Raku glazes are low-firing glazes designed to melt fast and then be shocked. What happens after the piece leaves the kiln matters as much as what happened inside it: the reduction in a bin of combustible material is what blackens bare clay, darkens crackle lines and pulls copper towards metallic lustre.
The crackle is the point
A raku glaze crazes because it is cooled violently, and the smoke from the reduction chamber drives carbon into every crack. The result is a white or coloured glaze webbed with black lines and bare clay burned to matt black around it. That contrast is the signature of the process and it is produced in the bin rather than the kiln.
Copper, and the unpredictable lustre
Copper-bearing raku glazes can produce metallic and iridescent surfaces when reduced hard and then sealed from air. The same glaze can give a flat green if it meets too much oxygen on the way out. Controlling that is a matter of how quickly the piece goes into the bin, how completely the bin is sealed and when it is opened, and even experienced raku potters treat some of it as luck.
Why raku is not functional ware
The body stays porous, the glaze is crazed through to it, and the surfaces are neither vitrified nor sealed. Water passes through, staining gets in, and nothing about it is suitable for food or drink. This site does not describe any glaze as food safe except by quoting a manufacturer's data sheet, and for raku that question does not arise.
Safety, which is different here
Raku is the one process on this shelf where the hazard is fire and heat rather than dust. It is done outdoors, with proper tongs, gauntlets, eye protection and a clear area, with the reduction bin lidded and controlled and water to hand. Reduction bins produce a lot of smoke, so neighbours and location matter as much as equipment.
Questions people ask about raku
What makes raku glaze crackle?
Rapid cooling as the piece is pulled from the kiln, followed by smoke driving carbon into the cracks so they show black.
How do I get copper lustre?
A copper-bearing raku glaze, a hard reduction in a sealed bin, and some luck. Too much air on the way out gives a flat green instead.
Is raku ware waterproof?
No. The body stays porous and the glaze is crazed through to it, which is why raku is decorative rather than functional.