Mollie Curry: Green, greener, greenest

Green, greener, greenest

By Mollie Curry on 03/06/2012

Almost every building project — whether it is new construction or a remodel/renovation — will use some kind of interior wall finish, often over drywall or already-painted surfaces. If you are thinking green, you’ll want to know that your options have expanded way beyond big-box store lines of “low” or “zero” VOC (volatile organic compounds) paints to include several degrees of greener possibilities.

Though those mega-brand paints are certainly an improvement in terms of health and the environment over “regular” paint, even the zero-VOC designation refers only to the tintable base. Adding colorant increases the VOCs, sometimes significantly. And they are still petrochemical-based products. Not only that, but just being designated zero-VOC does not mean nontoxic. For instance, acetone is just one of many chemicals not regulated in paints under the VOC requirements.

I am by no means a chemist, and the world of paint chemistry — whether for conventional paint or more sustainable, eco-friendly, healthier paint — is very complex, so I am going to stick to the basics. Several companies now make paints that are similar to “regular” latex (and their reduced-VOC versions), but are significantly less toxic. 
Toxic ingredients in conventional paint are the target of many of the alternative eco-paint companies — they have cut out the toxins for the benefit of human and planetary health.  Alternatives have been on the market for more than 20 years that replace many petroleum-based ingredients with plant-based ingredients, such as soy or linseed oil.

In terms of how they look on the wall, these paints are close corollaries of the petrochemical paints they were designed to replace. Besides being healthier and better for the planet, they do have other differences, sometimes in coverage ability, ease in cutting in, washability and cost.

Milk paint, which uses milk protein (casein) as the binder, is another option. It is an old technique that produces a very durable and nontoxic paint that has a different look than most modern-day paints — it tends to be chalky or mottled. A couple of companies sell it or you can make it yourself. The chemistry is pretty easy.

Finishes made from clay and sand 
Natural-building methods such as straw bale, adobe, cob and slip-straw also include remarkable wall finishes over conventional materials. These finishes look and feel different from the paints I’ve mentioned so far, and are some of the greenest alternatives possible: clay-based paints and plasters. 
These products have unique texture and depth, which create a certain quality of light. They also are completely natural — no petroleum, no VOCs, nontoxic ingredients, potentially local and, often, a low embodied-energy impact. They can help balance humidity and are mold-resistant. On the down side, these finishes will not withstand kids with toy trucks banging into them and do not let go of stains easily. They are more labor-intensive, too.

Clay-based paints 
Clay paint is relatively smooth (no sand) and has a matte finish. The manufacturer says it simulates the look of traditional plasters. It’s also a very environmentally friendly product. But it’s not your only choice for earthen paint: You can make your own or find a local artisan who knows how. 

The main ingredients in homemade earthen paints (aka “alis”— meaning “to smooth”) are powdered clay and fine sand or some other even finer aggregate like powdered chalk.  Shiny flecks of mica occur in some sand sources naturally or can be added for a subtle or dramatic sheen. Mica on the wall is surprisingly attractive and does not come across as glitzy. Another fun addition is finely chopped and screened straw.

Wheat paste (like old-fashioned wallpaper paste) and powdered milk (for the casein) are modern additions to the traditional recipe — they increase durability. If you use nontoxic pigments, the paint is completely nontoxic. That said, you never want to breathe dust while you are mixing — mineral dust can cause serious disease.

Clay paints give a variety of beautiful textures to what would otherwise be a plain flat wall.  Depending on ingredients and application techniques, the resulting finish can be quite smooth or with brush marks, sponge marks, finger marks, etc. It is pretty easy to create interesting textures without the final product being gritty or shedding. Burnishing with a sponge is a step that strengthens the surface and increases durability.

Earthen plasters are readily available 
One of the main differences between earthen paints and earthen plasters is the way they are applied — plasters are usually put up with trowels, which is somewhat more labor-intensive than doing earthen paint. Troweling takes a bit more skill than painting or sponging, but is still within the realm of possibility for handy novices. 

Though you can make your own earthen plaster, you can buy clay-based plaster in bags as well. You don’t have to source and measure and experiment — just mix with water. There is lots of information on manufacturers’ websites about how to do it and how to prepare various substrates.

Earthen plasters come in many colors and textures. A very smooth surface can be created as well as a more rustic one with many trowel marks and skips. A sponge finish requires less skill and leaves an interesting texture. Subtle mottling is common even when using only one color. Two or more colors can be artfully combined for even more amazing outcomes. The results are regularly stunning with both the manufactured product and homemade earthen plasters. You can see some of the possibilities at several local retail outlets.

