LIBRARY OF CONGRESS 0017 111 5140 • STATES DEPARTMENT OF AGRICULTURE .ru^,.. BULLETIN No. 552 Contribution from the Forest Service HENRY S. GRAVES, Forester SU^^-mTU Washington, D. C. PROFESSIONAL PAPER July 9, 1917 THE SEASONING OF WOOD/ By Harold S. Betts, M. E., in charge, Office of hidustrial Investigations. CONTENTS. Page. Importance of proper seasoning methods 1 Fiber saturation point and shrinkage 2 How wood may be injured in seasoning 11 Checking 11 Casehardening 11 Honeycombing 11 Warping 12 Collapse 12 Page. Air seasoning 12 Crossties, poles, and sawed timbers 12 Lumber 17 Rules for piling lumber 20 Kiln-drying 22 Types of kilns 22 Preliminary treatments 25 The process of drying 26 IMPORTANCE OF PROPER SEASONING METHODS. Practically all wood before being put to use is either seasoned in tbe air or dried in a kiln. The main objects of seasoning are to increase the durability of the wood in service, to prevent it from shrinking and checking, to increase its strength and stiffness, to prevent it from staining, and to decrease its weight. The sooner wood is sea- soned after being cut the less is the chance that it wall be injured by I the insects, which attack unseasoned wood,^ or decay before the time comes to use it. Wood that is to be treated with preservatives needs in nearly all cases to be seasoned as much as wood that is to be used in the nat/ural state. Wood has a complicated structure. The walls of the cells of which it is made up shrink and harden when moisture is removed from them, and unless timber that is to be air-seasoned is piled in the right way, or conditions in the dry kiln are maintained in accord- ance with certain well-defined physical laws, the material is hkely to warp or check, or in some way to be damaged seriously. Until recently proper methods of seasoning received comparatively little attention from manufacturers, and large losses, especially among 1 For assistance and si^gestions given in connection with the preparation of this bulletin, the author is _ I indebted to Mr. D. P. Sexton, of John B. Ransom & Co., Nashville, Tenn., and to Messrs. R. K. Helphen^ I Btine, jr., and N. de AVitt Betts, of the Forest Service. 2 The sapwood of seasoned hardwood is subject to attack and frequently to serious damage by powder. ! post insects. See Farmers' Bulletin 778, "Powder-Post Damage by Lyctus Booties to Seasoned Hard- I wood," by A. D. Hopkins and T. E. Snyder, 1917. 87732°— BuU. 552—17 1 ""„ ., s 2 BULLETIISr 552. U. S. DEPARTMENT OF AGRICULTUReA ^C woods that are difficult to dry, were the rule. Sometimes as mucn as 20 or 25 per cent of the seasoned himber was rendered unfit for the use intended by defects which had their origin in the drying process. Since the quality of the finished product can be impaired seriously by wrong methods, the importance of right methods becomes apparent. FffiER SATURATION POINT AND SHRINKAGE. Water exists in wood in two conditions:^ (a) as free water con- tained in the cell cavities, and (b) as water absorbed in the cell walls. When wood contains just enough water to saturate the cell walls, it is said to be at the ''fiber saturation point." Any water in excess of this which the wood may contain is in the form of free water in the Fig. 1.— Shrinkage as aSected by direction of annual rings; approximately twice as great tangentially as radially. cell cavities. Removal of the free water has no apparent effect upon the properties of the wood except to reduce its weight, but as soon as any of the absorbed water is removed the wood begins to shrink. Since the free water is the first to be removed, shrinkage does not begin, as a general rule, until the fiber saturation point is reached. In the case of eucalyptus and some of the oaks, however, shrinkage begins above this point. For most woods the fiber saturation point corresponds with a moisture content of from 25 to 30 per cent of the dry weight of the wood. Figure 1 shows graphically the difference between tangential and radial shrinkage. Shrinkage is due to the contraction of the cell Myalls, and sets up stresses which tend to cause the wood to