Building with Cob by Adam Weismann

Author:Adam Weismann
Language: eng
Format: epub
Publisher: UIT Cambridge Ltd.

The application of lime requires sensitivity to the weather and seasons

Cement can be applied in all weathers and seasons, whereas lime, to get the best results, can only be applied within a certain set of conditions (see next page). This encourages the fostering of a deeper relationship between you, the building, and the environment in which it is situated.

Lime has the ability to self-heal

Lime has the ability to self-heal any small, hairline cracks that may appear immediately after applying the plaster or render. This is because when the carbonation process occurs as it sets on the wall, (i.e. the calcium hydroxide in the lime reacts with the carbon dioxide in the atmosphere). Calcium carbonate is produced, which fills the small cracks. We also find that small cracks can be filled successfully with the slurry from the bottom of the lime wash bucket.

The production of lime and the lime cycle

The raw material for making lime is calcium carbonate, which occurs naturally in limestone, shell or coral. This is heated in a kiln to very high temperatures reaching 1,200°C. As it heats up, steam is driven off and then carbon dioxide, and a chemical change takes place, leaving calcium oxide in the form of white lumps. This is known as lump lime or quicklime, and is the raw material for making lime putty, which is what you will be using with sand to render/plaster your cob wall.

The process of making lime putty from the calcium oxide is called slaking, which involves adding water to the quicklime. You can purchase quicklime and do this yourself at home if you really want to go the whole hog, but it is a dangerous process which can burn your skin on contact or blind you if it gets in your eyes. It is much easier and safer to buy ready-prepared and matured (it must be matured for a minimum of four months – the longer lime putty sits around, the better quality it becomes) lime putty from a quality supplier here in the UK.

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