Burnt sand mastic was traditionally made by putting sand in the oven to thoroughy dry the sand, before adding it to linseed oil to create another old linseed oil based marvel, a traditional sealant used for centuries in historic buildings. It's a flexible, weather-resistant material perfect for sealing joints in masonry, around windows, and in other critical areas. If you own an old building, you need it to maintain integrity and prevent moisture ingress, preserving its historical fabric.