· Media > Published Articles > Alternative Building > Homemade Earthen Plaster

 

"The D Acres Cob Oven Project" by Joy Payton, Fall of 2002 and Summer of 2003

With a lot of much appreciated Help From:

Amber Wiggett
Ben Graham
Becky Bee
Josh Trought
and
Monika Chas

Contents

Information about D Acres

Chapter 1: Cob!!!

Chapter 2: Our Oven Design

Chapter 3: Deciding on a Site

Chapter 4: The Foundation

Chapter 5: Mixing and Building

Chapter 6: Plaster Plaster Plaster

Earthen Plaster Recipes

Chapter 7: What We're Doing Differently Our Second Season Building with Cob

Books and People

D-Acres Certified Organic Farm &

Evolving Self-Sustainable Homestead

D-Acres is an experimental center designed to provide a sustainable living arrangement for people interested and excited about living simply. Located at the edge of the White Mountain and Lakes District, it is a rural family enterprise with a broad mission directed toward "improved quality of life" through sustainable practices within an educational context. The organization and estate is owned and facilitated by the Trought family. Aunt Edith, our 93-year old matriarch, is a skilled artist and spirited elder who worked the land with her husband, and still resides on the property. The members of this small community explore skills relating to sustainable development, subsistence living, and small-scale organic farming. There is an emphasis here on community, both with the handful of workers on the property, and with local artisans, builders, and farmers who are involved in the center's work and social life.

Projects vary with the season, but all are related to the development and maintenance of our home and workplace. Assignments consider the skills and interests of the individual members and are agreed upon at the community meetings. The work-exchange program consists of 26 hours of work each week for room and board. Projects include (but are not limited to) organic farming, forestry, light construction, landscaping, and participation in the local farmer's market. There are opportunities to pursue masonry, mosaic and tile projects.

Evolving from very rustic living and working conditions, the Farm now has a modern support building with a state of the art kitchen, carpentry workshop, extensive research library, studio, yoga space, internet access, root cellar, laundry and hot showers! Alternative technologies include composting toilets, a greywater system, nongrid power production, solar showers, and cob (an earth-based building material) constructions such as outdoor wood fired ovens and cook top.

Accommodations consist of tree platform houses, raised tent platforms, and indoor lodging. Space in the main house is available for personal projects, and members have free range of the beautiful countryside property.

The land consists of approximately 150 acres of mixed hard/soft 3rd growth forest and 5 acres of meadows and gardens, which are continuously being improved upon for our use with various caretaking and farming practices. The farming ideology is still evolving through experimentation and research, with the goal to supply food and farm products at a sustainable rate. Numerous schools of thought are explored including permaculture, edible landscape, season extension, biodynamic farming, and farmscapes. Over 100 species of perennial plants have been introduced to the property, ranging from edible fruit and medicinal herbs to ornamental flowers. In addition, D'Acres cares for several animals, and holds a mutualistic view of husbandry. The animals participate in work activities that benefit them as well as the center.

The community and education elements of D'Acres hold its many operations together. With research and utilization of past and present technologies, the center continues to refine its efforts toward environmental compatibility, and improved quality of life.


CHAPTER ONE : Cob
Cob \käb\ n [ME cobbe leader] (15 C) 1: a male swan 2: dial Eng: a rounded mass, lump, or head 3: a crudely struck old Spanish coin of irregular shape 4: CORN COB 5: a stocky short-legged riding horse

Cob n. [prob. Fr. Cob] (1602) : a mixture that consists of unburned clay usu. with straw as a binder and that is used for constructing walls of small buildings

Cobber \käb-r\ n. [origin unknown] Austral (1895] : BUDDY

Materials

When getting materials there are ideals. They aren't necessary! Go for what's local. Cob is forgiving.

Everything will work out just fine.

What's in the mix?

Water

Air

Sand (about 75%)

Clay (about 25%)

Straw

water facilitates mixing

Air insulates and helps cob walls dry

Sand, ground up rocks, gives the mix compressive strength. The best kind of sand to use would be jagged/course and screened sand. You don't want ocean sand! It's way too smooth and rounded.

Compressive strength - If you were pushing on one side of a cob wall, and your friend was pushing on the other side the sand would make sure that poor wall stays just how it was before you and your friend started harassing it.

