by Carol Crews
A clay slip, known to some as an <i>aliz</i>, is used on an earth-plastered wall almost like paint is used on other surfaces. The purpose is to seal and beautify the surface. After it has become soiled or damaged, another coat may easily be applied to renew its fresh look. Traditionally, the Aliz was applied with a sheepskin in a very thin layer. This was done as a part of spring cleaning after a winter of woodsmoke, often using the skin of a spring lamb which had been killed for food. In Taos, the favorite material was a micaceous pearly-grey clay called Tierra Blanca found in a cave south of town. The finish work on adobe buildings was traditionally women’s domain, and many vintage photos portray this activity.
The oldest example I’ve seen of a clay slip not used in pottery was in an Anasazi cave in the Tsankawi ruins. On the floor under two inches of dust was an intact layer of peachy-colored clay sealing the coarse graininess of the tufa cave. In Africa and India the use of clay slips for decorative work on houses is well documented. Earthen architecture is the most basic and common known to man in most of the world, but the accessibility of cement and cinder-blocks are causing people to value their traditional methods less. They want to try out what they think is a more permanent and therefore better means of construction. This is very apparent in Mexico where people associate mud dwellings with poverty, and prefer the status of concrete buildings despite their failure to moderate temperatures.
Modernization has definitely affected the use of clay finishes here in Taos. Even at the Pueblo, some people are buying latex paint for their interior walls instead of using the old techniques. Wherever there is a leak the paint peels off, revealing dark mud beneath. Paint keeps the wall from “breathing” properly, or more technically correct, its ability to transpire moisture without it being trapped against a barrier of a more impermeable substance. Clay slips on the interior and mud plasters on the exterior allow an earthen structure to absorb moisture and then dry out again without creating major problems.
A big lesson was learned at the famous St. Francis de Assisi Church in Ranchos de Taos when it was coated with cement plaster in 1967. The plaster cracked and allowed moisture to go deep into the adobes and stay there, unable to dry out again. Large sections of the buttresses had to be rebuilt, and the community has now gone back to annual renewal of the mud plaster—which not only keeps the church building in beautiful condition, but strengthens neighborhood ties as well.
Before sharing the details of clay slips with you, I will briefly explain the basics of plastering the wall with mud.
Clay is essential for stickiness, so first you must locate some. Do a shake test by placing your sifted earth in a jar of water with salt added and shaking it up thoroughly. When it settles, the clay will be the top layer, the silt is in the middle, and the heavier particles of sand will have sunk to the bottom. Plaster dirt should be at least 20% clay. Even at this percentage, you may wish to add manure or flour paste to make it stickier.
When plastering straw bales, I find it easiest to first spread a thin layer of mud with a high clay content and no additional sand or straw on the bales to lock into the straw and provide a surface upon which the next layer can adhere. If you use this technique on tight bales, you can avoid using stucco netting. Even when chicken wire or lathe are used, the smooth mud will penetrate the metal and leave no air spaces to cause future cracks. You don’t have to wait for this to dry before adding the thicker layer of plaster with straw added.
On rough cob or adobe and for the next layer on straw bales, I like to use a plaster with high clay content and lots of straw mixed to a slippery, easily-spreadable consistency. This can be applied with the hands to a dampened wall and is very good for filling in depressions. (It gets rid of your own depression too, because it feels so good to sling that mud around.) The condition of your walls and how much shaping they need will determine whether to use long straw, chopped straw or a combination. The thicker the layer needs to be, the more long straw it should have. Don’t trowel this layer down smooth, but get it as flat as you can with your hands and let it dry out thoroughly. It will make lots of little cracks and provide a perfect surface for the next layer to adhere to.
Always try out an earth plaster you are not familiar with by making a test patch of several square feet. Clays differ in their shrinkage rates, and if it cracks too much you need to add sand and more straw. I’ve seen some plasters dry into the sort of cracks you find on the bottom of a dry lake bed and fall off the wall. This happens more often to a trowelled surface because there is less surface area to release moisture than if it’s left rough. If the dry plaster is very soft when you scratch it, it has too much silt or sand and needs more clay, fresh manure and/or flour paste, which makes long-straw plaster very good for the outside of a building.