The above-mentioned plasters are designed to go on conventional substrates, like drywall, in a very thin layer. However, there is also a wealth of possibilities in making your own bulkier, clay-based plasters from clay, sand and straw native to your area. Embarking on this adventure involves learning and experimenting — and way more work than buying it in a bag. But it is much cheaper, and through the process you become more connected to the world. (Mud therapy! Ultra-local and low-embodied energy!) This plaster can even be used to shape decorative borders around features such as windows, to build strong shelves and ledges or even to create bas relief sculpture.

So the next time you think about painting, consider some of these alternative wall finishes.  They all aim for a more sustainable and healthy world. If you are overwhelmed with the choices, ask someone with experience with the various brands and methods to advise and offer recommendations for your project — this could be retailers, contractors or skilled tradespeople. Some local artisans and companies offer the less-common methods as part of their services. Look up books on natural plasters and paints — they have wonderfully inspiring photographs of various projects. Many people learn how to do their own natural paints and plasters through reading and hands-on classes — be sure to keep an eye out for these if you are interested.

Mollie Curry of MudStrawLove has 15 years of experience with natural finishes. She and her partner, Steve Kemble, can paint, plaster and bas relief up your house, but also can design and help build your house — especially if you want straw bale, cob, or earthbag walls.They also teach workshops on all these methods.

Primer: How lime compares to cement and clay

You know those gleaming white buildings of Greece and the British Isles? They use lime in the form of plasters and paints. This special lime has been heated to produce a chemical change; it’s used as a protective and decorative finish — inside and out. Lime-based plasters and paints (known as lime wash) have stood the test of time on many historic buildings. Mortar is also made with lime and sand. “Burning” limestone to make building lime is a very old technology, dating back at least to Roman times — the tagline for one of the lime suppliers is “Since 30 BC.”

Making Portland cement is similar in that a high heat is needed, although the two products have important differences. Both produce plenty of greenhouse gasses during manufacture (from the burning of fossil fuels), which means they have a higher embodied energy than materials that do not use high heat in their production (like clay). Carbon dioxide is also driven off the limestone as a result of the chemical reaction that turns it into a reactive substance that can harden again when mixed with water. The difference is that the building lime slowly reabsorbs the carbon dioxide it lost and becomes limestone again, whereas the carbon dioxide that was driven off the Portland cement is not needed for it to become hard; it does not reabsorb it. So even more carbon dioxide enters the atmosphere from making Portland cement than from making building lime.

Lime-based plasters are softer than cement-based ones, but harder than clay-based ones.  Since lime-based stuccoes are more vapor-permeable than cement stuccoes, they are less likely to trap water behind them and cause deterioration of the substrate. This is also true of clay-based plasters, which are almost as vapor-permeable as lime. Both lime and clay plasters resist mold, but lime is more resistant because of its high alkalinity. 

When working with wet lime, the alkalinity is a danger to skin and eyes. You must wear protective gear or risk significant chemical burns. This is one way that earthen plasters and paints are easier to work with than lime and cement ones. Other factors are the “working time” and repairability. Earthen plasters are easier to deal with for both of those issues.  Lime and cement are less forgiving of unskilled workers.

Clay-based products do not need to cure, just to dry, unlike both lime and cement products.  Lime takes longer to cure and is more finicky about the level of moisture that needs to be maintained in order for it to cure well.

Lime wash and lime plasters do not need to remain white — pigments can be mixed in, as they can with earthen plasters and paints. Interestingly, brighter colors are possible with lime-based products, but the alkalinity is incompatible with some pigments, so only “lime-safe” pigments can be used.

On the “green-ness” scale, I see clay-based products as the greenest (lowest embodied energy, nontoxic), followed by lime-based, then cement-based. Resistance to impact and weather runs in the opposite order: cement, lime, clay. This matters less with interiors than exteriors, and buildings can be designed to use even the least weather-resistant coating (clay) effectively on the exterior. The greater vapor-permeability of the lime and clay are an advantage over the cement. If cracks occur and water gets in, it can evaporate out when the coatings are not cement. However, water trapped behind cement stucco has had disastrous effects on many buildings. Also, lime and clay tend to moderate odors and are less acoustically “hard” than cement. — M.C.