[Cob] crack in plaster

[Jill Hogan]

1 part lime, 3 parts and and 1 earth is a great mix. We trowel it on (metal trowel), scratch coat as thin as possible and then a second or final coat which we trowel on as well. The mixture is wet like a cement plaster. We do not keep it wet and we have extreme temps. If you are getting cracks it means there is to much clay in your earth mixture so reduce it to 1/2 part rather than one. 
We have great plaster mixtures. Good luck with yours
Jill Hogan
MAT
023 625 1533
www.mat.org.za
We are sponsored by Cape Lime, Robertson.
  ----- Original Message ----- 
  From: Bob Smolen 
  To: coblist at deatech.com 
  Sent: Tuesday, September 16, 2008 8:21 AM
  Subject: [Cob] crack in plaster


  I have a small section of cob wall that I have covered with 2 layers of a coating of 3 parts sand , 1 part clay, and 1 part lime. I spread this on in a fairly dry mix and let dry for a week during a hot , dry spell 80+ temps. I sprinkled the wall with water each day but the plaster developed many thin cracks (spider web like). These cracks formed almost as soon as the wall dryed,in a day or so. Nothing fell off and the coatings seem to be adhering well.  Sunday I plastered with a mix of 3 parts sand to 1 part lime. The coating was not real thick, say 1/8 in. in most places. This ccoating has developed a few spider cracks. Should I be concerned or is this normal? Will an all lime plaster be a good final coat?
  The sand I have is fairly coarse but I have screened it down to 1/8 size. 
  I forgot to mention that I had straw/dry grass mixed in the first coats but nothing in the last 3/1 mix.
  Thanks for your comments,
  Bob
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