There is an oily film on the surface of my tea

In short

This is not dirt, grease or residue, and washing the cup harder does nothing. It forms where tea compounds meet air in the presence of dissolved minerals, so it is a property of your water interacting with your tea rather than of either alone — which is why it appears reliably in some houses and never in others.

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. Hard water, which is the main variable

   Water chemistry

   Check: Does your kettle scale up quickly, and does the film appear on every tea rather than one?

   Fix: Carbonate-rich water is the strongest predictor. Filtering to reduce carbonate hardness reduces the film markedly. This is the same water property behind kettle scale and a flat-tasting green tea, so one change addresses three complaints.

2. 2. Standing time and surface area

   Technique

   Check: Does it appear on a cup left standing, or on a wide open pot, more than on a cup drunk straight away?

   Fix: The film forms at the air interface and builds with time. A covered pot, a narrower cup and drinking it sooner all reduce it. It is worst on a large open jug of tea left on a table.

3. 3. Strong black tea, which supplies more of the material

   Leaf qualityInferred by TeaHQ, not established

   Check: Is it a strong black tea or a stewed pot, rather than a light green or white tea?

   Fix: More extracted tea means more of the compounds involved. The film on a strong builder's tea in a hard-water area is close to unavoidable, and it is one of the reasons milk is added — it disperses it as well as softening the astringency.

4. 4. A trace of acid disperses it, which is the giveaway

   TechniqueInferred by TeaHQ, not established

   Check: Does a squeeze of lemon make it disappear immediately?

   Fix: If it does, this is what you have, and it confirms the diagnosis. That is a demonstration rather than a remedy — nobody wants lemon in every cup — but it settles whether you are looking at a water interaction or at something on the crockery.

5. 5. Actual detergent or grease, which behaves differently

   Equipment

   Check: Does the film appear immediately on plain hot water in the same cup, with no tea in it at all?

   Fix: Then it is the cup, not the tea. Rinse thoroughly and check the dishwasher rinse aid. A tea film needs tea to form; anything that appears on plain water is residue.

Tea types this applies to

• Black tea

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.
• Tea bag brewing — Pre-portioned tea in a permeable bag. The format was an accident of early twentieth-century sampling practice and became the dominant way tea is drunk worldwide, because it removes every decision except how long to leave it in.
• 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.

Related problems

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

• 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 tea gets drier and harsher as it cools — The first mouthful is balanced and the last one, several minutes later, grips the tongue and feels rough — from the same cup, with nothing added.
• My milk curdles or separates in the tea — Flecks or threads appear when the milk goes in, or the surface breaks into small islands, and the drink looks grainy rather than smooth.
• My tea disappears when I add milk — A tea that tasted fine on its own becomes a pale, vaguely sweet milk drink with no tea character left in it.
• Tea from the office boiler or the hot tap never tastes right — The same tea that works at home is thin, flat or stewed when made from a plumbed hot tap, a wall boiler or a catering urn.

Sources