[Cob] (cob) lime mortar

[Slan Lusan]

Hi Bob.
Are you sure it would have been lime? Sounds like, judging by its age and use of it as a barn could suggest maybe clay mortar was used and even possibly no mortar was used at all. Only buildings in scotland that age using a lime mortar were well appointed houses and larger buildings. Lime was readily available locally just about everywhere at that time but, they werent using it as a mortar in cottages, small houses and certainly not byres (barns) because of the cost. They quite often however only used lime as a limewash on stone and earthen cottages, small houses and byres and usually only because the occupiers landlord provided an annual bucketful to each of his tenants households in order to keep his properties maintained. 
On a 200 year old ruin on my croft the earth has done precisely what Mr. Switzer suggests it has sunk in places under the foundation over the years causing it to fail eventually. In many places on the ruin that is left the clay mortar is still in good condition. In fact, where i live there are several neolithic stone structures (burial cairns and brochs) still standing that are dry stacked and some have clay mortar as well. The workmanship though, of the stone masonry is impeccable i must add and all these buildings are round.
As was also suggested cement used as a mortar with stone will damage the stone and create moisture problems. You should always use a mortar softer than the stone. And the mortar should never be used to make up for poor stonemasonry. It is maybe also worth noting that Historic Scotland requires that all repairs to "listed" buildings are to be done without cement and with lime only and not for historical reasons but, because the damage caused to old buildings by repairs and maintanence using cement instead of lime is well known. The tens of thousands of stone and earthen houses in this country that are now extremely damp and stone eroded due to repointing in cement and/or re-rendered in cement are the testament to this.
-Stephanie