In India, the Middle East, and elsewhere, it was discovered long ago that old stinky mud works better than fresh mud, and so it is mixed in a pit with animal manure and other wastes and left to “ripen.” This causes the molecules of clay to line up as closely as they can, improving wet plasticity and dry toughness. The same principle is well known in pottery-making.
The next layer is the final coat before the Aliz, and if your adobe, cob, rammed earth, pumice-crete or straw/clay walls are even, as I know the last three surely will be, this is the only layer of plaster you may need. If you are using a found adobe-dirt clay, sift it through a quarter-inch or window-screen size mesh and add an equal part of medium- to fine-grained sand to it. Add about 1/4 to 1/3 the dry ingredients’ volume of chopped straw. This should be easily workable, sticky enough to adhere well to the dampened wall, and wet enough to trowel on easily but not so wet that it’s hard to pick up. Robert LaPorte recommends adding a small amount of cooking oil to the mixture to make it slide on more smoothly, and I find it does make a positive difference. Disks of plastic cut from the tops of coffee cans, yogurt containers or whatever, are a useful tool for going around curves and “bullnosing” around windows and doors. If you have gathered a more pure clay that is in lumps, it’s best to soak it and make a slip by stirring it into water. A paint mixing tool put on the end of a drill simplifies this job as long as the lumps are not so big they would jar the tool. Add fine sand to the thick clay slip until it is the right consistency—up to 70%, depending on the clay—and then add the straw to it.
To apply this plaster in preparation for a clay slip, you need a trowel, a tile sponge, and a bucket of water. I prefer to work from a bucket of plaster rather than a hawk so that I can use both hands and don’t wear out my arm holding the awkward hawk… though I know most professional plasterers would disagree. I wear rubber gloves, scoop some plaster onto the trowel and apply it to the dampened wall like icing on a cake. I cover about a square yard at a time without worrying about smooth perfection. Then I go on to the next yard. When I finish that, I go back to the first yard, which has had time to “set up” a bit, and smooth it out with the sponge. A floating tool could also be used, but this seems to leave the surface rougher. The sponge leaves a perfect, fine-grained “tooth” for the Aliz to bind with. If it’s still too wet to sponge smooth, wait a little longer. If you want to leave the plaster without a clay slip, you can trowel it hard and smooth at this point, spraying it lightly with a squirt bottle as you go.
A very fine finish plaster can also be made without straw, to be applied in a thin layer to a wall which is quite flat already. If clay is hard for you to find, you can purchase it at a pottery supply store. White Kaolin, which we use for slips, can also be used for plasters. If you’re less concerned about color, and planning to put a slip over it anyway, ball clay of a grey color is very good in plaster because it has greater plasticity and dries somewhat harder than Kaolin. Either mix the fine dry sand (70%) and clay (30%) together first and then add water, or mix the clay and water into a slip first and then add the sand. Don’t forget the splash of oil for workability. Proportions may vary somewhat depending upon your sand.
And now for my favorite part, the final clay slip, or Aliz. I use white Kaolin and ground mica as my major ingredients. Often I add a small amount of fine sand, especially to the first coat to make it thicker and fill in any small irregularities in the plaster surface.
I use cooked flour paste as a binder in the proportion of 20% to 25% of the liquid. To cook the flour paste, set a 2/3-full pot of water to boil on the stove. In a mixing bowl, whisk together equal parts cheap white flour and cold water. When the pot of water comes to a rolling boil, pour in the flour and water mixture and stir it well with your whisk. It should thicken immediately and become somewhat translucent. Don’t keep cooking it or it will scorch. The proportion of all the water to the flour is approximately four to one.
I understand that gum arabic also works well as a binder, but I haven’t tried it yet. I have used Elmer’s glue on occasion to give the first coat more strength over a weak plaster. Instant flour paste can be purchased as wallpaper paste. Milk products also act as a binder, buttermilk being best.
