I usually try to go to the ceramics showcase at Bussière Badil in the spring, when about 20/30 potters exhibit, and about 2 years ago I saw some delicate porcelain pots with what looked like engraving or carving. I asked the potter how he got this effect, and he was a little cagey, as you would be, if you had been working up a technique. He said it was done by painting the pot with a special paint, then wiping with a wet sponge. And that is basically it. The pot is thrown and turned, and while still green a design is painted using, in my case, button polish.
Care must be taken not to get the water all over the place, and not to wet the pot too much, or to wipe back the surface until the pot is too thin and collapses. And it is difficult to get a uniform thinness, to allow uniform translucency.
I have tried the same technique now with stoneware, a very different clay, I have ended up spraying the pots with a water jet, which is much more of a performance art.