check. As observed in a cross section of a piece of lumber, shrinkage in the tangential direc- tion is about twice as great as in the radial direction; lengthwise of ' The term "sap" sometimes is used wrongly to mean the moisture in wood, and at other times to mean the sapwood. Sap is formed, mainly in the early spring, in the leaves from water rising from the roots through the sapwood . In the leaves this water is converted into true sap, which contains sugar and soluble gums. The sap descends through the bark and feeds the tissues in process of formation between the bark and the sapwood. The heartwood contains no sap. //- /// D. : of D. J^L 23 1917 THE SEASONING OF WOOD. the lumber it is very slight. Table I gives the green, air-dry, and kiln-dry weight per cubic foot of the principal commercial woods, and Table II gives the per cent of shrinkage from a green to an oven- dry condition. Table I. — Average weights of various species of wood. Species and locality. HARDWOODS. Alder, red, Snohomish County, Wash Ash: BUtmore, Overton County, Tenn Black- en tonagon County, Mich Marathon County, Wis , Blue, Bourbon County, Ky Green- Richland Parish, La New Madrid County, Mo Oregon, Lane County, Oreg Pumpkin, New Madrid County, Mo. . White— Stone County, Ark Oswego County, N. Y Pocahontas County, W. Va Aspen : Rusk County, Wis Largetooth, Sauk County, Wis Basswood: • Potter County, Pa Marathon County, Wis Beech: Hendricks County, Ind Potter County, Pa Birch: Paper, Rusk County, Wis Sweet, Potter County, Pa Yellow- Potter County, Pa. Marathon County. Wis Buckeye, yellow, Sevier Cfounty, Tenn . . . Buckthorn, cascara. Lane County, Oreg. . Butternut: Sauk County, Wis Sevier County, Tenn Cherry: Black, Potter County, Pa Wild red, Sevier County, Term Chestnut: Baltimore County, Md Sevier County, Tenn Chinquapin, western. Lane County, Oreg. Cottonwood : Pemiscot County, Mo Black, Snohomish County, Wash Cucumber tree, Sevier County, Tenn Dogwood: Flowering, Sevier County, Tenn Western, Lane County, Oreg Elder, pale, Douglas County, Oreg Elm: Cork- Marathon County, Wis Rusk County, Wis Slippery — Hendricks County, Ind Sauk County, Wis , Weights (pounds per cubic root). V) Air-dry.2 (0 i') 28 {*) Green.' (') 47 51 46 47 43 41 41 56 54 51 59 56 59 45 47 46 33 53 56 61 (<) 46 50 65 55 65 • About 8 per cent moisture. 'About 12 or 15 per (»nt moisture. Average condition reached without artificial heating by material sheltered from precipitation, North Central States. maowiai 3 Average green material. * No figures available. Note.— Any individual lot of lumber in the condition specified in the column headings would proba- bly vary 5 per rent from the figures given with a possible variation of as much as 20 per cent. For exam- ple, young thrifty pmes wiU have a high moisture content when freshly cut and will probably weieh 20 oer cent above the average given. ' ^ ,, 6« *« pci 4 BULLETIN 552, U. S. DEPARTMENT OF AGRICULTURE, Table I. — Average weights of various species of wood — Continued. Species and locality. Weights (pounds per cubic foot). Kiln- dry. Green. HARDWOODS— continued. Elm— Continued. White- Potter County, Pa Marathon Coimty , Wis Gum: Black, Sevier County, Tenn Blue, Alameda County, Cal ■. Red— New Madrid County, Mo Pemiscot County, Mo Hackberry: Hendricks County, Ind Sauk County, Wis Haw, pear, Sauk County, Wis Hickory: Shellbark— Sardis, Miss Napoleon, Ohio Bittemut, Napoleon, Ohio Mockemut— Sardis, Miss Chester Coimty, Pa Webster County, W. Va Nutmeg, Sardis, Miss Pignut — Sardis, Miss Napoleon, Ohio Chester County, Pa Webster County, W. Va Shagbark — Sardis, Miss Napoleon, Ohio Chester County, Pa Webster Coimty, W. Va Water, Sardis, Miss Holly, American, Sevier Coimty, Tenn Hornbeam, Rusk County, Wis Laurel: California, Douglas County, Oreg Mountain, Sevier County, Term Locust: Black, Sevier County, Tenn Honey, Hendricks County, Ind Madroiia: Butte County, Cal - Douglas County, Oreg Maple: Broadleaf, Snohomish Coimty, Wash Red- Marathon, Wis Potter County, Pa Silver, Sauk County, Wis Sugar— Hendricks County, Ind Potter County, Pa Marathon, County, Wis