|Re: Zi Manufacture|
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Ancient zi beads, and reproductions that are accurately made, all have at least two applications or treatments, that have typical predictable results.
One treatment, that is similar to staining (and is called "caramelization" and/or "carbonization") makes the agate brown or black. This, traditionally involves saoking the stone in a sugar solution, and then heating it.
The other process (formerly called "etching"—though this is wrong) involves "painting" (or penning) a design onto the surface of the bead, using a solution that is made with washing soda (called "sal soda") plant juice, as a thickener, and possibly lead. Having been fired, this creates the white or pale lines, circles, and spots, that become a permanent part of the stone.
Although some zi beads have "dark patterns," most zi are dark with pale patterns. Also no "dyes" are used. (Dyes are organic compounds; whereas chemical decoration uses minerals to impart color. Dyes are also temporary, and will not soak deeply into agate.) It is not always clear whether the beads are decorated first, or stained first. There were probably different procedures and results.
You can read longer descriptions of these processes in my 2002 article for Arts of Asia.
Be well—and Happy 2020. Jamey