Immersed element: the electric kettle
An element inside the water heats it directly and is the most efficient arrangement available domestically, because almost nothing is lost to the room. It is also the least flexible: it heats water and nothing else, and the vessel is fixed. For nearly every tea drinker this is the correct default and the rest of this guide is about the cases where it is not.
Hob and flame
A gas flame or an electric ring heats the base of any vessel that tolerates it, which makes it the most materially permissive option. It suits stovetop kettles, saucepans for boiled and spiced preparations, and pots such as the Maghrebi berrad that go directly onto the flame. Its failure modes are boiling dry and heating a vessel unevenly. A flame licking up the sides of a pot is what destroys handles and lid knobs.
Induction, and the magnet test
An induction plate generates a current in the vessel itself, so the vessel does the heating and the plate stays comparatively cool. It is efficient and fast and it only works with ferromagnetic material. Cast iron and most stainless steel work; glass, clay, copper, aluminium and non-magnetic steel do not. The test takes a second: hold a magnet to the base, and if it grips firmly, induction will heat it. Do this before buying a kettle, not after.
Radiant electric plates
A glowing element under a ceramic or mica plate heats by radiation and conduction and therefore heats anything set on it. This is why the radiant tea stove rather than the induction plate is the common tea-table heat source in China: glass boiling pots, clay kettles and unlined iron all sit on it. It wastes more energy than induction and stays dangerously hot after switch-off with no visual signal, which is its one genuine hazard.
Charcoal and alcohol
Charcoal is slow, steady, laborious and produces carbon monoxide, so it requires real ventilation without exception. It belongs to practices built around it — Chaozhou gongfu, chanoyu, senchado — and is kept for the practice rather than for a difference in the water that anyone has demonstrated. An alcohol burner sits far below charcoal in output: it will hold a pot at temperature and will not boil a kettle in any useful time. Its flame is nearly invisible in daylight, which is worth knowing before reaching across one.
Thermal shock, the thing that breaks vessels
Most teaware failures under heat are shock rather than temperature: a cold vessel onto a hot plate, a hot vessel onto a cold wet surface, a dry clay body heated fast, a glass pot rinsed under a cold tap. Borosilicate glass tolerates high temperature and not sudden change. Clay tolerates neither if it is damp. Warming a vessel before filling it is a habit that matters more for thick and cheap ceramics than for thin and expensive ones.