Magnolia (evergreen), Winn Parish, La Oak: Bur, Sauk County, Wis California black— Butte County, Cal Douglas County, Oreg Canyon five, Butte Coimty, Cal Chestnut, Sevier Coimty, Tenn Cow, Winn Parish, La Laurel, Winn Parish, La Pacific post, Douglas County, Oreg Post- Stone County, Ark Winn Parish, La Red- Stone Coimty, Ark Hendricks Coimty, Ind Richland Parish, La Sevier County, Tenn (•) (') (') (') C) (') 0) (') 0) (') 38 54 56 45 46 48 50 45 47 (0 46 47 47 49 43 45 42 44 4S 50 41 42 (') (') (•) (') I No figures available. THE SEASONING OP WOOD. 5 Table I. — Average weights of various species of wood — Continued. Species and Jocality. Weights (pounds per cubic foot). Green. HARDWOODS— continued. Oak — Continued. Spanish— Lowland, Winn Parish, La Highland, Winn Parish, l>a Swamp wiiite, Hendricks County, Ind Tanbark, Willits, Cal Water, Winn Parish, La White- Stone County , Ark Hendricks County, I nd Richland Parish, La Winn Parish, La Willow, Winn Parish, La , Yellow- stone County, Ark Marathon County , Wis Osage orange, Morgan County, Ind Poplar, yellow, Sevier County, Term Rhododendron, great, Sevier County, Ttnn Sassafras, Sevier County, Term Serviceberry, Sevier County, Tenn Silverbell tree, Sevier County, Tenn Soarwood, Sevier County, Term Sumach, staghorn, Sauk County, Wis Sycamore: Hendricks County , Ind Sevier County, Tenn , Tupelo, St. John the Baptist Parish, La Umbrella, Fraser, Sevier County, Tenn Walnut, black, Kentucky Willow: Western black, Douglas County, Greg Black, Sauk County , Wis '. Witch hazel, Sevier County, Term , CONIFERS. Cedar. Incense — Lane County, Greg Weed, Cal '.'.''.'."." '."■■■ Port Orford , Douglas County , Greg Western red — Missoula County, Mont Snohomish County , Wash White, Shawano County, Wis Cypress: Bald, St. John the Baptist Parish, La Yellow, Lane County, Greg '. Douglas fir: Plumas County, Cal Humboldt Coiinty, Cal ]..[[[[.[..... Johnson County, "Wyo Lane County, Greg ' . " Chehalis County, Wash Lewis County, Wash Washington and Oregon Missoula County, Mont Fir: Amabilis — Dee, Greg Snohomish County , Wash .\lpine, Grand County, Colo Balsam, Rusk County, Wis Grand- Missoula County, Mont Douglas County, Greg Noble, MultnomahCouuty, Greg White, Madera County, Cal Hemlock: Sevier County, Tenn Marathon County, Wis Mountain, Missoula County, Mont ......[............ Western— Chehalis County, Wash Grays Harbor and Buckley, Vvasii ................. ' No figures available. 47 49 40 42 50 52 4:i 44 43 45 46 48 46 47 46 48 46 47 45 46 43 45 40 42 54 56 27 28 39 40 31 32 52 54 31 32 39 40 32 34 34 35 35 36 35 37 30 31 41 44 30 31 25 26 45 46 (■) (') 30 31 21 23 21 22 24 21 (') 33 (') (') (') (') 30 (') (') 31 31 32 35 36 31 32 35 37 26 27 22 23 24 25 28 29 27 ^'^8 25 26 31 32 24 25 30 32 (') 67 62 69 66 63 59 61 67 63 76 63 62 62 38 62 4t 61 44 53 41 51 53 66 47 58 51 51 59 (') (') (') 0) 52 36 28 45 (') (') 40 6 BULLETIN 552, U. S. DEPARTMENT OF AGRICULTURE. Table I. — Average lueights of various species of ivood — Continued. Species and locality. Weights (pounds per cubic foot). Kiln- dry. 1- Air-dry. 37 39 33 34 29 30 27 28 37 39 27 28 27 28 29 30 28 29 27 28 42 44 42 "« 39 41 38 40 35 36 38 40 32 34 35 36 43 "« 26 27 36 37 29 30 25 26 28 29 28 29 27 28 26 27 23 24 26 27 22 23 24 25 28 29 27 28 (') 25 26 29 30 37 38 43 45 Green. CONIFERS— continued Larch, western: Missoula County, Mont Stevens County, Wash Pine: Jack, Barron County, Wis Jeffrey, Plumas County, Cal Loblolly, Nassau County, Fla Lodgepole— Grand County, Colo Gallatin County, Mont Granite County, Mont Jeflferson County, Mont Johnson County, Wyo Longleaf— Nassau County, Fla Bogalnsa, La Lake Charles, La Tangipahoa Parish, La Hattiesburg, Miss Pitch, Sevier County, Tenn Pond, Nassau County, Fla Norway, Shawano County, Wis Shortleaf— Malvern, Ark Bogalusa, La Slash, Nassau County, Fla Sugar, Madera County, Cal Table-mountain, Sevier County, Tenn Western white, Missoula County, Mont Western yellow — Coconino County, Ariz Madera County, Cal Douglas County, Colo Missoula County, Mont White, Shawano County, Wis Redwood: Humboldt County, Cal Mendocino County, Cal