«The Cob Builders Handbook You Can Hand-Sculpt Your Own Home Table Of Contents: Acknowledgements and Warning INTRODUCTION What is cob? Why build ...»
If you want to leave the exterior of the cob walls as they are, without plastering, observe the walls for erosion during the rainy season. If you can detect any wear, you may want to plaster or paint with lime wash to protect the cob, or build a porch or verandah on to the weathered side of the house.
You can add new layers of plaster whenever the urge grabs you. Remember to dampen the surface of the walls before applying a new coat of plaster. If you add plaster to the inside walls too often, your house will fill up with plaster. If this happens, you have a bad case of plasteritis. Please write a ballad about it and sing it to the next Natural Building Conference in your area.
Preparing the walls for plaster
Some natural builders say to let the cob walls dry well before plastering them. The theory is that the walls might shrink and crack the plaster. Also the plastering slows down the drying rate of the walls. Some old books say to plaster the interior, move in, and plaster the exterior next spring.
The more similar the moisture content of the wall and the moisture content of the plaster, the better the plaster will stick. Wet the surface of the walls well. You can spray them every 5 minutes or so while you are getting stuff organized to plaster. This is your chance to make last minute changes to the shape of the surface of the walls.
Use a shovel and/or machete to carve the big bumps off the walls. Layers of plaster can be used to fill in minor dips and holes. Any gaps where the cob has shrunk away from the wood frames can be filled with plaster.
You can leave the shaggy straw that's sticking out of the cob wall. Most of it will get folded over during the plastering and will help to hold the plaster onto the walls. If some stray straw sticks through the plaster, cut it off with scissors.
To carry the plaster from where it's mixed to where it goes on the wall you can: use a wheelbarrow, drag it on the tarp, carry it in a plastic dishwashing tub, or on a plaster hawk. A hawk can be made out of a dowel screwed to a small piece of board or plywood.
Methods of application
Try putting the plaster on with all these methods. You will probably prefer one way and stick to that. Keep your tools and hands wet so the plaster won't stick to them instead of the wall. The general idea is to apply a thin layer (3/8 of an inch or less), in upward sweeps.
Spread it like frosting or icing on the walls
Polish the plaster when it is partly dry. You will soon learn how much to let it dry for optimum polishing. Keep an eye on the drying. You can burnish (polish) the plaster fairly soon after you apply it.
If it gets too dry before you get to it, you can re-wet it gently with a fine spray and then polish. This is where the pieces of plastic yogurt or cottage cheese containers come in. Cut 4 inch round or oval pieces out of the sides and lids of the containers.
You'll need quite a few because they do wear out. These can be used to apply the plaster and to polish it. It is especially important to polish areas that will get lots of wear, like around light fixtures, in the kitchen, and the cob furniture.
A smooth stone makes another good polishing tool. This leaves a lovely textured look on the walls.
Replastering Later When you are replastering after a few years of living in your home, you'll find that it is easy to patch worn areas. Fresh earthen plasters stick well to old plaster. Wet the area where you are replastering. Use drop cloths to protect your counter tops and floors from splatters.
Scoop the plaster onto the hawk, lean the hawk on the wall, and scrape a blob of plaster off the hawk onto the wall with your trowel.
Where to start plastering Some folks start plastering at the bottom of the walls and others at the top. I'm one of the start-at-the-top kind of people, because I don't like slopping on my finished work.
The logic of starting at the bottom is that as you add sections of plaster to the walls they overlap each other like shingles, encouraging the water to run off the wall instead of into any slight seams that might form where the different applications of plaster join.
How thick do I make each layer of plaster? The first layer of plaster needs to be thick enough to fill in the dips and can be quite thin over the high points. If the dips are deeper than 1/2 an inch, you may need to fill them with two or more 1/2 inch layers of plaster.
A general rule for plaster thickness is: the smaller the particles in the plaster ingredients, the thinner the layer. The thinner the plaster, the less it will crack. Do some small tests of different thicknesses in out-of-the-way places.
