Fourteenth century jointing mortar in old Finnish fortifications has remained very durable to the present day. An effort has been made to imitate such mortar by adding calcinated clay to ordinary lime mortar. The results of two different laboratory tests of these imitations have been contradictory. Further investigations are going on to find suitable joint -- and plaster mortars for the maintenance and repair of old buildings in Finland. See also AATA 19-1792.