Spruce: Engelmann— San Miguel County, Colo Grand County, Colo Red- Coos County, N. H Sevier County, Tenu Sitka, Chehalis County, Wash White- Coos County, N. H Rusk County, Wis Tamarack, Shawano County, Wis Yew, western, Snohomish County, Wash (') (0 0) 51 42 60 47 54 33 47 41 39 37 (1) 0) 0) 45 54 42 54 49 42 45 53 50 54 39 44 53 49 51 39 38 39 48 30 ' No figures available. THE SEASOisTiNG OF WOOD, 7 Table II. — Average shrinlage of various species of wood. Species and locality. HAKDWOODS. Alder, red, Snohomish County, Wash . . . . Ash: Biltmore, Overton County, Tenn B lack- On tonagon, Mich Marathon County, Wis Blue, Bourbon County, Ky Green- Richland Parish, La , New Madrid County, Mo Oregon, Lane County, Oreg Pumpkin, New Madrid Countv, Mo. . White- Stone County, Ark Oswego County, N. Y Pocahontas County, W. Va Aspen: Rusk County, Wis Largetooth, Sauk Comity, Wis Bass wood: Potter County, Pa Marathon County, Wis Beech: Hendricks County, Ind Potter County, Pa Birch: Paper, Rusk County, Wis Sweet, Potter County Yellow- Potter County, Pa Marathon County, Wis Buckeye, yellow, Sevier "County, Tenn. . . Buckthorn, cascara, Lane County, Oreg. . Butternut: Sauk County, Wis Sevier County, Tenn Cherry: Biack, Potter County, Pa Wild red, Sevier Coiinty, Tenn Chestnut: Baltimore Coimty, Md Sevier County, Term Chinquapin, western, Lane Coimty, Oreg. Cottonwood : Pemiscot County, Mo Black, Snohomish County, Wash Cucumber tree, Sevier Comity, Tenn Dogwood : Flowering. Sevier County, Tenn Western, Lane Comity, Oreg Elder, pale, Douglas County, Oreg Elm: Cork- Marathon Coimtv, Wis Rusk County, Wis Slippery— Hendricks County, Ind Sauk County, Wis White- Potter County, Pa Marathon County, Wis No figures available. Shrinkage from a green to an oven-diy condition (per cent of dimensions when green). 12.6 12. C 10.4 IS. 9 13.2 15.5 13.4 (■) 0) 4, ^6.2 (») 0) (') (') 7.6 0) 0) 8.1 (') 9.5 Note.— Oven-dry means entirely free from water. The shrinkage from a green to a kiln-dry condition (8 per cent moisture) is generally about 75 percent of the shrinkage to anovon-dry condition, the shrink- age from a green to an air-dry condition (12 to 15 per cent moisture) is generally about 50 per cent of the shrinkage to an oven-dry condition. BULLETIN 552, U. S. BETAETMeNT OP AGEICFLTLTRE. Table II. — Average shrinkage of various species of wood — Continued. Species and locality. Shrinkage from a green to an oven-diy condition (r>er cent of dimensions wlien green). HARDWOODS— continued. Gum: Black, Sevier County, Tenn Blue, Alameda Comity, Cal Red- New Jfadrid County, Mo Pemiscot County, Ato Hackberry: Hendricks County, Ind Sauk County, Wis Haw, pear, Sauk County, Wis Hickory: Shellbark— Sardis, Miss Napoleon, Ohio Bitterriut, Napoleon, Ohio Mockernut— Sardis, Miss Chester County, Pa Webster County, W. Va Nutmeg, Sardis, Miss Pignut- Sard is. Miss Napoleon, Ohio Chester County, Pa Web!', W is Magnolia (evergreen ), Winn Parish, La Oak, bur, Sauk County, Wis California black — Rutte ComUy, Cal Douglas County, Oreg Canyon live, Butte County, Cal Chestnut, Sevier County, Tenn Cow, Wiim Parish, La Laurel, Winn Parish, La Pacific post, Douglas Coimty, Oreg Post- Stone County, Ark Winn Parisli, La 13.9 4.4 22.5 7.6 (') (') 14.0 4.2 13.8 4.9 (') (n 1.7.6 7.4 20.9 7.9 0) (') 16.5 6.9 18.9 8.4 (>) ^'\ (') 0) 15.0 5.6 15.3 6.3 16.9 6.8 21.2 8.5 16.0 6.5 18.4 7.9 (») (>) 15.5 6.5 (') (') 16.2 4.5 (') 8.2 (•) 0) 14.4 5.6 9.8 4.4 8.6 0) 16.2 5.1 17.6 5.5 11.6 3.7 (') (') 12.5 3.8 12.0 3.0 14.3 4.9 14.7 4.8 (1) (») 12.3 5.4 12.7 4.4 13.6 4.1 (') 0) 16.2 8.0 16.7 5.5 19.4 5.9 19.0 3.9 (') 0) 16.0 5.7 16.5 5.2 1 No figures available. THE SEASONING OF WOOD. Table II. — Average shrinkage of various speaes of iroor^/— Continued. 