The Drip Line Is The Product
People don't buy this cup because it holds coffee. They buy it because the glaze runs down the wall and stops somewhere different on every piece. Reactive glaze is why the cup photographs well and why your QC conversation gets difficult, because the feature and the defect come out of the same kiln. Your job as a buyer isn't to remove the variation. It's to define which variation you'll accept before the first carton ships.
Write The Tolerance, Not The Wish
"Similar to the sample" isn't a specification. Decide how far the drip line can sit from the rim, how much bare clay is allowed at the foot, and whether pinholes on the interior are acceptable. Then photograph three pieces: the lightest, the darkest, and the worst one you'll take. Send those photos with the PO. A supplier who argues about tolerance at the sample stage will argue harder in a claim, and a written range ends the conversation before it starts.
Where Reactive Glaze Actually Fails
Two places. The handle joint, where glaze pools and then chips when the cup is knocked, and the foot ring, where a thick glaze layer can craze after a few dishwasher cycles. Ask how the handle is attached, pulled and joined by hand or cast in one piece with the body, and ask whether the foot is wiped clean before firing. A wiped foot also means the cup sits flat instead of rocking on a saucer, which is the complaint nobody photographs.
Weight, Rim And The Cafe Test
A wide bowl cup drinks differently from a tall one, and a cafe notices in the first week. Check the empty weight, the rim thickness, and whether the cups stack without the handles colliding. Then do the boring test: fill one with hot water and hold it for a minute. If the wall is too thin, the customer feels it and puts the cup down. Reactive glaze hides a lot on a shelf. It doesn't hide a cup that burns your hand.