Don't work the plaster too much while it's wet. This brings the adhesive materials in the plaster to the surface, away from the wall where they are needed. If you plan to add another layer, don't worry about how pretty it looks. In fact, leaving it rough gives the next layer more to hold onto. If you want to level off the surface, scrape the edge of a board across the partially dry plaster.
Make sure you wet the last coat of plaster (or the wall) before you add the next coat. Put the last layer of plaster on with care because this is the coat you will see.
When you've finished plastering for the day, try to leave the edges of the latest
You can make relief sculpture on the walls with the plaster. Try some sculpting as you plaster. You can always chop it off later if you decide you don't like it. Sculpting around door and window openings can be very beautiful.
A whisk broom (straw hand brush) is a handy tool for plastering. You can use it to whisk off the loose stuff on the cob or the last layer of the plaster. Keep it in a bucket of water near you, to splash water onto the dry wall. After you've put the fresh plaster on, brush with the broom to \ smooth the plaster and to give it a lovely texture. A handled broom can be used tor each the parts of the wall that are over your head.
Embedding decoration into the plaster Plastering is a work of art. You can set pretty tiles, stones, sticks and bits 'n pieces into the plaster. Some people add a little Elmers glue to the plaster behind the decorations and tiles.
On the outside surface, you may want to embed small stones, seashells or pieces of mosaic tile along the bottom 18 to 30 inches of the wall. This will help protect the walls from water splashing off the roof and from the worst of the windblown rain.
This can be a wonderful opportunity for the artist in you and can add an elegant as well as practical touch to your home.
You can make beautiful textures inside and out, by printing or cutting into the wet plasters. Experiment! You can always smooth it back over if you don't like the results.
Polishing the plaster To erase unwanted bumps and lumps, go over the plaster with a damp sponge or rag when it's partially dry and 'sand' off the raised areas. (Burlap works well.) To flatten a large area, take a piece of 2x4 or a wooden trowel and rub it in a circular motion over the plastered wall.
Plaster agricultural bags as soon as possible because they will break down quickly in the sunlight. You'll be surprised how well the plaster sticks to the bags. For the tricky areas, like the underside of where a bag overhangs, plaster where you can, then let it dry. Your next application of plaster will stick to the dry plaster and form a bridge to the next area of dry plaster.
On brick or stone walls Natural plasters will work well on any natural surface.
On straw bale walls You can use these plasters on straw bales too. Spray the walls lightly with water before you plaster them. When an earth plaster is the right consistency, it will stick beautifully to the straw, especially when it is put on by hand. You won't need any chicken wire or lath. Plaster right onto the straw. Use two or three layers of plaster on the straw and finish it just like you would a plaster on a cob home. The earthen plasters breathe well, which keeps the straw from rotting.
Because lime is caustic and breaks down organic matter, it's probably best not to use it directly on the straw. If you want to use a lime plaster, you could put a layer of earth plaster on first, then try the lime. Or use an earth plaster and a lime paint.
Thick layers of plaster on a straw bale wall give it structural support and added thermal mass.
I have very little experience with gypsum. I did use it once at a workshop, but after breathing the fumes as I put it on, I felt sick and headachy. Because of that and because I'm so happy with earth plasters, I haven't bothered to learn anything else about gypsum. I'll tell you the little that I know. Gypsum is one of the main ingredients in plaster of paris and some people are allergic to it. The Gypsum we used came out of a bag at the building supply store. We followed the instructions and mixed it with water and sand. It set up slowly and can be mixed with an earth plaster.
The instructions said for indoor use only.
Builders' lime Plaster (not gardening lime) Caution: lime burns skin and eyes! Wear glasses and gloves.
Lime mixed with sand and water was the most common plaster used on traditional European cob buildings and as a mortar for stone buildings. It was also used in the plasters and mortars of the ancient Anasazi and other Native Americans.