9 Species and locality. Shrinkage from a green to an oven-dry condition (per cent of dimensions when green). HARDWooDs^tontinued. O^k— Continued. Red- stone County, Ark Hendricks County, Ind Richland Parish, I/a Sevier County, Tenn Spanish — Lowland, Winn Parish, La Highland, Winn Parish, La Swamp white, Hendricks Coiuity, Ind Tanbark, Willits, Cal ". . Water, Winn Parish, La White- Stone County, Ark Hendricks County, Lnd Richland Parish,"La Winn I'arish, La Willow, Winn Parish, La Yellow^ Stone County, Ark Marathon County, Wis Osage orange, Morgan County, Ind Poplar, yellow, Sevier County, Tenn Rhododendron, great, Sevier County, Tenn Sassafras, Sevier Coimty, Tenn Serviceberry , Sevier County , Tenn Silverbell tree, Sevier County, Tenn Sourwood, Sevier County, Term Sumach, staghorn, Sauk Coimty, Wis Sycamore: Hendricks County, Ind Sevier County, Term Tupelo. St. John the Baptist Parish, La Umbrella, Fraser, Sevier County, Tenn Walnut, black, Kentuckv Willow: Western black, Dduglas County, Oreg Black, Sauk County, Wis Witch hazel, Sevier County, Tenn C0N1FER,S. Cedar: Incense — Lane County, Oreg ^..^ Weed, Cal Port Orford , Douglas County, Oreg Western red— Missoula County, Mont Snohomish County, Wash White, Shawano County, Wis Cypress: Bald, St. .John the Baptist Parish, La Yellow, Lane County, Oreg Douglas fir: Plumas County, Cal Humboldt County, Cal .Tohnson County, "Wyo Lane County, Oreg Chehalis County, Wash Lewis County, Wash Washington and Oregon Missoula County, Mont Fir: Amabilis — Dee, Oreg Snohomish Coimty, Wash ' No figures available S7732°— Bull. 552—17 2 14. 13. (') 15. 16. 16. 17. (') 16. 15. 14. 16. 16. 18. 14. (0 8. 11. 16. II). 18. 12. 15. (•) 1.3. 14. 12. 13. 11. 13. 13. 18. 7.6 8.6 7.0 11.5 (') 11.7 (') 10.9 13.2 12.5 12.3 (') (') (') 14.1 0) 4.2 3.7 3.7 5.2 4.5 5.5 (') 6.2 4.9 4.8 6.4 5.0 (•) (') 4.1 6.3 4.0 6.7 3.8 6.3 5.0 .5.2 4.4 4.4 5.2 2.9 2.2 (') 2.5 2.5 2.1 (') (') 3.7 5.7 4.4 6.0 (') (>) (') 4.5 (•) (') 8.3 8. J 8.3 10.8 8.7 10.0 8.3 9.0 9.2 9.5 9.6 9.7 (') 6.9 8.7 6.2 10.8 7.6 8.9 (') 7.3 7.9 7.9 12.5 7.1 9.0 8.2 (') 4.6 5.6 4.7 6.0 (') (') (') 0) 6.9 I 6.6 7.6 7.4 8.3 10.0 10 BULLETIN 552, U. S. DEPARTMENT OF AGRICULTURE. Table II. — Average shrinkage of various species of wood — Continued. Species and locality. CONIFERS— continued. Fir— Continued. Alpine, Grand County, Colo Balsam, Eusk County, Wis Grand — Missoula County, Mont Douglas County, Oreg Noble, Multnomah County. Oreg White, Madera County, Cal Hemlock: Sevier County, Tenn Marathon County, Wis Mountain, Missoula County, Mont Western— Chehalis County, Wash Grays Harbor and Buckley, Wash. . . Larchj western: Missoula County, Mont Stevens County, Wash Pine: Jack, Barron County, Wis Jeffrey, Plumas County, Cal , Loblolly, Nassau County, Fla Lodgepoie — Grand County, Colo Gallatin County, Mont Granite County, Mont Jefferson County, Mont Johnson County, Wyo Longleaf — Nassau County, Fla Bogalusa, La Lake Charles, La Tangipahoa Parish, La Hattiesburg, Miss Pitch, Sevier County, Tenn Pond, Nassau County, Fla Norway, Shawano County, Wis Shortleaf— Malvern, Ark Bogalusa, La Slash, Nassau County, Fla Sugar, Madera County, Cal Table- mountaui, Sevier County, Tenn. . Western white, Missoula County, Mont. Western yellow— Coconino County, Ariz Madera County, (.'al Donglas Coimty , Colo Missoula County , Mont White, Shawano County, Wis Redwood: Humboldt Co^unty , Cal Mendocino County, Cal Spruce: Engelmann — San Miguel County, Colo Grand County, Colo Red- Coos Coimty, N. H Sevier County, Tenn Sitka, Chehalis County, Wash White- Coos County, N. H Rusk County, Wis Tamarack, Shawano County, Wis Yew, western, Snohomish County, Wash. . . Shrinkage from a green to an oven-dry condition (per cent of dimensions when green). Volume. 1 9.0 10.8 10.9 (') 13.6 10.2 n.6 9.2 10.8 (') (') 13.2 (') 10.4 9.9 12.6 11.3 11.9 11.8 12.0 10.1 12.2 (') 12.8 12.8 11.0 11.7 11.2 1L5 0) (') 12.7 8.4 10.9 11.5 9.2 n.5 9.9 9.3 7.8 10.3 10.5 (') n.8 11.2 14.8 13.6 •9.7 2.5 2.8 3.5 (') 4.9 3.4 2.3 4.4 (0 3.4 4.4 5.5 4.2 4.6 5.0 6.0 3.6 Tangen- tial. (') 5.1 5.4 6.0 4.8 4.8 5.1 4.6 f.! 5.9 2.9 3.4 4.1 4.1 4.9 3.8 3.5 2.2 u 3.0 3.7 (■) 0) 3.7 3.7 4.0 ' No figures available. THE SEASONING OF WOOD. H HOW WOOD MAY BE INJURED IN SEASONING. CHECKING. Checking is caused by unequal shrinkage. If the outside of a piece of wood dries considerably faster than the inside, the surface in time will contract until it can no longer extend around the comparatively wet interior, and so