I first came across this technique during a trip to Seagrove North Carolina, a town with more Potters Per Capita (PPC) than anywhere else I know of. I stopped by the studio of Eck McCanless, whose work I was interested to see for his amazing crystalline glazes, but he told me he had moved on from crystal glazes to "Agateware" which was much more exciting. He gave a demo of how he does the technique, and I bought a cup with marbled patterns incredibly resemble an owl, and that cup is currently on my mantle in sight as I write this.
It wasn't until a few years later in the studio that I suddenly decided to try this technique, and I quickly realised that the first few pieces I made were, without thinking, very similar to the shape of the cup I'd bought from Eck, I immediately changed up the design when I realised, oops!
These pieces I also referred to "Agateware" which I consider similar to "Swirlware", by my definition Swirlware is a technique of mixing two or more clays and letting them spiral together on the pottery wheel, and Agateware is when we then carve through those spiralled clays to get interesting patterns to emerge.
Often these patterns resemble wood grain, or a Rorschach Test, and often all sorts of imagery seems to appear in the patterns. I usually like to carve vertical facets into the piece, this creates almost symmetrical patterns and with some effort we can just barely trace the spirals of clay as they climb up the piece.
After making a lot of Agateware pieces, one day I was standing behind my display of these at an art show and another potter walked up and said "Oh, you're making Neriage pots" and I hadn't heard the term, at the time Eck McCanless was the only person I'd known to use this technique, not that I thought it was original of course, just I hadn't ever seen it done through studying ceramics in college and having had a job at a ceramics museum.
Neriage and Nerikomi are two very similar techniques involving coloured clays. According to Robbin Hopper, the rough translation is "Neri" means "to mix" and "age" means "to pull up", pulling up referring to the act of shaping clay on the pottery wheel. And "komi" means "to press into" so Nerikomi refers to marbled clay hand-building techniques often involving molds, while Neriage is marbled clay thrown on the pottery wheel.
Many times I've been asked by other potters if I've had issues with the clay cracking along the seems where its marbled, and the answer is never once has even the tiniest hint of a crack occurred. I do carefully select clays for this technique tho. Technically we'll have good luck joining two different clay bodies together, that is much easier than getting a clay slip to apply well and have the same shrinkage, especially throwing clays will be similar to one another.
Personally, I always use two clays for this technique, a very dark brown clay with lots of iron, and a porcelain that has the same firing temperature. I selected these clays which both have 10% shrinkage over the course of drying and firing, and it's a good idea to check the shrinkage rates match. But more important is to make sure the peak firing temperatures match. If you make functional ware to be durable and safe for microwave/dishwasher use, then both of these clays need to become vitrified at the same temperature.
The other consideration is to select clays that don't have any course materials, no grog or sand, when these get carved those coarse materials will drag through and leave scratch marks. A porcelain can be used and coloured with mason stains, that's a good way to decide what colors you want and not have to worry about the shrinkage and firing temperatures having to match.
These pieces always look fine with a clear glossy glaze, but I've been really enjoying using crystalline glazes over them. Usually the glass is clear and the crystals are opaque and shimmer in the light, they can create a slight opacity that I really enjoy.
It is a gamble, because these pots are so time consuming, from trying to throw the piece to get the ideal density of spiralling and carefully carving just the right amount, plus putting handles on these mugs is super tricky cause I carve the piece dry then carefully resaturate it to add a handle, there are easier ways but they just don't look as good. And finally a gamble with the crystalline glaze which could obscure and cover those patterns that were so much work. But for the ones that turn out well, truly unique and impossible to ever replicate, with intricate details that can only be noticed when you turn the piece around in your hands, that a photograph can never do justice for, I think that gamble is worth it.