The tea did not change
Pour one brew into three different vessels and the three will not taste alike, which is disconcerting the first time it is noticed and is not a trick of suggestion or expectation. The liquor is identical by construction; what differs is the delivery. Three things vary between drinking vessels, and each has a mechanism that can be stated plainly: how fast the liquid cools and therefore what temperature it reaches the mouth at, how the space above it presents aroma to the nose, and how much liquid arrives at once and where in the mouth it lands. None of these is mystical, all of them are large enough to notice, and all of them are under the drinker’s control. Knowing about them means a cup can be chosen to suit a tea rather than grabbed from whatever is nearest, and it also means that anyone comparing two teas should be careful to use the same vessel for both.
Temperature at the lip
A thin porcelain cup takes almost nothing from the liquid on the way in and then cools it quickly through its large exposed surface. A thick stoneware mug takes a great deal on the way in and then holds what it has, so the tea arrives cooler and stays warm longer. A wide bowl loses heat rapidly from its very large surface and is drinkable within a minute. Since perceived sweetness, perceived bitterness and the release of aroma all vary with temperature, three vessels deliver three different points on that curve from the same pot. It also explains why a small cup suits an aromatic tea: the drink is finished before it has cooled far, so every sip is taken near the temperature the brewer intended, whereas a large mug spans a wide temperature range between the first sip and the last and effectively serves several different drinks in succession.
Aroma, and what the opening does
Most of what is loosely called flavour is smelled rather than tasted, and how much of it reaches the nose depends on the volume of space above the liquid and the width of the opening it has to escape through. A narrow-mouthed vessel concentrates volatiles in a small headspace and delivers them in a column to the nose as you drink, which is the entire design principle of the tall aroma cup used in Taiwanese practice and the reason it exists as a separate object from the drinking cup. A wide bowl disperses aromatics into the room, which is pleasant to sit over and gives noticeably less at the moment of drinking. A mug with a large opening and a long cooling time loses a good deal of its aroma to the air before the drink is finished, which is one reason the last third of a mug of good tea is less interesting than the first.
Sip size, and where the liquid lands
A small cup delivers a mouthful that can be held, moved around and swallowed deliberately; a mug encourages a large swallow that goes straight to the back of the mouth. Astringency in particular is a tactile sensation that develops with contact time across the whole surface of the mouth rather than a taste registered at a point, so the same liquor sipped slowly from a small cup reads as more textured than the same liquor drunk in gulps from a mug. Sweetness and the returning aftertaste behave similarly, since both need a moment to appear. This is a large part of why gongfu practice uses small cups, and it is not merely ceremonial or a way of making a small volume of tea go round a table: the vessel enforces a manner of drinking that makes the differences between infusions perceptible at all.
The inside of the cup, which is not only aesthetic
A white interior shows the liquor colour accurately, which is why professional tasting sets are white porcelain and why cupping is done in standardised vessels. A dark, patterned or textured interior makes colour unreadable — and since colour is a poor guide to strength anyway, that is a smaller loss than it first appears, but it also removes the ability to see cloudiness, sediment, an oily surface film or an uneven pour, all of which are genuinely informative. For everyday drinking it hardly matters and a favourite cup is worth more than an accurate one. For assessing a tea, comparing two teas, or working out what a water fault is doing, a plain white interior is worth having and costs very little. The traditional preference for dark glaze with certain teas is an aesthetic tradition rather than a diagnostic one, and the two purposes are simply different.
Testing it on your own shelf
Brew one pot, pour it into whatever three vessels are to hand, and taste them in quick succession before the temperatures have diverged too far to compare. The differences are usually obvious immediately, and the exercise is far more informative than reading about it, because it calibrates the size of the effect for your own equipment and your own palate rather than for someone else’s. It also tends to correct the two opposite errors people make about this subject: dismissing it as suggestion, and overstating it as the most important variable in brewing. The useful conclusion is not that one vessel is best but that the vessel is a variable — worth holding constant whenever two teas are being compared, and worth changing deliberately when a tea is not showing what it should.