Thinset mortar
Also called: thinset, thin-set, tile mortar, modified thinset
A cement-based tile adhesive mixed from powder, used for glass, stone, large tiles, floors, wet areas and anywhere heat or moisture is a factor. Stronger and more moisture-resistant than premixed mastic, and the right choice whenever you're unsure.
Thinset is Portland cement, fine sand and water-retention additives; “modified” thinset adds polymers for better bond and flexibility and is what most backsplash jobs call for. It’s mixed to a peanut-butter consistency, left to slake for ten minutes, spread with a notched trowel, and cures by hydration over 24 hours. Use white thinset under glass and light stone — grey shows through.
Mastic (premixed organic adhesive in a tub) is the alternative for backsplashes. It’s easier: no mixing, immediate grab, longer working time. But it cures by air-drying, so it never fully sets behind non-porous glass tile, it softens with heat behind a range, and it fails in wet areas. Manufacturers limit it to ceramic and porcelain tile under about 8 inches on dry walls.
The rule: mastic is fine for a ceramic subway backsplash on painted drywall. Everything else — glass, stone, mosaics, large-format, the wall behind the cooktop — gets thinset.