Why red tea tolerates heat
Red tea is fully oxidised, which makes it the most heat tolerant category. It will survive a full boil, and most of it wants something close to one.
That tolerance is convenient, and it is also why the category gets brewed carelessly. The leaf gives up everything quickly, and the window closes fast.
What temperature controls here is the balance between the oxidised polyphenols that give brightness and body, and the leftover catechins that give astringency.
Water at 95 extracts the first well and the second slowly. Water at 100 for six minutes extracts both, and the cup turns harsh.
The practical point is that red tea cares less about the exact number than green does, and more about how long the leaf stays in.
The comparison worth holding on to is with green tea. There, hot water is the enemy. Here, it is time that does the damage.
Temperature by red-tea style
Most Chinese red teas, including Keemun and Dian Hong, want 90 to 95. That is hot enough for body and cool enough to keep the sweetness.
Lapsang Souchong, which is smoked over pine, wants a full boil. The smoke needs the heat to come out properly.
Darjeeling first flush is the real exception. It is only lightly oxidised and it wants 85 to 90, like a green-leaning tea.
Assam and other strong, malty origins want 100, particularly if you are adding milk.
Broken grades and tea bag blends are built for boiling water, and they are the one case where it is unambiguously correct.
Faced with a tea you have not met before, start at 95 and adjust. It is the safest single number in the category.
A thermometer removes the guesswork for very little money. Kettles that cut off early are common, and 90 is not the same cup as 95.
Water quality and hardness
Red tea is brewed strong, which makes it more tolerant of minerals than a delicate green, but not immune to them.
Total dissolved solids of 50 to 150 parts per million is the sensible range. Above 200, the liquor turns dark and flat.
Hardness matters more here than you might expect, because calcium reacts with the oxidised polyphenols and dulls the brightness.
Around 30 to 80 milligrams per litre as calcium carbonate is comfortable. Much above that and the cup loses its edge.
If your water is hard, a filter jug is a cheaper fix than buying better tea, and the difference is easy to taste side by side.
Minerals and temperature are independent variables, and fixing one will not rescue the other.
Practical brewing steps
Bring water to a boil and let it stand for 30 seconds, which lands you near 95.
Warm the pot first. A cold ceramic pot drops the temperature by five to ten degrees the moment you pour.
Use 5 grams per 200 millilitres and four minutes, then lift the leaf out immediately.
For a gaiwan, use 5 grams in 120 millilitres, boiling water, and 15 second steeps. It will run six or seven rounds.
Taste at three minutes rather than four. You can always pour later, and you cannot undo an over-brewed pot.
Keep a note of the dose that worked for each tea. Red tea is the category where that habit pays off fastest.
Warm the cup as well as the pot. A cold mug takes five degrees out of a 200 millilitre pour, which is more than it sounds.
Common mistakes
Leaving the leaf in is the biggest mistake. A pot with the leaf still in it keeps brewing, and four minutes becomes ten.
Under-dosing is the second. A teaspoon in a mug is about 2 grams, which is well under half of what the tea needs.
Boiling water on a delicate Darjeeling is the third. It is the one red tea that behaves like a green one.
Brewing weak and adding milk is the fourth. Milk covers a thin brew rather than fixing it.
Fixing astringency by lowering the temperature instead of shortening the time is the fifth, and it usually makes the cup thin rather than smooth.
When a cup is wrong, change one thing at a time. Time first, then dose, then temperature.