Jakub,
I see that this vessel (if it is the large vessel that is featured when you click the link) was made by Rafi Farchi who is a friend of mine that lives in Israel and is a long-time user of Woodturner PRO. Looking at it, it is pretty easy to see how it was done.
The primary feature ring consists of the three rows in the center of the vessel (the ones with the biggest diameters. That ring started out as a single ring made up of tall, square segments made from two species - one that forms the background and another that creates the circle. A hole is drilled in the background species and the circle is glued into that hole. Typically, you would cut the hole on the drill press and then take those pieces and mount them on the lathe in a four-jaw chuck or something similar and then slightly use a tool to angle the hole slightly. Likewise, the circular plug would be slightly tapered to fit the hole so that the circle is glued without a trace of the joint.
Those segments are then mitered so that they form a single segmented ring. That ring is then mounted on the lathe using a faceplate with MDF mounted to it. It can be mounted with double-sided tape or hot melt wax. It is then parted of to create three rings. These rings are then used as the center three rings and added as you would any segmented ring.
The ring above and below this group of three rings was made identically but slightly smaller to make a smaller diameter ring. That ring was also parted into three rings on the lathe but the middle ring was not used in this vessel (probably was saved for another vessel, though). This identical process was followed for the rings above and below these two rings as well.
The next three rings above and below were added all at once. A long board was made by gluing three species on top of one another. This board was then tipped up on its side and cut into segments so that the outside of the ring was one species, the center was a second species and the inner side of the ring was a third species. The diameter of this ring was critical so that when the globe was turned, the outside of the ring would show the desired amount of the outside species.
The same approach was done for the row above it (the top row) that is a sandwich of seven strips of wood glued together (there would have been two of these 'sandwich' boards made and used alternately for every other segment.
All of the curvature so see in this vessel (except for the center circles) would have been achieved as a natural consequence of turning a bowl with a profile that changes dimension. Since the segments have straight lines but the profile changes, it creates lines that curve. If you were to look straight down from the top of the vessel, you would see that all the curved lines are actually straight.
With the excption of the circles inserted into the background segments, this would actually a pretty easy vessel to create. The hardest part of the design is to get the diameters of the various rings exactly correct. It is interesting to note, though that this entire vessel was made from only five segmented rings. The hardest part of all would have been turning the vessel. I suspect that Rafi would have turning it in two halves - the top and bottom and then joined the two halves at the end.
Please let me know if you have any questions.
Lloyd