Clay is feldspar (a common rock) broken down on a chemical molecular level. Chemical weathering. Clay is sticky and elastic. The more water your clay absorbs the more it will shrink when it dries, the more chance it has of cracking. Less water absorption is better. You know you've found clay when it's really hard to get the shovel inn. It won't crumble. When you do cut it with a shovel the surface is shiny, smooth and darn sharp looking. Clay adds sheer strength to your mix. So, if you were trying to pull or push on on side of the cob wall she wouldn't budge.

SILT IS NOT TO BE CONFUSED WITH CLAY. SILT IS ALMOST AS FINE AS CLAY, BUT IT IS NOT STICKY AND ELASTIC. It is finely ground rock - finer than sand - not broken down on the chemical molecular level like clay. Silt is not what you want.

Straw is not to be confused with hay. Two different things. Straw is hollow, like a drinking straw. The color is yellow. The grain has been removed. Hay (what you don't want) is still pretty green. It is used to feed goats, oxen, etc. If you use hay in your mix the walls will rot. The straw you use should remain dry up until the point when you add it to your mixture. It is used as a reinforcing fiber. Straw adds tensile strength and insulation value to your mix. The more insulation you want - the more straw you add (creating little pockets of air adds to your insulation value.

Tensile strength: If you and your friend got sick of pushing on the wall to test the strength the sand adds, you might start testing the strength the straw adds. Each of you would stay on your own side of the wall and start pulling. If there's straw in the mix the darn thing won't move a bit.

CHAPTER TWO: Our Oven Design

Last winter Josh learned about the design we are using while at an eco village in Argentina. There are many other designs for earth ovens. The design we are using seemed to be the best for our needs- to bake a large amount of bread with a small amount of firewood. We hope it will be a good way for us to curb our reliance on fossil fuels, becoming more independent.

Firewood will be loaded through the lower rectangular door and sit on a floor made of straw- free cob. The fire will burn below a metal drum. Inside of the metal drum is where the bread will be baked. There is a deflector between the drum and the fire to make sure no flames make direct contact with the drum. There is a space of about six inches between the cob and the exterior drum surface. Our hope is the heat from the fire will circulate in this space around the drum, heating it evenly, baking the bread inside.

We went to the amazing father and son wending team, Dyers Welding (603.523.4442) and commissioned them to build the metal structure of the oven. They put together something outstanding- made of reused materials!

CHAPTER THREE: Deciding on a Site
Things to keep in mind when you are locating an oven site include:

Is it close to the kitchen?
Where will you prepare the foods that will be baked?
Is there a water source near by? This is necessary not only for the mixing of cob but also for fire safety.
Is the ground level or slightly sloped? You do not want to build on ground which has too much of a slope.
Will the oven be built under a pre-existing roof or overhang? If not, a roof will need to be built to keep the cob structure as dry as possible.
Where will the firewood be stacked? The wood needs to stay dry too!
What are the fire hazards?
Can there be music on site? It's great to have music while building and baking.

CHAPTER FOUR: The Foundation
We filled the masonry foundation walls with clean builders sand. We filled up to the top of the stones and watered it to compact it. Cob is a very heavy material. If you build on your sand and rubble without compacting it first there is a possibility the oven will begin to sink and crack over time.

CHAPTER FIVE: Mixing and Building with Cob
Now that the foundation is all set we want to start building with cob! The soil you want is subsoil, NOT topsoil. Topsoil is full of living organisms, which will make your walls rot over time. First figure out the proportions of clay to sand in your subsoil. Who knows, it might have perfect cob mix proportions (around one part clay to about three to four parts sand). To see what kind of subsoil you have, put a handful in a glass jar. Fill over the soil with water. Put a lid on the jar and shake it up!!! Set it down and see what happens. The sand particles (the largest particles) will settle first- within a few seconds. The silt will settle second. The clay will be the last thing to settle. It may take minutes, hours, even days, months, YEARS to settle!

Our soil looked like about ten parts of sand to about one part clay. More clay has to be added in order to make it a good mix! We got a trailer load of clay from a near by excavator, Neal McCarther. People you could try if you are in search of clay are excavators, contractors, gravel pits, etc. Before you have anyone deliver "clay" to your site- go check out what they want to deliver! A lot of people mistake silt for clay. Imagine a big truckload of silt being delivered to site!

