Foam, and the compounds associated with it
A vigorously poured or whisked tea produces a foam, and the compound group associated with it is the tea saponins, a class of surface-active compounds present in the leaf and in the seed. Surface-active compounds lower the surface tension of a solution and stabilise the thin films between bubbles, which is what allows a foam to form and to persist, and the same class of compounds is responsible for foaming in a number of other plant materials. This is a well-supported association rather than a convention. The saponins are also described as contributing a slight bitterness and a certain textural quality, which places them among the minor contributors to a cup rather than among the defining ones. What is worth knowing practically is that foaming varies substantially between teas and between preparations, so the presence or absence of a head is not in itself a quality signal, and a tea that foams more is telling you something about its composition and how it was poured rather than about how good it is.
Matcha, where the foam is the object
The one preparation built around this property. Whisking matcha in a bowl with a bamboo chasen incorporates air into a suspension of ground leaf, and the surface-active compounds present stabilise the resulting foam. Usucha, the thin preparation, is whisked to a fine even head; koicha, the thick preparation, uses far more powder and much less water and is kneaded rather than whisked into a smooth paste with no foam at all — the two are different preparations of the same material with different targets, and it is worth knowing that a head is the goal in one and not in the other. Different schools of Japanese tea practice also differ in how much foam is considered correct for usucha, which means a photograph of a densely foamed bowl is a statement about a tradition rather than a universal standard. Whisking technique, water temperature, the fineness of the powder and how thoroughly it was sifted all affect the result, and a poor head is more often a preparation problem than a materials one.
The oily film, and what it is not
A thin, iridescent film sometimes appears on the surface of a strong tea, and it is a different phenomenon from foam. It comes from lipid material in the leaf and from the surface-active compounds already present, spread as a very thin layer that produces interference colours in the same way a film of oil on water does. It is more common with certain teas, with high leaf ratios and with very hot water, and it is a normal property of a well-extracted liquor rather than a sign of contamination. A related and distinct effect is a film left by a vessel that has previously held something oily, which is a cleaning issue and is distinguishable because it appears regardless of which tea is brewed. The general point is that a visible film is worth identifying rather than worrying about: a thin iridescence that appears with a strong brew and not with a weak one is coming from the leaf, and one that appears with every brew including plain hot water is coming from the cup.
Scum, which is a water problem
The dull grey-brown film that forms on the surface of a mug of black tea, and which clings to the sides of a cup and the inside of a pot, is a different thing again and it is largely a property of the water rather than of the tea. It is associated with hard water — water carrying dissolved calcium and other minerals — where the mineral content and compounds from the tea form an insoluble surface layer. This is why the same tea produces a heavy film in one part of a country and none in another, why it is worse when water has been boiled and held, and why filtering the water addresses it where changing the tea does not. It is also why the effect is so strongly associated with black tea in hard-water regions, and why visitors to such places notice it immediately. The remedies are all on the water side: a filter jug, a different water, or a shorter interval between boiling and brewing. Distinguishing this from the leaf-derived film is straightforward, since the mineral version is dull rather than iridescent and appears whether the brew is strong or weak.
Brown staining, sediment and the rest of the surface phenomena
Two further things get grouped with these and belong elsewhere. The brown staining that accumulates inside a teapot or a mug comes from the oxidised polyphenol material depositing over repeated use, is accelerated in hard water, and is a cleaning question rather than a brewing one — the catalogue keeps a separate troubleshooting record for it, including the argument about whether a seasoned pot should be scrubbed. Sediment in the bottom of a cup is a filtering question, being fine leaf material that passed the strainer, and it is expected with a deep-steamed Japanese tea and a broken grade and unexpected with a whole-leaf tea through a fine filter. Neither is a fault in the tea. What all five phenomena on this page have in common is that they are visible, that people reasonably assume something has gone wrong, and that the actual causes are unrelated to one another — which is why identifying which one you are looking at is the whole of the diagnosis.
Why any of this matters to a taster
Because the surface of a liquor is part of what a taster reads and part of what changes the drink. A foam holds aroma above the liquid and changes how much reaches the nose on the first sip, which is one reason a whisked bowl and an infused cup of the same material read differently. A film alters the surface and slightly changes mouthfeel. And the mineral film of a hard-water region genuinely changes the drink, since the same water that produces it also flattens aromatics and adds a drying grip, which is the largest single under-diagnosed influence on how tea tastes in any given house. Anyone whose teas have all seemed to change at once should suspect their water before their storage, and brewing a known tea with a different water is the ten-minute control that settles it. That is the same recommendation the mineral and disagreement pages make, arrived at here from the least likely direction, which is the film on the top of a cup.