First off, what is keystoning? Keystoning is a cutting technique used on square tesserae being laid along a curve, that eliminates the problem "v" shaped gaps which otherwise occur like this:
When you keystone your tesserae, they follow a continual curve, fitting very close to each other, from top to bottom of each tile like this:
How long has keystoning been practiced? If you look back at the ancient mosaics, you will see keystoned tesserae on all the curved areas in these mosaics. This technique has been around since mosaics got their start.
Why do you need to learn the keystone technique for mosaics? Keystoned tesserae maintain parallel edged goutlines perpendicular to a curve. Example:
What cuts do you need so your tiles go around a curve or circle? The objective is to eliminate "V" shaped gaps so that each tesserae meets at the top and bottom corners. Shown below are 4 shapes you can give to tiles that will make them fit a variety of curves.
The first is a square tile and fits in curves that are flattening out. The second cut may be used for more gradual curves. The third shape may be used for making a tighter radius (smaller circle). The last is used for more extreme curves. The basic rule is: the tighter the circle, the sharper the angles must be cut.
The main thing to mastering this technique is practice. Try practicing the various cuts until you get the hang of it.
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