There are a lot of things you can do to make sure you have a good mix. Any of the cob books listed on the last page will have all of the information you need.

Bring the dirt as close to the building site as possible. This goes for all of your materials. You want to do as little lugging as possible.

The metal structure gets placed in the sand bed we've made for it.

The curved top and back of the oven get covered with wire mesh. This will support the cob as we build the walls up around the structure, leaving about six inches of space for hot air to flow between the outside of the metal drum and the inside of the cob wall.

Mixing and building with cob is by far the easiest part of this whole process!!! The best way to learn is to get out there and start building. The only way I've ever mixed cob is with my feet on a tarp. I've heard of people using rototillers on a cement pad to mix up big batches of cob quickely. There are many reasons why using the rototiller meathod has it's advanages and disadvantages. We're going to be foot mixing all of our cob. Dump the sandy soil and the clay onto the middle of the tarp. Add a little bit of water. Start dancing on it! "Turn" it often! Every ten seconds! The more often you "turn" it the better! If will get mixed up more quickley and evenly. What it means to "turn" the mix is to take the corners of the tarp and pull them so that you are rolling the cob mix onto itself, creating something that looks like a big mud burrito. Add water to your mix slowley- it's easy to end up with a much more goopy mix than you had anticipated. It is easier to mix when it's really wet, but the building process is slower. The walls won't be able to get built up very high when your cob is super wet. Too much ooging will occur (see the next page for more about ooging). Adding straw is the last part of the mixing process. Add as much straw as you can!!! Keep shaking it out onto the mix, stomping it in, turning it in. Keep going until you can't possibly add any more straw. Turn the finished straw filled mix to form a cob burrito. Next you'll need to get down onto the tarp with your burrito. It's time to form little "cobs". Grab a big hunk of cob off of the burrito and knead it like you would bread dough. Make it into a nice little loaf. Keep doing this until the whole cob burrito is gone and all you have left are a whole bunch of little cob loafs ("cobs"). Next form a line of people going from the cob wall to the pile of cobs on the tarp. Pass the cobs from on person to another until they get to the wall. If you are working by yourself you could drag the tarp over to the wall or put the cobs in a bucket and bring the bucket to the wall.

Line the cobs up on the wall.

Then start sewing it in- either with your thumbs or a stick. Keep in mind when sewing that you want to wall to be uniform.You want it to all be one big piece of cob, no longer the little individual cobs. The straw should be used like thread to stitch all of the mud together.

Keep the edges of the wall at a 90 degree angle while you are building.

OOGING.

When ooging happens, simply trim the wall with a machete to make it straight again. A cob wall should start out wide at the bottom of the wall and gradually get thinner as you build up. There are tools you can make to make sure it's going up at an even angle the whole time you're building. You can read more about that in any of the cob books listed on the last page.

The reason for doing this is that the walls are stronger and more sturdy. It's like a tree- the thickest part of the trunk is the bottom, otherwise it couldn't support itself.

CHAPTER SIX: PLASTER PLASTER PLASTER

We will be using Poop Plaster (recipe on the next page). We will mix it on a tarp, just like the cob was mixed. The plaster layer(s) make everything smooth and pretty. You can get the materials as fine and smooth as you can imagine. It depends on how much time you're willing to spend on it. We will be putting our sand through a window screen and the clay through ¼ inch wire mesh. The ox manure we will be using is fresh enough that we don't need to put it through a screen. Plaster should be applied to walls that are wet and scratched up a little bit in order to ensure a good bond. It can be applied with hands or tool. Regular old plastering tools work great, of course. Yogurt container lids work well also. Good tools for smoothing are damp spounges, damp paintbrushes, a round flat thin piece of plastic (a yogurt containter lid with the lip/ridge cut off works beautifully!), smooth stones, or what ever else you can think of. If your plaster cracks you've got too much clay, which is making it drying too quickley.

Cob needs to be able to breathe. This means latex paints, concreate, etc. are all out of the picture when finishing cob.

Earthen Plaster Recipes

With all of the plasters, putting materials through a fine screen will make for a smooth plaster. A few coats of plaster can be applied. Always wet down and scratch up the surface to be plastered.

Lime Plaster
2 Parts Sand

1 Part Clay

1 Part fresh ox manure (or sifted horse manure)

1/5 Part Lime

*Water!

