LIGHT & SHADE: Moldings on the outside of a building are almost always illuminated by direct sunlight. Interior moldings, on the other hand, are usually illuminated by indirect reflected light or electric lighting. To compensate for the general weakness of interior light, you might want to make still more adjustments, since moldings only exist tot he degree that they produce effective planes of light and shadow. This is especially crucial on the interior, where, in the absence of external miters, you must rely on the tonal play of shade and shadow to define the profiles at hand.
Space doesn’t permit us to go into detail on the dynamics of light. We can make a few basic points, however, by comparing the OVOLO and ECHINUS profiles (upper left and middle right, fig. 12). Assuming light falls in parallel rays from above, the gradation from light to dark on the OVOLO will occur in uniform increments because all points along its surface are equidistant from its geometric center. With the ECHINUS, the gradation is far more complex because it is constructed according to the non-circular geometry of the ellipse. An ellipse doesn’t have geometric centers, and therefore — unlike the OVOLO — its curvature is constantly changing, making the shift from light to dark more subtle and less monotonous than the OVOLO.
Particularly noteworthy is the little highlight at the top of the ECHINUS, known as the QUIRKING, which is where a curve returns upon itself at its ends. You can see it even more exaggerated in this CYMA REVERSA, (lower left of Fig. 11) which can also be made from portions of an ellipse. The QUIRK can add much to a molding, in addition to the light effects it produces. For instance, it creates greater contrast between curved and straight at the extremes of a molding, in contrast to the OVOLO, where the profile assumes a vertical orientation just as it meets the adjacent fillet. The QUIRK also anticipates and makes a more organic connection between contiguous elements than does the rather abrupt perpendicular termination of the OVOLO into the flat surface above.
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[via Traditional Building Magazine]
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Thanks for the woodwork example. I understand the difference between a good finish shape and a quick finish shape.