Lime Plastering a Cob Oven

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Part Four shows how to apply a lime plaster to your cob oven. Lime plaster is made with 3 parts mason's sand and one part lime putty. I use Mississippi vertical hydrate lime, purchased as a powder, and I let it soak in water for a minimum of 6 weeks, until it is the consistency of sour cream. The longer it sits in water, the creamier and nicer to work with it becomes. LIME IS CAUSTIC! So be sure to wear protective clothing and gloves. Never touch it directly! My lime plaster bible is the book "Building with Lime" by Stafford Holmes and Michael Wingate.
You can also keep your oven dry with a roof or with clay plaster. Clay plaster will perform better in weather if it has wheat paste or manure added to it. Either way, plan to spend an hour or two repairing the clay plaster once a year.

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30 май 2012

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Комментарии : 44

@Suburbanstoneage 4 года назад

This video was very helpful! Emphasizing the difference between the lime coating drying vs. Curing really it helped me visualize and understand how the process worked. Thank you!

@barbgreene8497 11 месяцев назад

Thank you for this excellent visual as I’m at this same stage in completing my cob oven. ❤️

@truthman10200 12 лет назад

Been waiting for this video. Thanks to you and glad youtube emailed me telling me it was available!

@kundikishore 4 года назад

Me too same here 👍

@patrickharper9297 7 лет назад

Nice build

@buildnaturally 11 лет назад

I would first make the oven smaller. For one, it will heat up faster, which would be good at a festival. But also, a smaller oven will weigh less. I would also make sure your insulating layer is made of straw that is just barely coated in clay slip (until it just looks like straw covered in chocolate milk). Minimizing the clay means minimized weight. I would also think about the transportation...shock absorption to minimize vibration & something to keep the oven from sliding around.

@buildnaturally 12 лет назад

You are quite welcome! Thanks for watching!!

@greyes701 11 лет назад

Great videos, beautiful work, and thanks for sharing! I was just wondering, I and a friend are interested in building a cob oven. My friend is interested in taking it to festivals and/or markets. Basically, I was wondering about your input as far as a change from the build seen here to where the oven would be transportable, on a flat-bed trailer, maybe, I guess.

@txturbo930 10 лет назад

Great videos. Very Helpful. Thank you. I'm curious why no chimney? Is it not needed with these demensions, or does it heat up faster and hold heat better with no chimney?

@nicovanlent6927 11 лет назад

Thanks for the very instructive videos, they proofed very helpfull. I just finished my cob oven. Now it is drying (under a temporary roof, it is raining again here in The Netherlands). When it is dry i'll plaster it. Can I use a plaster on a gypsum basis? Will that allow the oven to breath?

@dariocanarte6842 4 года назад

Hi! Very helpful, thanks. I have a gorgeous cob oven an I was planning to put a roof over it but I want to know more about the plaster method first. Can I use limewash (the one that you get in lowes)? If so, how can do it? What are the steps to follow to make that lime putty? Is there any method that requires less time, I may be leaving for the military soon and I want to do it before I leave. Thank you and thanks for sharing!

@buildnaturally 11 лет назад

I never recommend cement stucco/render for any application. It is brittle and not breathable, so will crack, let moisture in, erode the surface of your clay/cob, and then fall off.

@glvedio 11 лет назад

wowww..blue rubber glove...:-) :-) :-) TQ so much..nice rubebr glove video

@buildnaturally 12 лет назад

Lime in this case refers to hydrated lime putty, which is calcium hydroxide soaked in water for a number of weeks. (Slaking in this case refers to soaking the powdered lime in water to re-hydrate it.) The powdered lime is purchased (see above for the type I use). You then need a big bucket or a small pit (with a lid!) to soak the lime in. I added some additional information about the lime and the plaster recipe, as well as the book I use as my lime bible in the notes just below the video.

@georgeprice3492 8 месяцев назад

Thank you for your very informative and well-produced videos. I also am at this stage on my cob oven. We live in the northern Rocky mountains and used clay, sand and rocks available in the land of our farm and straw that we had already on hand. I plan on using a fine-screened sand/clay plaster. My question is can we apply the plaster in freezing weather or should we wait until Spring?

@valtersantos2207 11 лет назад

i loved the videos. i'm going to build one soon. in my country the weather is very dry and about 35 Celsius. for this condition how long should i let it dry before i can fire it up?

@That-Wanderer 6 лет назад

Hi, I know I'm quite late to this, but how long will the lime plaster last ? How long is it waterproof? Will I need to go back over it over a certain amount of time

@Marialla. 12 лет назад

Thanks!

@gedhuffadine5796 2 года назад

How long will it last great job

@Marialla. 12 лет назад

I always wondered about cob plaster. Thanks for posting! At the risk of sounding painfully ignorant... what is lime, exactly? I mean, you can often get sand and clay and straw by digging around your building site, but where does lime come from? And how do you "slake" it, exactly?