Paper Pulp Plaster
1 Part Clay

2 Parts Paper Pulp

*Water!

to make paper pulp soak ripped up paper (any kind of paper will do) in water for about ten days. Turn the paper into a pulp by stomping on it like you do the cob mix, put it in a blender, or an old washing machine! There's got to be an endless amount of ways to make a pulp out of the soaked paper. Experiment!

Poop Plaster
3 to 4 Parts Manure (the fresher the better)

1 part Clay.

*Water!

*Your mix should have enough water to make it goopy and easy to shmear.

Plaster can be mixed on a tarp, in a wheel barrow, or anything else you can think of. I like to mix it with my feet, but I've also seen people use a big electronic paddle (I think it's normally used for mixing house paint).

What we're doing differently our second season building with COB

The biggest realization I've had between last season and this season is;

Building with cob is intuitive. Taking time to recognize what you already know is invaluable.

With building the foundation we aren't digging down to the frost line. We aren't digging down at all!
The rocks for the masonry are all from on site.
We aren't using cement mortar. Instead we are dry stacking the stones and putting cob in where there are big gaps.
For filler between the stone masonry walls, instead of using commercially sold gravel and sand we are using small rocks from on site and our ashes from firing up the oven.
WE'RE ADDING PAPER PULP to the cob mix!!! This spring I went down to the Ozark Mountains in Arkansas for Mud Camp (also known as the Women's Natural Building Symposium and Extravaganza)! There, Becky Bee, the Queen of Cob, shared that paper pulp is now being added to cob mixes. The result is harder walls! Apparently adding the pulp to the mix makes it possible to make the walls much thinner as well. It is being experimented with over in New Zealand. Any paper can be used! I was told 10 parts cob mix to 1 part paper pulp. We'll probably experiment a little with those proportions. It's so exciting! We're using glossy colored newspaper and junk mail. To make the pulp we ripped up the paper, and soaked it in water for a little over 10 days. Now we need to make it into a pulp. We are going to try stomping on it in a pit like we have done with the clay. Which brings us to what we are doing differently with the clay…
Last season when we tried mixing somewhat hard chunks of clay into the sand. It worked, but we're going to try something else. Another thing I picked up in Arkansas was how to make a mixing pit with some straw bales and a tarp. It goes like this:
Put the clay chunks in the mixing pit and add a good amount of water.

Take your shoes off and jump in! Stomp on the clay and mush it around until it

becomes a nice thick uniform goopy slip.

We're borrowing tarps. If there is something you think you need and you don't have it- either figure out you probably don't actually need it, or borrow or trade something for it with a friend or family person near by. We're borrowing the tarps from a few friends in the area.
This season we are building benches!!! There are different things we need to know. One thing is how to make them as comfortable as possible. Another thing is how to make their finish a little tougher since they are close to the drip line of the roof. A trick to making a bench comfortable is to make it possible for feet to go under it a little bit.
To make the finish a little more durable we are going to rub linseed oil and then a

linseed oil – wax mix into the finished plaster

The last thing I can think of- we're starting SOOO much earlier in the season. By the time we were finishing the oven last year it had started to snow. Warm weather is much more enjoyable for cobbing!!

Books on Cob

If you are thinking about making a cob oven definitly read Build Your Own Earth Oven by Kiko Denzer

If you want to know a lot about cob, especially if you are even slightly interested about the idea of building your own cob home, here's a list of some books to get you started.

The Cob Builders Handbook by Becky Bee. Becky's book is simple, fun, easy to understand, perfect.. There are a group of women who have read this book and then built a cob house for themselves.

The Hand Sculpted House by Ianto Evens, Linda Smiley, and Michael G. Smith *** This book is GOOD. I mean GOOD!

The Art of natural Building Joseph F Kennedy, Michael G. Smith, and Catherine Wanek, editors

The Cobber's Companion: How to Build Your Own Earthen Home by Michael G. Smith

A couple of other people to check out
Amber Wiggett and Ben Graham at Spiral Works (they don't have a book yet, but they're doing good things)

These two have amazing hearts, talents and visions. They got me started with cob. I have nothing but good things to say about them.


upcoming events Upcoming Events
Social Networking D Acres of NH Blog D Acres of NH Facebook Page D Acres of NH Vimeo
GoodSearch: You Search...We Give!