In the United States, lime has gone out of fashion, so people there can't go to the local lime kiln to get lime or to the building supply store to get lime putty. People in the U.S. usually use dry powdered lime. This is an inferior product and loses quality when it sits around in the dry state. If that's all you have, you may want to use an earthen plaster and just use the powdered lime putty to make paint.
Making a lime puttye
If all you can get is the powdered lime, mix it in water to a sour cream consistency to make a lime putty. Add the lime to the water slowly and stir it well. Let it soak for a few weeks before using. This putty can be mixed into plasters or mortar. While it's soaking an important chemical reaction occurs. Once it is mixed with water, the quality shouldn't deteriorate and the lime can be stored wet indefinitely.
Recipe for a lime Plaster 1 part lime putty (See above for how to make this putty) 3 parts sand of different particle sizes 1/3 to 1/2 part fiber (cow hair was traditional in Britain) Remember to protect your skin and eyes! Mix the putty and the sand well (you can use a cement mixer), cover it well and let it sit for two weeks. Then add the fiber to the amount of plaster you can use each day. Lime is caustic and corrosive to organic matter. Hair is an excellent fiber to use with lime because it is very hard to break down. Chopped up polypropylene baling twine works well too.
166 Lime plaster takes more skill to put on than earth plasters. Practice in some out of the way spots on the wall. Wet the walls well before plastering with lime. Apply 2 or 3 thin layers, always wetting the previous layer before adding a new one. The proper way to put the first coat on and get it to adhere well to the wall is to flick it onto the wall with a backhand stroke off a little shovel (like the kind for cleaning out ashes from the stove). When it is partly hard, go over it with a piece of wood in a circular motion to even out the surface. I tried this and never got the hang of it and I didn't like the caustic splatters, so I just put it on with a trowel and that worked OK. Make the layers as thin as possible. Do not fuss with it too much once it's on the wall.
Let it dry until it is firm to the touch but not all the way dry before you add the next layer. You can rub the plaster with a piece of wood between each coat to even it out.
Scratch the previous coat when it's still damp to make it rough so the next layer will stick better.
Lime is like concrete, it needs to cure slowly. In English reference books about lime plasters, they suggest hanging wet burlap sheets a few inches from the freshly plastered walls for a week or more after plastering. Shade the fresh plaster from the sun. Do not plaster on dry, windy days.
A lime plaster makes a hard looking surface similar to a cement stucco.
About lime additions to earth plasters Lime added to an earthen plaster makes it harder. It also lightens the color. Adding lime makes the plaster caustic, so remember to protect your skin and eyes. If you are adding lime putty to your earthen plaster, make test batches with different proportions of lime putty. Put them on the building, let them dry and see which mixes work well.
Look for cracks. If you do add lime to your earth plaster, let it dry as slowly as you can. Wet the walls well before you put the plaster on. High humidity days are good for slowing down the drying process.
Alis and Paint Alis (pronounced "a-lease" with the accent on lease) is a term referring to a thin plaster or thick paint.
The ingredients in cob, earthen plaster, alis, natural paints and washes are much the same. One graduates into the other as the amounts of water and particle sizes vary.
Do not use paints that seal and prevent breathing on cob walls! (Or on any wall made of natural materials.) The acrylics and emulsions in most modern paints will block the vital breath of a natural home! This will cause destructive moisture to be trapped in the walls.
If you paint the walls with a light color, the house will feel bigger and the light walls will reflect the sunshine and brighten up the indoors. This is always a hard decision for me because I also love the warm soft look of the earth plaster or alis.
Wear protective clothing and glasses. Because lime is so alkaline, it disinfects and discourages critters.
Mix water with lime putty until it is the consistency of skim milk. That is the recipe!
(See page 162 in this chapter for how to make a lime putty out of powdered lime.) This will keep very well, so make enough to cover your walls and save some for touch-ups or future coats.