1 Introduction Natural plasters are beautiful, non-toxic to live with (though not always to work with) and steeped in tradition. The act of plastering is generally enjoyable, even addictive for some, but very hard work. It''s also serious business - along with roof and flashing details, the plaster skin of a building protects the materials inside from degradation by water, wind, sun and animals. The job of the natural plasterer today is to take millennia-old techniques and materials, combine them where appropriate with contemporary materials and tools, and employ them safely and efficiently on a modern construction site. This book gives detailed direction on how to do this. Many natural builders have collaborated to share their expertise in this book; the cross-section of recipes, and the approaches to plastering they reflect, make this book a valuable resource for beginner and professional alike. Why use natural plasters Before we launch into nine chapters on how to use natural plasters, it''s worth taking a moment to reflect on why we would want to use them. There are two kinds of situations in which we''d be likely to use natural plasters: to cover a natural wall system, in which case the permeability and flexibility is important, if not essential; or over other wall systems, such as a stud wall sheathed with wood lath or drywall, where natural plasters add beauty and are a non-toxic alternative to paint or other wall finishes.

Here''s a short list of some of the advantages of natural plasters: In our increasingly sealed homes indoor air quality is important, and there''s a growing body of evidence that the chemicals we surround ourselves with can cause harm in relatively low concentrations. Natural plasters are free of these environmental toxins. Natural plasters connect us to our heritage, and have a track record going back thousands of years. We know that they work, and how they interact with other natural building materials, including wood. Unfortunately some of this knowledge has nearly been lost, but as a natural plasterer you can help keep this knowledge alive. Natural plasters have greater flexibility and vapour permeability than most synthetic materials. They tend to protect the materials they are bonded to from moisture damage. They are essential as a coating for many forms of natural building, and can be beneficial for many forms of conventional construction.

Natural plasters typically have a low embodied energy - the energy it takes to mine, process, and transport them. They may be locally available, contributing to local economies and further reducing their ecological impact. They are beautiful. There is evidence that human happiness is tied to our connections to the natural world, and natural plasters likely contribute to human well-being by introducing natural products, forms and textures into homes. Natural plasters can help regulate temperature and humidity in homes, improving comfort and reducing the need for air conditioning and heating. How to use this book While at heart this is a recipe book, to be a successful plasterer you will need to understand the materials, and how they interact with environment, substrate and design. The opening chapters of this book describe the materials, how to design for them, how to prepare the walls, and how to mix and apply natural plasters in general. It''s tempting to jump straight into putting mud on the walls, but the preparatory steps leading up to that moment are more likely to determine success or failure than the days spent plastering.

Chapter 3 is probably the most important chapter in this book. Before you begin make sure you have chosen the best plaster for your application, and that your house is designed appropriately. Too often we have been called to repair plasters that weren''t appropriate for the site or design of the building. This may cause the plaster to fail quickly, or even worse can cause damage to the underlying building materials. When you''re ready to plaster, chapters 5 through 8 tell you exactly how to process and use earth, lime, gypsum and cement plasters, including recipes for a wide variety of plasters. A plaster recipe is only a starting point. When you use a recipe from this book there will be a learning stage while you come to understand the properties of the plaster; how to lay it on the wall; how thick it can be applied; how long it needs to set up before a finishing pass; whether it needs burnishing or compression; how many coats are needed. Much of this information can be gleaned from the recipe, but other things you''ll have to learn by doing.

This all becomes more complicated in the real world, because when you change the substrate, or use a different aggregate in your plaster, or if the weather is different while you''re working, you will have a different result. The first time you use a recipe always do tests Then if possible try it on a wall at home before plastering a whole room or a whole house. Get to know the plaster and understand how it works with your locally available materials. Take notes: how much water did you add to the mix; how long does it take to set up to ''leather hard''; did you burnish it, and if so after how long; at what depth did you apply it; did you use a steel trowel; did it crack? Check the coverage rate - rates in the recipe are only given as guidelines. Good notes are essential when you start modifying recipes, which will happen sooner than you expect. When you change a recipe, try to change one thing at a time. If it''s cracking, try adding aggregate, or changing the type of aggregate, or add fibre, or simply apply it in a thinner coat Finally, be cautious. The world needs cowboys, but don''t be a cowboy on someone else''s house, try to do your inventing on a wall of your own house, or preferably an outbuilding.

Use your mistakes as learning opportunities. Take it seriously, but have fun too *** 5 Earth plaster base coats About base coats The base coat provides the majority of the strength of earth plasters. This strength comes from the depth of the application (typically around inch) and from the relatively high fibre content (commonly 10-30% of the mix). In general we can think of base coats as falling into two categories: clay-straw plaster, which might have a ratio of around 1 clay: 1 sand: 1 fibre; or clay-sand plaster, which might have a ratio of 1 clay: 3 sand: Clay-Straw Plaster Typical Ratio 1 clay: 1 sand: 1 straw **Application** Hand application, then levelled with a float or trowel **Advantages** Very strong, resistant to cracking, bridges transitions well. Can be applied by an inexperienced or volunteer crew. **Indicated when** Substrate is weak, or transitions may cause cracking; or when volunteer labour is available. Clay-Sand Plaster Typical Ratio 1 clay: 3 sand: **Application** Trowel application, in two passes (see XX) **Advantages** Faster to mix and apply **Indicated when** Substrate is solid; an experienced crew is available; labour costs are high and/or schedule is tight; additives such as manure are available. Preparations Most preparations for earth plasters will be the same as for any other natural plaster (see chapter 3), but there are some exceptions.

Use curved corners for most outside-corners, because earth plasters are the weakest plaster and square corners are very prone to damage. Stuff and fill voids with slip straw or cob - bare straw stuffing may pull out of the wall during plastering. Avoid using metal in the prep, even galvanized metal, because earth plasters tend to cause rust. Metal fasteners will be inevitable in many situations (use coated or galvanized), but large areas of metal lath may cause problems. Because clay is hydrophilic, wood may not always need a barrier from the plaster to protect it from rot. Mixing and application details Base coats are usually mixed in a mortar mixer (p. xx). When a clay slip is used, start with all of the clay slip in place of water, then half the sand etc.

The final consistency of the plaster should usually be sticky and fairly wet, but not sloppy. If a mortar mixer isn''t available, or only a small amount of mix is needed, it can be mixed by hand. It is easiest to add the fibre last, after mixing all the other ingredients with a paddle mixer. The fibre usually needs to be mixed in by hand, or with a hoe etc. Earth base coats need pressure to bond well to the substrate. There are couple of ways to achieve this. The first is hand application; there''s nothing like hands to work plaster into the substrate. After applying roughly the desired thickness of mud by hand, it may be leveled and compressed, usually with a wood float.

This is the best (and often only) way to apply a very straw-rich base coat, though it is usually slower than trowel application. The second way to get a good bond to substrate is to use a ri.