You have probably scrolled past one. A mug with a tidy checkerboard on it, or a bowl with stripes so even they look printed. The natural guess is paint, or a decal, or something clever done with glaze, because that is how nearly every patterned object you own was made. Now imagine snapping that mug in half. The checkerboard is still there, running through the broken edge the way stripes run through a candy cane. The pattern is not on the cup. The pattern is the cup.
First the potter builds a loaf
The technique is called nerikomi, a Japanese term for building with colored clays. It starts with slabs of clay in two or more colors, stacked like a sandwich and pressed together until no air is left between them. The block is cut into strips, and the strips are turned, restacked, and pressed again. Do that a few times and the cut face shows a grid, each square a column of clay running the full length of the loaf. Two colors can end up as a hundred squares.
Then comes the part that looks like a deli counter. A thin wire is drawn through the block and a slice lifts away with the pattern on its fresh face. Each slice is a few millimeters thick. The slices are laid into a mold, or curved by hand into the wall of a cup, and joined at the seams. Nothing is drawn or painted at any point. By the time there is a cup to look at, the design has been finished for a while. The real work happened in the block.
Newer word, older craft
The term nerikomi only came into common use in the 1970s. The craft is far older. The Cleveland Museum of Art dates the kneaded two-color technique in northern China to the Tang dynasty, which began in 618.
Two names, and no clean answer
There is a sister word, neriage, and careful sources contradict each other about which word covers what. Neri means to knead or mix. Age means to pull up, as in pulling clay upward on a spinning wheel. Komi means to press in, as in pressing a slab into a mold. By that reading, neriage is wheel work and nerikomi is hand-building. But writers in Japan and elsewhere swap the two or use them interchangeably, and the potter Thomas Hoadley reports being told the terms are mixed up in Japan as well.
The craft is much older than either word. The Cleveland Museum of Art dates kneaded light-and-dark clay in northern China to the Tang dynasty, and some accounts trace marbling back further still, to Egypt and Rome. Japan has surviving marbled pieces from the Momoyama period, which ended in 1600, and from the Edo period that followed. In 1993 the potter Matsui Kosei was named a holder of an Important Intangible Cultural Property, Japan's Living National Treasure designation, for neriage.
Every mistake is sealed inside
Colored clays have to shrink at close to the same rate. Clay pulls in as it dries, then pulls in again in the kiln, and different recipes do it by different amounts. If the dark clay shrinks more than the light one pressed against it, the seams tear and the block comes apart, sometimes weeks after it was made. That is why potters usually start from one clay body and tint portions of it with powdered stains, instead of buying separate colored clays and hoping.
The other enemy is smearing. Every cut, press, and scrape drags color across the line it was meant to respect, so the crisp edges you see online come from practice and a lot of thin shavings. And there is no fixing it from the outside. You can wipe a bad brushstroke off a pot before it dries. You cannot reach into a wall. Worth saying plainly: this is hand-building rather than wheel throwing. Same room, same clay, a different set of skills.
You will not build a checkerboard block in an afternoon, and most studios do not teach this one. But a two-hour drop-in class in Chicago or Boston puts your hands on the material these potters are arguing with, which is usually where the interest starts.