The bark can be peeled readily when the sap is flowing. In winter, when the exterior of the tree is frozen, the bark can be removed only when heat is applied. During a prolonged thaw, however, this may be accomplished without the application of heat. Bark peeled from the tree during a winter thaw, and early in the spring or late in the fall, usually adheres strongly to the inner rind, which comes away from the tree with the bark. The act of peeling, however, puts a strain on the bark, so that only tough, well-made bark can be removed under these conditions. This particular characteristic caused Indians in the east to call bark with the rind adhering "winter bark," even though it might have been peeled from a tree during the warm weather of early summer. Since in large trees the flow of sap usually starts later than in small ones, the period in which good bark is obtainable may extend into late June in some localities. Upon exposure to air and moisture, the inner rind first turns orange-red and gradually darkens with age until in a few years it becomes dark brown, or sepia. If it is first moistened, the rind can be scraped off, and this allowed it to be employed in decoration, enough being left to form designs. Hence winter bark was prized. To the eastern Indians "summer bark" was a poor grade that readily separated into its paper-like layers, a characteristic of bark peeled in hot weather, or of poorly made bark in any season. In the west, however, high-quality bark was often scarce and, therefore, the distinction between winter and summer bark does not seem to have been made. Newfoundland once had excellent canoe bark, as did the Maritime Provinces, Maine, New Hampshire, and Quebec, but the best bark was found back from the seacoast. Ontario and the country to the immediate north of Lake Superior are also said to have produced bark of high quality for canoe building. The bark of the paper birch was preferred for canoe building because it could be obtained in quite large sheets clear of serious blemishes; because its grain ran around the tree rather than along the line of vertical tree growth, so that sheets could be "sewn" together to obtain length in a canoe; and because the bark was resinous and not only did not stretch and shrink as did other barks, but also had some elasticity when green, or when kept damp. This elasticity, of course, was lost once the bark was allowed to become dry through exposure to air and sunshine, a factor which controlled to some extent the technique of its employment.