chemical strengthening
Chemical strengthening reaches the same goal as thermal tempering, a surface locked in compression, but by chemistry instead of a thermal shock. The idea is delightfully direct: swap the sodium ions in the glass surface for potassium ions, which are bigger. Cramming larger ions into the same-sized holes crowds the surface, and that crowding forces the outer layer into compression, the exact stress state that makes glass hard to crack.
In practice a glass part is bathed in a molten salt, usually potassium nitrate KNO3, at around 400 to 450 degrees C, which is safely below the glass transition so the shape never distorts. Over hours, potassium ions from the bath diffuse in and sodium ions diffuse out, ion for ion. Because a potassium ion is about 30 percent larger than the sodium it replaces (radii near 1.38 versus 1.02 angstroms), the swap wedges the surface tight and can build compressive stresses of 500 to 1000 MPa, considerably higher than thermal tempering achieves. The trade-off is depth: ion exchange only penetrates a thin skin, often tens of micrometres, called the depth of layer, whereas thermal tempering compresses a much thicker zone.
This method wins wherever glass is too thin for thermal tempering, which cannot build enough stress across a thin section. That is why the chemically strengthened cover glass on phones and tablets, the aluminosilicate glasses sold under names like Gorilla Glass, is made this way: thin, tough, and scratch-resistant. The honest limitation is that the compression layer is shallow, so a deep scratch or an impact flaw that pierces straight through it lands in the tension underneath and the strengthening is defeated, which is exactly why a dropped phone can still spider-crack from a sharp edge hit.
The cover glass on a smartphone is chemically strengthened aluminosilicate, only about 0.7 mm thick. A thin sheet like that cannot be thermally tempered effectively, so it is bathed in molten potassium salt to stuff its surface with oversized ions. The result survives being sat on and dropped on carpet, yet a hard hit on a grain of sand at the corner can still start a crack that dives past the shallow compression layer.
Ion exchange packs oversized potassium into a thin glass surface to build high but shallow compression, ideal for thin phone glass.
Chemical strengthening gives higher surface compression than thermal tempering but only in a shallow layer, so it resists scratches and bending yet can be beaten by a sharp impact that punches through the thin compression zone.