The alkali used to etch glass is hydrofluoric acid (HF). It's important to note that HF is not an alkali; it's a strong acid.

Here's why HF is used for etching glass:

* Reactivity with silica: Glass is primarily composed of silica (silicon dioxide, SiO2). HF reacts with silica, breaking down the glass structure and creating a rough, etched surface.

* Selective etching: HF etches glass more effectively than other materials, making it ideal for creating intricate designs and patterns.

Important Safety Considerations:

* Extremely dangerous: HF is highly corrosive and can cause severe burns. It can also be absorbed through the skin and is extremely toxic.

* Proper handling: Always handle HF with extreme care, wear appropriate personal protective equipment (PPE), and work in a well-ventilated area.

* First aid: In case of contact, immediately flush the affected area with large amounts of water and seek medical attention.

If you are considering etching glass, it's essential to understand the risks involved and use HF responsibly.