will be torn apart in checks. Checks often are classified as end checks and face checks. End checking or spUtting during seasoning causes nearly as much loss as face checking. CASEHARDENING. Casehardening or surface hardening occurs when the surface of wood becomes set in a partially dry condition while the interior is still wet. This condition results from too rapid surface drying. If the interior of a casehardened piece of wood dries further, it tends to shrink, while the "set" condition of the surface tends to prevent it from doing so. As a result, stresses are set up in the piece. Plate I , figure 1, shows sections cut from casehardened boards, with a strip sawed from the center of each section. In A, the stresses cause the prongs to curve inward and bind on the saw. If the stresses are reheved by treatment with steam, as may be done sometimes, and the board dried a second time, the resawed prongs, as shown in B, will curve outward, owing to a reversal of the stresses. This is termed ''reverse casehardening." ^ Plate I, figure 2, shows the form taken by resawed pieces of kiln- dry boards steamed for different lengths of time. In No, 1 the prongs curve inward, owing to casehardening. No. 2 and No. 3 also show a casehardened condition as indicated by the strips curving inward. In Nos. 4, 5, and 6 the casehardening has been eUminated by longer steaming and the resawed strips are straight. No. 7, which has been steamed stiU longer, shows a condition of " reverse case- hardening," in wliich the resawed strips curve outward. Sections cut as shown in Plate I may be used also to determine the distribution of moisture in lumber, whether casehardened or not. If not casehardened, such sections wiU curve inward as they dry if the lumber is wetter on the inside than on the surface, and outward if the reverse is the case. If the lumber is uniformly dry, the prongs wiU remain practically straight. HONEYCOMBING. Honeycombing or internal checking occurs in casehardened pieces when the interior continues to dry and the surface remains fixed. In such cases splits appear in the interior. Plate II, figure 1, shows examples of honeycombing in casehardened pieces. iFor further discussion see " Problems in Kiln-Drying Lumber," by H. D. Tiemann, Lumber World Review, Sept. 23, 191o, 12 BULLETIN 552, U. S. DEPAETMENT OF AGKICULTUKE. WARPING. Warping or twisting in lumber is due to unequal shrinkage. Some woods are much more subject to warping than others. The trouble can be prevented to some extent by careful pihng, both during drying and afterward. Plate II, figure 2, shows badly warped pieces of lumber. COLLAPSE. In some woods, notably western red cedar and redwood, when the very wet wood is dried at a high temperature, depressions appear on the surface of the boards, presumably due to the coUapse of the plastic cell walls in certain places. If, however, the woods in ques- tion are heated above the boihng point while wet, the steam generated in the nonporous cells causes the wood to bulge on the surface. Plate III shows collapse and bulging, or ''explosion," as it is termed by the discoverer ol the phenomenon.* AIR-SEASONING. Though the use of dry kilns is increasing steadily, the bulk of our wood is still seasoned in the open air. If kept in the air long enough, the moisture content of the wood finally comes into equihbrium with that of the surrounding atmosphere, and the wood is said to be air- dried. The rate of drying varies, of course, with time of year, species of wood, size and form of piece, and method of piling. Certain of these factors may be controlled or utihzed in a way to hasten the drying process and lessen the hkehhood of defects appearing in the material. CROSSTIES, POLES, AND SAWED TIMBERS. The data in figures 2 to 12, inclusive, collected by the For- est Service^ in various parts of the country, show the rate at which crossties, poles, and sawed timbers of several species lose mois- ture when freely exposed to the atmosphere. In some cases it was not possible to weigh the pieces for several days after they were cut. FresUy cut timber loses weight very