[Cob] crack in plaster

[Bob Smolen]

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