It's a very, very old Japanese pottery method. Essentially, the Potter shapes the piece, gives it a glaze coating...most of mine had a copper glaze..and fires it in a kiln. When it is removed, it is placed in (often) an aluminum garbage can filled with combustible material like straw. The lids to the can most probably have glass in them so the Potter can monitor the piece. When the hot piece of pottery meets the combustible material it makes a fire. The Potter closes and opens the lid as the fire burns the glaze. I'm confident an actual Potter would be much more informative.
I like the ones with the blues, mauves-to-red and golds because I feel I can see the flames on them as they are being worked. :)
My late-husband and I purchased our first one in Calistoga, California in 1990. I carried it in my lap the entire trip back to Texas.....as you can see it was no small package.
From that point I was hooked! All my other pieces are much smaller and were gifts or acquired on various trips but I enjoy each and ever one.
The little tiny urn on the glass shelf and the little plate at bottom were gifts....the one to the right of the tiny urn came from Sedona and was also a gift.
Now when I'm at festivals I see Raku in all colors and glazes, but I still love the 'firey' hues. :) :)
...until the next time from Tillie's Nest....have a great weekend.