rapidly in warm dry weather. Ties in some species lose 10 pounds in 24 hours. The rates of season- ing of the various species may be compared by the general trend of the curves. When the curves reach a horizontal position, the ma- terial may be said to be air-dry, unless this happens at a time of year very unfavorable for seasoning. The ties were seasoned in piles of 50 each, and were exposed with- out cover. The ties on the top of each pile, however, were placed close together and served as a rough roof. The curves are platted 1 H. D. Tiemann, in charge Section of Timber Physics, Forest Products Laboratory of Forest Service. 2 See " The Air Seasoning of Timber," by W. H. Kempfer, Forest Service, in Bui. 161 of the American Railway Engineering Association. Bui. 552, U. S. Dept. of Agriculture. Plate I. Fig. 1 .—Sections Cut from Casehardened Boards. Fig. 2.— Resawed Sections Cut from Casehardened Red Gum Boards Steamed for Different Lengths of Time after being Kiln Dried. (1) No final steaming; (2) and (3) 18 minutes final steamins;; (4). (5), and (6) 36 minutes final steam- ing; (7) 3 hours final steaming. Bui. 552, U. S. Dept. of Agriculture. Plate II. Fig. 1.— Honeycombed Oak Timbers, the Result of Casehardening. FiQ. 2.— Badly Warped Boards. Tlie trouble here is due to a poor arrangement of stickers and to the piling together of boards of unequal length. THE SEASONING OF WOOD. 13 l-BLACK PINE -j-prn Piwr i to \ 1 ;- DOUGLAS F IR 1 \ *- V \ LiJ \ \ »— \ v^ ( 3 \ .\ \ V A V ^. 100 1 N>^ ^ *"" 2 4 6 8 TIME SEASONING -MONTHS 2 4" TIME SEASONING' Fig. 2. — Southwestern ties, (o) Seasoning of ties at Pecos, N. Mex.; cut in January and February; (b) seasoning of ties at Pecos, N. Mex. ; cut in August, September, and October. (Black and red pine are local names for western yellow pine; black pine refers to young trees and red pine to mature trees). 6 8 10 TIME SEASONINO- MONTHS Fig. 3. — Northwestern ties. Seasoning of Indgepole pine ties at Bozeman, Mont., Douglas fir at Sandpoint, Idaho (curves U and 12), Pasco, Wash, (curve 15), and Tacoina, Wash, (curve 16); and western larch at Sandpoint, Idaho, cut in January and February; Tacoma ties in December and January. 14 BULLETIN" 552, U. S. DEPARTMENT OF AGRICULTURE. 200 I/C- HEMLOCK- UNPEELED •80 I9C - < 5 160 1 u L \ N L L.J k. N ^— < >— , ^^ '— >i a; W ■^-^ "^ k J7c S"20 LlJ 5 1 1 M WtiH < V o N H H 1 "~^ •— « I8c tta 6 8 lO 12 TIME SEASONING -MONTHS Fig. 4.— Seasoning of hemlock ties at Escanaba, Mich.; cut in August and September. 240 220 200 1, 23-LOBLOLLY PINE-TE X. V 23- 24-SHORTLEAF PINE K. \ 25-LONGLEAF PINE 2g-i HRi ni IV PiNr.MiCQ. V > A \ s k \ ^. ^ ICO \ k> ^ (m; 8^ 1 - (AO V N W k k -*- , ^25 at \ ^ ^ C:: ^3' f • ^0- ^h »* ion \ ^ »— ■oZi > s. i V H 1 eo 60 _| ) > » « > ■ J ) 1 6 1 2" TIME SEASONING -MONTHS Fig. 5. — Seasoning of ties at Silsbee, Tex., and Ackerman, Miss.; cut in January and February. THE SEASONING OF WOOD. 15 6 8 10 la TIME SEASONING -MONTHS Fig. 6. — Seasoning of hardwood ties in Southern States; cut in October, November, and December. t^ae •"28 = J4 \ \ \ \ \ L — 1 r a M ^— < Lr JMN CUT r rr wl^ m m i=^ »=s* *— ' 2 4 6 e 10 12 14 le ~ II TIME SEASONING -MONTHS FiQ. 7.— Seasoning of southern white cedar poles at Wilmington, N. C. 6 8 10 12 14 i§ TIME SEASONING - MONTHS Fig. 8.— Seasoning of western red cedar poles at Wilmington, Cal, 16 BULLETIN 552^ U. S. DEPARTMENT OF AGRICULTURE. ^. — 1 n s w k' \ h H '--., N k. r N M v,^ w ruMULT \N INTE Rcll""^ a H k. 1 1 I 1 T ? n ? i "1 3 6 8 10 12 14 16 13 TIME SEASONING - MONTHS Fig. 9.— Seasoning of chestnut poles at Thorndale, Pa. 40 to o \ \ [ *-> a. k. \ \ai lTu^ IN C JT M H fc = r N -A — --H ■w,-. 1^^ 3 V, INTE RCl H P L ^ H k> d h N6 1 ;uT ^ H > MEf CU 20 ( . e ( i 1 a 1 2 1 4 riM£ SEASONING - MONTHS FiQ. 10.— Seasoning of northern white cedar poles at Escanaba, Mich. TIME 8IAS0HIN6 - MONTHS Fig. 11.— Seasoning of Douglas fir timbers at Eugene, Oreg. Bui. 552, U. S. Dept. of Agriculture. Plate Collapse and Bulge in Very Wet Lumber Dried at a High Temperature. Bui. 552, U. S. Dept. of Agriculture. Pl^te IV. Fig. 1.