Portuguese historic buildings were mainly built using lime mortars, not only as renders, but also as bonding material, together with stone and bricks. Preserving this heritage requires, as in other countries, discovering traditional techniques and adopting them, as much as possible, to new realities and experiences. This project involved research on traditional techniques used as renders and plasters in the 20th century buildings of Alentejo, a region of southern Portugal. There are a few traditional kilns still working, smalt family firms that go on from one generation to another, which were visited to deepen knowledge of lime making. This, plus consulting projects and oral information, including inquiries made to masons and other workers who used lime mortars, led to a more experimental level. The study of physical, chemical and mineralogical lime constituents also provided a better knowledge of lime. Mortars were tested in the laboratory with mixes obtained during the research and applied in old and new buildings.