Probably the most important part of the job. Finishing the building trying to keep it water proof and yet breathe. Lime plaster forms a light durable surface, resistant to weather and mold. Especially useful if applied to earth walls as it shrinks and swells in a similar way to earth. It protects the walls from rain, but it also allows the walls to dry out. Humidity control. Using Hydrated Lime (Type S and N) it will set and harden through drying out and absorbing CO2 from the air. (Air lime). They are the softest, most breathable limes available. Having said that, there are varying qualities of type “S” lime, depending on the source. The type “S” I am using for the demo has been slaking for over a year. (Slaking in water. Add the powder to the water. N.B.) The next task is to find good quality lime and use a barrel oven (80 gal. With an Adobe oven piggy backed on top for food cooking!) to make air lime? Its a technical engineering nerd thing for me to make lime putty. That’s what we do though…. I have a twin that does the same thing… I purchased a stucco sprayer. See “resources” for information. Fantastic. As a test the cob oven was stripped down and the base shot with a lime/sand/wheat straw mix. The straw was soaked overnight and could have been cut shorter?
Image on left shows first coating directly onto polypropylene bags. Stuck like crazy!
So this is the Mortar Sprayer purchased from mortarsprayer.com. Nice folks to deal with and great backup. It runs well at about 145 p.s.i. and I run on a 100 ft. 3/8 ” airline. I may put a 1/2″ line in front of the smaller line to have 150 ft.from the compressor coverage. All you do is make a mix in a cement mixer as per the video, and scoop it out of a drum/wheel barrow that you pour the finished mix into. I purposely mention and use a cement mixer as there are those out there who suggest that only a “mortar mixer” will work. Seeing is believing baby… The mix, by volume (say buckets) … used 1 type “S” lime, 2 sand and one wheat straw that had soaked over night to soften it. It splutters and coughs a little when mixing, so wear protective clothing, especially eye protection. I had a bottle of vinegar water on hand , just in case. (Just in cases?) Lime has a PH of 12, so its caustic and will burn quite badly. Eye protection is essential as you will see when it splutters a little. The snake in the background did no harm as it is made of plastic…. Cat deterrent. Wishful thinking. Ha ha.
Check out the sprayer in action in the short clip: By volume, 2 sand, 1 lime some straw. It’s a thin coat and was later sprayed again. The straw tends to discolor the final appearance, but bear in mind that straw will only be put in the first inch or so. Final coats will be sand and lime only. Natural Hydraulic Lime will set under water. Typically NHL3.5 and NHL5 is used. We will be using Lime that sets in air. Typically type “S” and type “N”. Please research the matter. I also got some very good advise from Randy. We are talking spraying the adobe on: The mix in general should have the finish coat weaker than the initial coat. Start with a stronger mix, then gradually go to a weaker mix by adding more sand. So the first application about 1 inch thick would be 1 Lime 2 Sand by volume, then end up with 1 Lime to 2.5 Sand on the third coat. (One would assume the second mix would have 21/4 sand?.. tut tut…) Sorry can’t help it….. Anyway, allow the layer to cure at least 2 weeks between coats. I wanted to know if we could mix and match NHL 3.5 with type “S”. That is: start with a NHL3.5 mix on the first coat and do a final coat of type “S” blended with NHL 3.5, to cheapen the overall plaster cost. Perhaps not good practice he suggested. The reason we use a stronger mix on the interior (first coat) is that if a weaker air lime mix is used it will not carbonate as well as it would on the surface. there are some papers related to the rate of carbonation such as this one.
The vacuum has a metal impeller, so it should have a good life? We will see. Next trick is to use it picking up the straw in the trailer. Save bagging twice and it’s shredded. The finished product has a few long straws in it but after overnight soaking, it should be fine. We operate the spreader at 145 p.s.i. and that should shoot it out no trouble? In the background is the initial layout for the rocket heater for the lime making. The bread oven was removed for a rebuild and upgrade.
Spraying the straw/sand/lime mix. This clip shows the Lime/sand/ little straw mixing. It’s applied over the remainder of the base in the clip above. Applied to bare bags, chicken/turkey (for Floridians) mesh covered bags and clay based adobe plaster. It was also sprayed vertically and upside down to test adhesion and operation of the sprayer. Two people doing the job would work great. One mixing the other spraying. Very happy with the overall performance! Time saver is what it’s about really. The straw will be washed and soaked. A Borax solution will be added to the mix. Mold in Florida. The lime will take care of most of the unwanted I’m sure. That mixer has worked very hard and has mixed all the dome bags. Just the right size. Brother John insisted on buying another. So I did. It’s still in the box at $180. He owes me? We wanted a thin initial mix, about 1/2″ .. to tell the truth, I only had 2 buckets of lime to play with. Curing: .. Ignore this free advice at your own risk! Air Limes need to go through dry and wet cycles to cure properly. It is recommended to drape burlap near, say approximately 1″ – 2 ” from the face of the lime mix sprayed. This allows air flow while keeping the local humidity high and protected from drying out from wind and sun. Carbon dioxide, dissolved in water, is needed for the carbonation process. So climates having cycles of wet and dry is good for the process. In drier places, you may have to wet down a little? The curing practice is important if you want a good final finish. The dome has two sets of “sticky outies” man… used during construction for scaffolding and awnings. They can be used to drape burlap as Randy recommends. Sand: The plaster needs to be able to key to the surface well. So smooth (round) sand is a no no. Use well graded sand, having many different particle sizes. Also, use a wooden trowel when applying your plaster. Metal trowels will tend to lift the lime particles to the surface and promote cracking. So… if you can obtain sea shell sand (95% calcium remember) would this not be the way to go for a brilliant finish? If that is what you want. Good tip from Kaki and Donald is to pre-moisten an earthen surface using water that has been treated with a small % of lime. The outer clay surface will create a chemical bond between the earthen substrate and lime plaster. As we go along, I will post actual footage of the whole process including slaking etc and building the rocket firing devices. Lime Wash. Great publication by Peter Mold and Richard Godbey. Extracts are included here. Limewash may be made from lime putty by the addition of water. About 15% lime to 85% water. It should be mixed long enough to assure that all the lime is in suspension. Many thin layers is better than fewer thicker layers. Alternatively, put about 30 gallons of water into a trash can and add 50lb. of hydrated line. Stir with a mechanical stirrer. This gives a lime wash mix without going through the putty stage. Upon application, it is translucent and as it dries, it becomes opaque. If you would like to color your limewash, try using oxide-based pigments as is used in masonry mortar. Pale and pastel shades work best. I used proper lime wash brushes whilst living in Turkey and hope to find decent brushes in the States, probably Mexico, where adobe is pretty widespread. Lime wash tools are dirt cheap overseas, as is bagged ready to use lime wash. The homes that are lime washed there get a coat inside and out, every year. Fills any cracks, takes care of mold too. Its a fantastic product.