The problem is, many different kinds of mastics have been used over the course of a century, making it difficult to predict their exact ingredients and the methods that might soften them. Start with hot water. On floors, long handled garden edgers are a break from scraping with putty knives. Try heat or cold where water doesn't work. Modern mastics and those containing linseed oil may soften with heat from a lamp or hot-air gun.
Some readers report freezing mastic with dry ice breaks the bond with the substrate.
Resort to chemical solvents last. Start with the most innocuous solvent and test in an inconspicuous area before proceeding or increasing strength. Isopropyl alcohol, paint thinner, xylol, lacquer thinner, or paint stripper may yield results. Work with plenty of ventilation and proper safety precautions. All Rights Reserved.