The cow dung- mud plastering of the second house took a pretty long span of time for the completion although it had qualities such as better water resistance, better termite resistance, better in absorbing impacts and better antiseptic properties. Also to level the bumpy surface of the cob walls, we have to apply small quantities of the mix, layer by layer [NO CHISELLING], and that lead to a much longer time period for plastering. So it was decided to go for COB PLASTERING (mud plastering using cob mix) for all the 3 houses in the prototype community houses.
Earth, chopped straws of length 10cms and rice husk are added in the ratio 4:1:1. It is stamped and mixed well, to get the right consistency.
The wall surface should be made moist before applying the plaster.
Mix is applied and spread using hands or trowels.
The surface is then smoothened by applying water and brushing it. Cracks will appear for 2-3 days, so we have to cure it by hammering with a wooden hammer gently. Other ways to seal the cracks is by brushing with brush wet with water, or by applying small quantities of cob if the cracks are big.
1. The plaster is not water proof. So to tackle that, a cow dung wash or lime wash is applied after plastering. For cow dung wash won’t stay for longer periods and it’s not having a pleasing color, lime wash is preferred over cow dung wash.
2. If hair line cracks appear then they should be gently hammered with a wooden hammer with rounded edge. This will seal the gaps and we can achieve a crack free surface.
The important things we learnt:
The process was quicker compared to the previous mud-cowdung plastering experiment.
The bond between the walls and the plaster is much stronger as both are of the same material.
The walls will be much smooth, and leveling was way too easy.
We discovered that there is a slightly better way to do this by replacing rice straw with rice husk. This method avoids cracks and has improved workability. The procedure for this method is quite similar to the cob plaster. The technique is as explained below.
• The wall (especially if it is Cob) is chiseled in case of large irregularities.
• The portion of wall to be plastered is always wet properly with a water brush beforehand.
• An “attachment” or “scratch” coat allows the successive layers to adhere to the wall comfortably. This layer is very thin and has irregularities for the leveling coat to adhere well. The mix of attachment layer consists of 1 part of sand, 1 part of rice husk and 5 parts of soil.
• The next coat uses plaster of a slightly thicker consistency and evens out the undulations. The leveling mix consists of 5parts of quarry dust, 5parts of soil and 1 part of rice husk.
• A trowel and floater are used to even out the thicker mix over the wall.
• The final coat is applied quite thin and is gives the desired finish to the wall.
• If cracks appear, either too much water has been used or there is not enough sand in the mixture. Major cracks mean the plaster is hollow and needs to be redone, minor cracks can be filled by brushing them over with thick, muddy water or by gently hammering with a wooden hammer with rounded edges.