@eogg25 9 лет назад

I was just wondering, as hot as these ovens get, doesn't the straw eventually break down and you would have a maze of worm like holes in the clay, making air pockets which is an insulator.

@buildnaturally 9 лет назад

eogg25 if you have straw on the interior, then yes, it will burn out eventually. However, the straw on the exterior should remain in tact, as long as you don't have large cracks on the interior of the oven (where the fire can get through to the straw layer). The insulating layer is mostly straw and very little clay, so if the straw all burned out, that layer would just collapse. But then you could always easily and quickly redo it. :)

@pamcolechadwell1302 10 лет назад

Hello great video's thank you very much for sharing. I have a question can you please help me? Do I just buy regular garden lime then what do I do with it to make the plaster? Thank you very much for any help...

@buildnaturally 10 лет назад

Hi Pamela, No you can't just use garden lime. It is not the correct chemical, and will just result in a dusty mess if you try to make plaster out of it. You need good quality, high calcium hydrated lime. Chemically it is called calcium hydroxide. You can read more here: www.buildnaturally.com/EDucate/Articles/Lime.htm

@pamcolechadwell1302 10 лет назад

Thank u very much

@AustinWilhite 6 лет назад

Can anyone speak to the differences in dolomitic vs high calcium hydrated lime for making lime putty?

@buildnaturally 6 лет назад

dolomitic lime has high magnesium in it and is usually process as "type s" lime through an autoclave process that renders the magnesium neutral (otherwise it goes through a carbonation process after your lime has already hardened and causes little pinholes to explode in the surface). High calcium hydrated lime is the purest form of lime...calcium hydroxide. High calcium lime is the best for strong, breathable, durable, consistent results.

@timothyberg999 6 лет назад

I noticed you waited a few months before applying the lime plaster. Can you do it more quickly or does the oven need to be completely dry and fired a few times?

@michaelstansell1001 5 лет назад

This is my very question :) Any answers or suggestions? Since one goal of lime render is to control absorption of water too quickly out of the render it is usually recommended to wet the cob before applying as seen in the video. However, can you also just apply the lime render to a "green" oven where the cob itself is not yet dry and therefore has high moisture content?

@reverendmississippi 11 лет назад

would cement rendering be ok

@Jim-K-Baker 4 года назад

How well dose this hold up will it eventually crack?

@pamcolechadwell1302 10 лет назад

one more question for ya, I live in east Tennessee near the city of Knoxville where can I get the right lime? or do I have to order the lime? Thank you

@buildnaturally 10 лет назад

I use a high calcium lime from Mississippi Lime and I soak it (rehydrate it) in water for at least 6 weeks. You can check their website for a local distributor. The product is sometimes called "food grade lime". But if you tell them you want "fresh high calcium, calcium hydroxide that was fired in a vertical kiln", then they should be able to direct you to the proper product.

@all_food5999 8 лет назад

what happens if I use a layer of leaves, mulch, to prevent water penetration? Will it work?

@buildnaturally 8 лет назад

+All_Food leaves and mulch would hold water against the surface, not cause water to shed off the surface more quickly...so no, I would definitely not recommend that

@17091ira0072 10 лет назад

we've built one of these in our garden to go with a cob summer house, and at the moment everything is lime whitewashed, but my family and I would like to add a splash of colour and designs, do you recommend any particular paint types?

@buildnaturally 10 лет назад

you can add pigment to your next coat of white wash :-)

@17091ira0072 10 лет назад

buildnaturally thanks :) we'll try that come summer

@buildnaturally 11 лет назад

Gypsum is fine over clay in terms of breathability, but it isn't very weatherproof...it will erode as it rains. In that case you might actually be better off just using clay plaster to finish and then just patching the plaster each year.

@buildnaturally 11 лет назад

Wow, I'm not sure I've ever timed it. I would try to get it to dry in about 3 weeks...in your case, that might mean slowing it down by covering it with something...plastic or damp burlap or something. If it's drying quickly and not cracking, then you can probably fire it up in a week or so. Some people fire the oven right away, but fast-drying clay cracks more, so that is your risk if you use it too soon. But then again,.cracks can always be patched!

@madreelish 8 лет назад

خوش فرن

@terrabugz 10 лет назад

We should also know that lime is not a fool proof protective coating ...right? Lime over earthen materials STILL needs protection especially benches and flat exposed surfaces. Lime over lime based base materials is a bit more durable. If lime could be integrated in the cob mix outer layers...gradually this might improve the durability? Traditionally cob walls are vertical surfaces...where water runs off quickly. Regular maintainence is essential and to be expected. mudwasp

@buildnaturally 10 лет назад

For sure! Any flat surface will eventually let water through, and if that water freezes inside the cob, it kindof explodes the cob apart (because ice is bigger than water. My favorite waterproofing is actually a clay plaster with manure in it. You need to reapply periodically, but it's very breathable, while also quite water-repellent. Also, you can just build a roof over the whole thing to protect it. :)