Mastic

Mastic is the sticky layer under old flooring. One asbestos testing firm describes black mastic as the dark, tar-like adhesive used to glue down vinyl and linoleum floor tiles for much of the 20th century.

You cannot tell by looking. The same source says color and texture are not reliable signs, and that only laboratory analysis of a sample can confirm asbestos. Until you have results, treat it as if it contains asbestos, and do not sand, grind or aggressively scrape it.

This matters for any floor project in an older Missouri building. Pulling up tile is easy. Scraping the glue is where fibers can get loose. Hard material that gets sanded or ground can become regulated, as the entry on non-friable asbestos explains.

Test the tile and the adhesive. They can test differently. If either is positive, hire a certified crew for removal. Prep work such as concrete grinding comes afterward, once the layer is cleared.