It’s a good time to summarize what we learnt from our cob building project, while it’s still fresh in my mind. Overall, it went pretty well; but there are a couple of things I’d do differently next time around.
- Give yourself plenty of time – and make sure you’ll be able to complete the project during a mostly dry part of the year. We had to take a long break during the winter wet season, which was quite frustrating.
- Start sourcing material early; line up backup sources.
- Mixing cob takes time and effort – it’s great to have some help with this. Sometimes multiple small batches can be quicker to mix than one giant batch.
- Make some test bricks to determine the correct soil/clay/sand ratio. Pure bagged clay requires much more sand than clay-rich soil. You want minimal cracking while still achieving good strength.
- Perlite and empty bottles make for an excellent insulation layer. It also seems quite stable when mixed with clay slip – a test hole showed no settling of perlite after 6 months.
- Fully dry and fire your thermal dome, before building the insulating layers. This allows you to patch any developing cracks while they’re easily accessible. You may need to repeat this process a few times.
- Incorporate some type of fiber in the cob mixture for the outer shell for reinforcement and crack resistance. We found that a simple clay/sand mixture inevitably cracks, no matter how high the percentage of sand is. Fibers like straw, horsehair, coco coir, etc. help prevent that.
- Water Glass is very useful for sealing the interior of oven chamber (to prevent sand from falling into your food) and patching up small cracks. It’s easy to apply and hardens to a tough glass-like (though brittle) consistency. It’s not a good choice for large cracks though.
- Any type of dripping, falling, or splashing water will quickly erode cob. Make sure you have planned for adequate weather protection; and your design incorporates a water-resistant surface treatment. Lime wash is surprisingly tough and really easy to apply.
- Be aware that the chimney gases are super hot. Unlike a single chamber design, you’ll see actual flames shooting out of the top of a 3’ chimney. Get a high-temperature insulated chimney stack. Type B gas vent pipes won’t do the job.
- Have a plan for securing your chimney from tipping under wind pressure. Cob doesn’t have much tensile strength, so it can easily crack around the base of the chimney.