There are black spots inside my teapot or infuser

In short

Every one of these is a place that stays wet after the rest of the vessel has dried, and that is the whole explanation. The distinction that matters is between staining, which is cosmetic and belongs there, and growth, which does not — and the two look similar in a spout you cannot see into.

Likely causes, in order

Most likely first. Answer the question under each one before moving on — several of these look alike in the cup and are told apart only by what was done to the leaf.

1. 1. Staining rather than growth, which is the commonest case

   Equipment

   Check: Is it a smooth even brown or grey-brown film that follows where the tea flows, and does it feel like part of the surface rather than sitting on it?

   Fix: That is tea stain and it is not a hygiene problem. It has its own record. On unglazed clay it is actively wanted; on porcelain it comes off with bicarbonate of soda and a soft brush if you dislike it.

2. 2. Growth in the spout, where nothing dries and nothing is visible

   Equipment

   Check: Is it discrete dark spots rather than an even film, in the spout or under the lid seal, and does the pot smell musty when first opened?

   Fix: A spout holds a plug of water after every pour. Shake the pot out over the sink, stand it on its side or upside down, and store it with the lid off. To clear what is there: a warm solution of bicarbonate of soda, a bottle brush through the spout, and a thorough rinse. Repeat rather than escalating to bleach on anything porous.

3. 3. The gasket, which is where travel bottles fail

   Equipment

   Check: Is this a flask or travel infuser, and have you ever taken the lid apart?

   Fix: Silicone gaskets lift out and almost nobody removes them. Underneath is a permanently wet, dark, unrinsed space. Take the lid apart, wash the parts separately, dry them open, and reassemble only when dry.

4. 4. Leaf left in the vessel

   Technique

   Check: Do you empty the pot straight after use, or does the leaf sit until the next session?

   Fix: Wet leaf left in a closed pot is a small damp compost heap. Empty and rinse immediately after the last infusion. This is the habit that makes the rest of it unnecessary.

5. 5. Unglazed clay, where this is the one problem that does not resolve

   Equipment

   Check: Is the vessel unglazed clay, and has it been stored closed and damp?

   Fix: Porous clay grows mould inside the body rather than on it, and boiling and rinsing help without finishing the job. Air-dry with the lid off after every session. Where it has already happened, the honest options are to dedicate the pot to a robust tea or to retire it.

Brewing methods this applies to

• Western brewing (teapot or mug) — A small amount of leaf in a large volume of water, steeped once for a few minutes and then strained off. The approach most English-speaking drinkers already know, and the one most teas sold in the West are made for.
• Gongfu brewing (Yixing pot) — The same short-steep, high-leaf approach as gaiwan gongfu, carried out in a small unglazed clay pot that seasons over time and is dedicated to one tea.
• Mug infuser brewing — Loose leaf in a basket inside a mug — the most common way loose tea is actually made, and the method most often responsible for people concluding that loose leaf is not worth the trouble.

When this cannot be fixed

Unglazed clay that has grown mould cannot be reliably cleared, because the growth is in the body of the material rather than on its surface. Glazed and metal vessels can always be brought back.

Related problems

Sharing a cause category, a tea type or a brewing method with this one.

• My tea goes cold before I finish it — The cup is lukewarm within a few minutes, and in a gongfu session the later infusions arrive noticeably cooler than the early ones.
• My tea tastes of chlorine, metal or the kettle — A chemical, swimming-pool or metallic note sits over the tea, and it is there whichever tea is brewed.
• My teapot came out of the dishwasher damaged or tasting wrong — A pot or cup returns from a dishwasher cycle dulled, with faded decoration, a fine web of surface cracks, or a persistent taste that was not there before.
• There are brown stains inside my teapot and cups — A brown or grey-brown film builds up on the inside of the pot and in the bottom of cups, and rinsing does not shift it.
• There is an oily film on the surface of my tea — An iridescent or dull skin sits on the surface of a cup or pot, breaking into patches when disturbed and clinging to the side as the level drops.

Sources