Second build abroad

[blueseamonkey]

Hello everyone. I'm just beginning my second cob oven build in the Philippines, though I am from Sheffield originally. My first oven was built at a house I was renting and had to leave several years ago.
The new one I am building with some of the lessons learnt from that first build.
It will be larger (60cm internal diameter), lower (30cm height) and better insulated. It will be a dome with an arched doorway (35cm wide, 19cm high) extending about 25cm forwards from the dome edge.
The materials I am using might not be familiar to all of you, but the end result should be pretty close to the cob mix you use in Britain. I am using termite mound clay, mixed with volcanic sand, coconut husk fibres for strength and rice hulls for insulation.
It is being built under a steel roof in an outside kitchen, mounted on a reinforced concrete platform.
The oven base I moulded a week ago from a 15cm thick cob mix, the sand and paper form will be added tomorrow, with the cob mixture over the top.
The one thing I am a little unsure about is whether or not to use a chimney at the front. With my last oven, I used to cook with the fire still burning at the back and sides and was planning to possibly do the same for this one. The old oven had no chimney, which meant I could not use a door without the fire going out. I was thinking a chimney would allow me to keep a low fire burning with the door closed, and keep some of the smoke out of my kitchen. But I know that adding a chimney can make the chamber lose heat faster once the fire goes out. What are people's thoughts on chimneys, are they useful or not?

[blueseamonkey]

A couple of pictures of the base so far

[downunderdave]

Oct 7, 2022 13:53:04 GMT blueseamonkey said:

Hello everyone. I'm just beginning my second cob oven build in the Philippines, though I am from Sheffield originally. My first oven was built at a house I was renting and had to leave several years ago.
The new one I am building with some of the lessons learnt from that first build.
It will be larger (60cm internal diameter), lower (30cm height) and better insulated. It will be a dome with an arched doorway (35cm wide, 19cm high) extending about 25cm forwards from the dome edge.
The materials I am using might not be familiar to all of you, but the end result should be pretty close to the cob mix you use in Britain. I am using termite mound clay, mixed with volcanic sand, coconut husk fibres for strength and rice hulls for insulation.
It is being built under a steel roof in an outside kitchen, mounted on a reinforced concrete platform.
The oven base I moulded a week ago from a 15cm thick cob mix, the sand and paper form will be added tomorrow, with the cob mixture over the top.
The one thing I am a little unsure about is whether or not to use a chimney at the front. With my last oven, I used to cook with the fire still burning at the back and sides and was planning to possibly do the same for this one. The old oven had no chimney, which meant I could not use a door without the fire going out. I was thinking a chimney would allow me to keep a low fire burning with the door closed, and keep some of the smoke out of my kitchen. But I know that adding a chimney can make the chamber lose heat faster once the fire goes out. What are people's thoughts on chimneys, are they useful or not?

I've always been interested in using anthill as a material for oven building, as well as an interesting experiment with my six year old grandson, so recently I used some to process it to establish the portions of clay to sand. The termites use clay and sand and their own saliva to bind the materials together, to build their homes, so do much of the processing of the materials used for us. I was really surprised to discover hardly any evidence of clay when the dry material was broken up and slaked in water. I'd expected, as you do with a cob mix from the ground, that the material would form a layer of the larger, heavier material (sand to sit on the bottom with a layer of clay on top followed by clear water sitting over that. There was virtually no clay in the mix. On pouring off the clear water from the surface and drying the heavier material it was simply fine processed sand  with very even grain size not exceeding 0.75mm when dry sieved. On listening to an interesting program re "glue" produced by slugs and frogs to be synthesised for use in healing wounds without stitches, the researcher stated that once the "glue has dried it has lost its adhesive properties if ground up again. I believe that this is exactly what happens with the termite mix. Perhaps the proportion of clay to sand particles in different termite mounds varies considerably  from place to place, but I suspect not. This would suggest that some powdered clay should be used as a binder since the termite saliva is inactive for re-use. I think you should do some tests on the material you plan to use. I for one would be most interested in your results.

[blueseamonkey]

The termites round here must do something very different to your local ants as they build their nest then. The 6 sacks of reddish termite mound my brother-in-law kindly dug up for me are largely clay, with just a few plant roots and a little sand mixed in. People here use it to make terracotta type pottery. I have had to mix it: 2 parts termite mound, 2 parts sand, 1 part coconut husk for the thermal mass layer, then the same mixture but with 2 parts rice hulls added for the insulation layer. This link is where I got the idea of using the termite mound from, although my oven design is very different to theirs.
selfsufficienthomelife.com/building-a-wood-fired-termite-cob-pizza-oven-with-rocket-stove-base/

[blueseamonkey]

A few more pictures from today's work. Sand form built, covered in manila paper, then the thermal mass layer of cob added.

[downunderdave]

Oct 8, 2022 13:15:35 GMT blueseamonkey said:

The termites round here must do something very different to your local ants as they build their nest then. The 6 sacks of reddish termite mound my brother-in-law kindly dug up for me are largely clay, with just a few plant roots and a little sand mixed in. People here use it to make terracotta type pottery. I have had to mix it: 2 parts termite mound, 2 parts sand, 1 part coconut husk for the thermal mass layer, then the same mixture but with 2 parts rice hulls added for the insulation layer. This link is where I got the idea of using the termite mound from, although my oven design is very different to theirs.
selfsufficienthomelife.com/building-a-wood-fired-termite-cob-pizza-oven-with-rocket-stove-base/

Thanks, that is most interesting. I will have to repeat my tests by taking some more samples to see if my results are repeated.
Remember that for a good cob mix the clay to sand proprortion should be around 3:1 by volume. This is very easy to measure by mixing a sample with water, pour all of the mix into a glass jar, leaving it for 24 hrs then measuring the height of the layers of sand and clay.

If the proportion of clay is too high the shrinkage will be excessive, leading to severe shrinkage cracking, If the clay portion is too low the mix won't hold together. Using coconut husks for reinforcing and rice hulls for insulation is a great idea. For your insulation layer the insulation value of the mix is higher as the insulating aggregate (rice hulls) is increased. You only need enough clay to hold it together, because you can render it over with the thermal layer mix, after it has dried. You may be able to go as low as 10:1 Look at the attached table which explains the strength to insulating value relationship.

Your pics certainly show that the mix looks pretty workable, good luck.