Tag Archives: clay slip

Earth plaster & clay slip for straw bale homes

The material used on the outside of straw bale homes is made of clay, sand, and organic matter. These days it’s called earthen plaster, but this mud mix has been used for about centuries in the form of wattle and daub, rammed earth and adobe. Until mud bricks were first fired and construction methods evolved, clay was an essential ingredient for home building. JSCW previously reported on sustainable building methods which have made a comeback and even here in the temperate rain forest of southwest British Columbia. (Click here to see our article on cob homes or here to begin reading our series on the history of bricks.) Today, I am concentrating on earth plaster and clay slip, the substance troweled and painted on the finished surface of a home built with these methods. This form of building is eco-friendly, inexpensive and easy to fix. The surface plaster differs from stucco which is made of lime and sand or Portland cement, sand, and water. When I was looking for a source of earth plasters and more information, I turned to a company that makes one of the clays I use, Plainsman Clay, in Medicine Hat, Alberta. Plainsman appears to have a good selection of earth plasters and it certainly knows its clay. After clicking on the link above, scroll to the bottom of the page to see natural colorants. Another writer enlightened me about the final coating on traditional strawbale home and was I ever surprised! And, naturally, once I learned about it, I found it very appealing. “A clay slip, known to some as an aliz, is used on an earth-plastered wall almost like paint is used on other surfaces,” says Carol Crews on the Last Straw site. “The purpose is to seal and beautify the surface.” She uses kaolin and ground mica as the basis of her clay slip, to which she adds flour paste. The kaolin would certainly make a smooth finish. She cautions people to use a mask to prevent inhalation of particulates, a familiar warning. (Over the weekend, I made sure I was wearing a mask when I used the Dremel to grind down the edges on a clay mold I’d made.) If you are interested in seeing actual straw bale homes in use, take a look at this International Straw Bale Registry Project for homes in your area and to learn whether you can drop in to visit the owners or whether you have to make an appointment. In my area, there is a straw bale house on Denman Island. You can read about it on the owner’s blog, Stuff from Denman Island. It is a straw bale house that has mixed lime plaster on its outer walls. Yet, from what I’ve been able to tell, if your eaves are deep enough and your foundation good enough, you won’t have trouble with earth plaster if your house is built correctly. A most interesting touch with  straw bale homes is the truth window, through which viewers can see the building material. I love the very idea of it and whatever metaphors can be divined. As my husband said, it would make a good name for a book.


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Good Impressions, Part 5: The ceramic legacy of Moorish Spain

Cuerda seca, lion in a landscape, 17th century. Source: Wikimedia

Cuerda Seca: While investigating an Art Nouveau method of tilemaking, cuenca, I learned about the technique from which it originated: cuerda seca. The latter is a Portuguese word meaning ‘dry string.’ Coated string was used to make a line of black resist to prevent glazes from running. Originally, the string was saturated in animal fat and minerals that blackened when fired, like iron or manganese. The fat soaked string was heated, placed on the piece to outline the design, then burned off in firing, leaving a black line that separated colors. Cuerda seca is an Islāmic method of tile decoration that was brought to Spain by the Moors. Wikipedia states that “the craft is still in use in the Arab world” with two main traditions, Egyptian and Moroccan Zalij. “This origin explains the unmistakable Arab influences in many tiles: interlocking curvilinear, geometric or floral motifs.” In Spain,  Seville became the tilemaking center where cuerda seca tiles were made. Christie’s sold a tile panel from Toledo for 10,000 Euros at the Decorative Arts Sale, held in Amsterdam in June, 2011.

Vase with palm tree. 8th–9th century CE, Iran. Source: Wikimedia

Cuenca: In the 19th century, cuerda seca was again popularized during the Art Nouveau period. The style evolved during that time. Sometimes designs were simply outlined with black. Cuenca, a Moorish method of outlining with slips and engobes also became popular then. In fact, it became a hallmark of ceramics during the Art Nouveau period. Cuenca means ‘basin;’ a reservoir was created by the trailed slips into which glazes were pooled. There is a tie-in with the Arts and Crafts period, too, as style eras overlapped. It was during this time that the cuenca method also came to be known as tube-lining. Because of the popularity of bungalow style, since the 1990s, there has been a return to the production of these decorative methods and motifs. Motawi, Historic Style, and Du Quella Tile & Clayworks, and are but a few potteries that sell ceramics made in the cuenca style. Today, safer methods are used for cuerda seca, too, which has also experienced a comeback. Now, black wax resist is used to outline designs. Using Aftosa’s product, black lines will remain up to Cone 8. You can even learn the method through the Oerth Tile Works in Alexandria, Virginia, or watch their youtube video about the process. These decorative styles and methods have spanned centuries and are still actively used. That’s what I call staying power! In the early 1990s, my mother gave me an Art Nouveau vase. Little did I know then that I would come to learn about the method with which it is made, cuerda seca, or that I would be writing about it today….

My Art Nouveau cuerda seca vase

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Making Slip for Slipcasting

This week at the studio, Otto talked about having made slip to use in plaster molds. A little piece he’d slip cast was on the work table when I came in. It’s been so long since I’d done any slip casting, it made me wonder what the ratio of clay to water should be, how it is made smooth enough to pour, etc. So, I found a couple of articles to share that concentrate on these aspects and more. I think the last time I did slip casting was with my Mom’s friend Nancy when I was seven or eight! Maybe it’s time to give her a whirl. Slip casting is a good article covering the process from start to finish and includes formulas. Basically, the ingredients consist of clay in dry form, water, water glass (sodium silicate). I’m a fan of allexperts.com and found some good information from one of their ceramic experts. He said, “any clay can be turned into a slip. Cut the wet clay up into small pieces and let it dry out completely. Put the dry pieces in a container and add water until it just covers the dry clay. let it sit, do not stir, you will be able to see the clay disintegrating as it absorbs the water. Within a few hours it will be ready, next day is better, mix up and use.” This is exactly what I wanted to know and I have some pieces I never bisqued for one reason or another. So, I think I’ll just break them up and follow Sam’s advice. Here is another site that gives good basic information. It is not an in-depth article, but covers the process of slip casting rather well. I cannot vouch for the veracity of the article, though, in terms of the length of time needed to mold a piece, because these articles have conflicting advice and I don’t know what to believe. Another article suggests 20-40 minutes, whereas the ‘another site’ article above says eight hours or longer, depending on the size. That sounds like an awfully long time. I guess I’ll just have to experiment and follow-up with Otto. I like the idea of slip casting something, then altering it to make it more of my own. Since a piece is leather hard when unmolded, I would still have the time to make changes. This video below on “Clay for the Slip Casting Process” gives more depth. Based on what this ceramist says, I would have to add a deflocculant like water glass or soda ash to make slip the right consistency. Also, I have to consider whether the clays I’m considering using are appropriate, a white and a red clay. According to the video, porcelain is a good choice. Because there are many questions still unanswered, I am going to have to ask the centre’s artist-in-residence more about slip casting to get a better idea and I will report what she says at a later time.


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