Drying Timber and the Five Distances of Trees
How wood cracks in seasoning, and how a tree shrinks to a point toward the horizon
This leaf from the sixth book of the Treatise on Painting turns from a preceding note on a river's east-to-west course to the seasoning of timber: one chapter explains why drying wood cracks straight or twisted depending on where the tree grew in its wood, and another advises boiling wood or sinking it in a river so it will not split. A longer chapter then divides the distance from the eye to the horizon into five equal stages, describing how a tree's leaves, twigs and branches dissolve from true shapes into scattered points and finally into a single dot. The passage sets out Leonardo's scheme of aerial perspective for rendering trees at increasing depth. No drawing accompanies the text on this page.
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Why drying timber cracks straight or twisted
Leonardo observes that the cracks wood makes as it seasons depend on where the tree grew: a plant more remote from the edges of its wood splits with straighter cracks, while one grown nearer the southern margins of the wood cracks more twisted. The timber's grain is treated as shaped by its position among its neighbours.
Boiling or soaking wood so it will not split
To keep wood from cracking as it dries, he advises boiling it a long time in ordinary water, or leaving it long at the bottom of a river, until it consumes its natural vigour. Only when this sap-like force is spent will the seasoned timber stay whole.
Five equal distances of a tree toward the horizon
The nearest trees give the eye their true shapes, with the lights, lustres, shadows and transparencies of individual leaves. At each greater distance the leaves, then the twigs, then the larger branches shrink into scattered points, until at the fifth and last distance, the horizon, the whole tree is reduced to a single point. Leonardo divides the span from eye to horizon into five equal parts.