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Estimation of crown height

In estimating proportions, especially in inferring crown height from the table size, the crown angle should always be taken into account. Only when a certain table size is combined with a given crown height can the ideal crown
angle of 34.5% be achieved.

This angle influences the ratio of the crown height to the table size, so that the greater the crown angle, the greater is the table in relation to the crown height.

Example:
Crown angle Table Crown height Crown angle Table Crown height
30° 56% 14%   30° 65% 10%
34.5° 56% 15%   34.5° 65% 12%
40° 56% 18%   40° 65% 14%

In order to arrive at a figure which is closer to an exact estimate of the crown height, is it therefore essential to have taken account the angle of the main crown facets in relation to the girdle plane (termed the crown angle) and to include it in the estimate. For this purpose the brilliant is examined in a position perpendicular to the table plane. Even a minor inclination leads to erroneous reflection images.

In this procedure an examination is made of the size of the main pavilion facets underneath main crown facets, and their further course is followed through to the middle of the table.

Fig 222 Estimation of crown angle with the aid of the pavilion facets
Fig 222 Estimation of crown angle with the aid of the pavilion facets

If the edges of the. main pavilion facets underneath the table are connected optically by a straight line with the edges underneath the main crown facets, the crown angle is around 2.5°.

 If the crown angle is greater than 30°, for example, the tip of the lower main facets appears to be opened to the table in a V-shape. In other words, the edges of the main pavilion facets are somewhat wider apart under the main crown facets, and there is no uniform perspective line as in the case of an angle of 25°.

 With an ideal crown angle of 34.5% it is possible to see. that the main pavilion facet at the table edge forms a clear step. The new line observed in the main crown facets corresponds exactly in width to the image under the middle of the table. This becomes clear in the following figure.

 The greater the crown angle, the clearer is the interruption in the main pavilion facet between the table and main crown facet. This is shown in both. of the following diagrams.

Crown height Estimation of crown angle