Steep black tea for 3 minutes 30 seconds at 95°C (203°F).
Black tea is fully oxidized. The leaf is withered, rolled hard enough to break its cells open, then left to react with air until it turns copper and finally dark brown. That reaction converts the fresh catechins of a green tea into theaflavins and thearubigins, which is where the malt, the deep color and the brisk edge all come from. Assam is malty and strong, Darjeeling lighter and closer to muscat grape, Ceylon citrusy, Keemun smooth and faintly smoky. Most tea drunk in the West is black tea, and most of it arrives already broken small inside a bag.
Black tea wants the hottest water of any true tea, 95°C (203°F), just off a full boil. Warm the pot first if you are making more than a single cup — cold porcelain can pull the water down twenty degrees before the leaf ever sees it. A teaspoon per 250ml cup, 3 minutes 30 seconds at medium, 2 minutes 40 seconds for a lighter cup and 4 minutes 35 seconds for a properly strong one. If you take milk, brew toward the strong end: milk mutes the tea, and a delicate steep disappears under it entirely. Add the milk afterward so you can judge how much the cup actually needs.
Letting the bag sit. A bag left in the mug while you drink keeps extracting tannin long past the point where anything good is left, and the last mouthfuls turn dry and puckering. Take it out when the timer sounds. The second mistake is water that is not hot enough — black tea genuinely wants near-boiling water, and an under-heated brew tastes thin and papery rather than bitter. The third is squeezing the bag against the side of the mug, which forces out exactly the harsh compounds you would rather leave behind in it.
Black tea is the poorest re-steeper on this list. Full oxidation and hard rolling mean the leaf gives up most of what it has in the first infusion, so a second runs pale and a little flat. Whole-leaf black teas — a good Darjeeling, a Yunnan gold — will manage one more decent cup if you add a minute to it; anything out of a bag will not. This is not a failing. It is simply what full oxidation does, and it is the same property that lets black tea keep for years in a tin while green tea fades in months.
Steep black tea for 3 minutes 30 seconds in water at 95°C (203°F), just off a full boil — 2 minutes 40 seconds for a lighter cup, 4 minutes 35 seconds if it is going under milk. Black tea is fully oxidized, so it takes hotter water than green or white without scorching, but it turns dry and puckering if the bag is left to sit.
Add it after if you are brewing in the mug, so you can see the color and judge how much the cup needs. Milk first was a habit of protecting thin porcelain from boiling water, and it makes no difference to modern mugs.
Because the leaf stayed in too long, or you squeezed the bag. Both push tannin extraction past the useful point — lift the bag out when the timer sounds and let it drain rather than pressing it against the mug.
Roughly 40 to 70mg per 250ml cup, about half a cup of coffee. It is the most caffeinated tea on this chart, partly from the leaf itself and partly because it is brewed hot and strong.