired up again over weekend, hearth got to 350 - 400c after about 90 minutes. I wish now that i'd had some dough ready to test timing for test pizza! The vermicrete didnt change temperature on the surface (which i guess is the idea) but still a way to go as it didnt retain its heat with door on for too long. Door on at 275c with embers and it was 160c 3 hours later. It was still 65c the next morning. So, still a way to go but it looks like we will be ready for my wifes birthday pizza party on 28th May, fingers crossed for the weather. Render coat can wait until after that!
|
|