To mix the Aliz, you will need a container at least as large as a 5-gallon bucket, and a big whisk or a paint-mixing attachment on the end of a drill. Start with 3 parts water to 1 part cooked flour paste in the bucket to approximately 3/5 full. Use a saucepan, coffee can (or whatever) for a scoop, and start adding the ingredients proportionally. Recipes vary according to the surface and whether people love mica or not, but generally I use three scoops of white Kaolin, two scoops ground mica (or more), and one scoop (more or less) of fine sand.
Sand is mainly for the first coat, or if mica is not available. Be careful not to breathe the fine particles of dust and mica, or wear a dust mask while mixing. Keep adding these ingredients until the mixture is the same thickness as heavy unwhipped cream. You may have to add a bit more water to achieve this. Sodium Silicate is an ingredient used in slip-casting to keep the particles of clay afloat in the water, and is useful in this context as well. A very small amount is required. It will also thicken the mixture somewhat, as will powdered milk which also makes the final product a little tougher.
Colored clays or pigments may be added to create different colors. The colored clays would replace Kaolin in the recipe, and pigments should be soaked in water, if not actually ground, to prevent color spots from showing up. The earth pigments sold to color concrete are quite suitable for our purposes too.
If mold might be a problem, add a little dissolved borax powder.
I love to add larger chips of mica to the mix, but they are not commercially available as far as I know. (If anyone out there knows of a source, please let me know.) If you can find large flakes of mica with no rock attached and want them smaller, chop them up in the blender with plenty of water. Bits of chopped straw are a popular addition to the Aliz too.
Bags of ground mica are available through KMG Minerals (PO Box 130, Velarde NM 87582; phone 505 852-2727) at $10 per 50-lb. bag, but shipping costs are high. The V-115 is the largest grain currently available, and gives a visible sparkle to the surface. The 1-1-17 is the next largest grade and has a more subtle sheen. Mica-200 is finely powdered and offers no sparkle at all, but improves the texture and thickens the mix. If it’s easily available, mica makes a lovely addition to finish plasters as well as clay slips, and adds to their workability. It is like a molecularly flat sand which is smooth instead of gritty. It’s a major ingredient in joint compound and is also used as a lubricant in drilling oil wells. I have appreciated this quality of mica when sculpting with sticky mud. Just dip your hands in a pan of mica frequently to keep the mud from sticking to them. (KMG should hire me as a salesperson.)
You will need a few tools to apply the Aliz to your wall: a 3- or 4-inch-wide brush, a one-inch brush for edges (natural bristles are best), two small buckets, and a fine-grained tile sponge. Sheepskins work, but I find them slippery to hold on to and the coat of slip is not as evenly applied.
Do not moisten the wall first. Make sure it is completely dry, because damp plaster will leave water stains. Start brushing at the top of the wall so you don’t mess up your fresh work with drips. Cover the floor with drop cloths. If the wood of your window edges and lintels is rough, tape it first to save yourself cleaning work. If it’s smooth and painted, the slip can be wiped off easily while it’s still wet. Use the small brush for edges, or another way is to mix some of the slip with extra sand in a small container and apply it to edges with a palette knife. In small curvy areas, sometimes I mix the Aliz a bit thicker and apply it with my hands, and often at the bottom of the wall I use the sponge to dip out some slip and slide it up the wall, starting at the base.
Most walls require two coats. Make sure the first one is completely dry before applying the second one. When the second coat starts to dry and look mottled, becoming “leather-hard,” take your sponge and a bucket of warm water and polish the finish in circular strokes with the squeezed-out sponge. Rinse and squeeze out your sponge often so that it will cleanly polish off the mica flakes and bits of straw (if you have chosen to put them in). Polishing smooths out the brush strokes and gives the surface a finer texture. You can even dry-polish it again later with a rag to get off any last bits of sand and polish the mica to a greater sheen.
When you are finished with your job, save the leftovers by drying them into “cookies”. These may be stored, then soaked and used to repair any small damages.
I can’t emphasize enough the need for trials and experimentation in this type of work. There are many variables in Mother Nature’s materials, so never take them for granted. I haven’t gone into the various additions like psyllium husks, cactus juice and micro-fibers here, but they are things that can be used. Happy Mudding!
Carol Crews, Gourmet Adobe; HC 78 Box 9811, Ranchos de Taos NM 87557; ph (505) 758-7251