— Cement as Foundation Material. These blocks extend 2 feet above and 2 feet into the ground. FiQ. 2.— Another Type of Permanent Foundation. Steel rails are embedded in cement piers. The himlier is fully 2\ feet above the ground, insuring excellent ventilation. THE SEASONING OF WOOD. lY from the average weight of the ties. The weight per unit volume could not be used, as in many cases the volumes of the ties were not 50 4.8 O 46 tt LONGLEAF PINE V ^44 \ s, \ ^ 1- v> -^ „^^ 6X1 2' AN DffX 16- O 42 bJ 5 40 -0-. ^>- . __ < — — -19 14 •- .._ — ^ -•- — -■ -19 15-- - — -• -1 ' iO, ) I t ( 3 1 1 2 1 4 16 18 20 TIME SEASONING -MONTH Fig. 12.— Seasoning of longleaf pine timbers at Madison, AVis. obtained. Tlie poles were seasoned on skids in the open. The sawed timbers were seasoned in open piles under shelter. Sawed lumber generally is dried by being piled in stacks with air spaces between the boards. In forming the stacks the boards usually are laid flat, with strips called stickers between courses or layers. Fiu. i3.— i^umoer piiea siaewise on cement ana metal rounaatlona. 18 BULLETIN 552, U. S. DEPARTMENT OP AGRICULTURE. A space also is left between each board in a layer and the adjacent board to provide for the circulation of air throughout the stack. Flat or horizontal piUng may be of two kinds: (a) With the ends of the boards toward the alley — endwise pihng, and (b) with the sides toward the alley — sidewise piling. Figures 13 and 14 illustrate the two methods. The stacks are arranged to slope from front to rear, Fig. 14. — Lumber piled lengthwise on wooden foundation. and to lean forward so that water dripping from the top falls to the ground without trickhng down over the courses below. With either method of piling the stacks should be so located in the yard that the prevaihng winds blow through them rather than against the ends. Most lumber manufacturers and dealers use the endwise method of piling. A number, however, have adopted the sidewise method, — ^'==3i=:l^«^J==fl=3t^ ^^^^ Fio. 15. — Method of providing drainage under lumber piles. which has certain advantages in the matter of air circulation. In endwise pihng the stickers obstruct the passage of air from back to front of a course, while in sidewise pihng the passages from front to rear are clear. Water wliich forces its way into the pile is more efficiently drained in sidewise piling, and the likelihood of sticker rot and discoloration due to the accumulation of moisture, dust, and dirt against the stickers is lessened. THE SEASONING OF WOOD. 19 The bottom boards in a stack rest on skids, which in turn rest on foundations, preferably of stone, cement, or metal. Pieces contain- ing rot should never be used for foundation timbers or skids, or allowed to remain in the pile. The vicinity of the pile should be kept clear of weeds. The use of cement and metal foundations is especially feasible in retail lumber yards and in those maintained by wood-using factories. In retail yards, where economy in space often is the essential thing, the piles are high and a particular space usually is allotted to each class or species of lumber. In factory yards lumber often is held for a number of years before being used. In such cases the frequent renewal of wooden foundations under lumber piles entails consider- able expenditure of time and money, to say nothing of the danger of infecting lumber by bringing it in contact with partly rotted foundation timbers. For these reasons foundations of a more per- manent character are constantly growing in favor in retail and factory yards. Plate IV shows foundations of this kind. Sawmill yards, on the other hand, often contain several million feet of material and cover several acres. Lumber coming from the saw generally is piled wherever most convenient, provided it is placed at the distance from the mill required by insurance companies. Economy in storage space generally is not essential, and piles of the same spe- cies and kind of lumber are Ukely to be found in a number of different sections of the yard. In addition, the stock is constantly being turned over, thus giving an opportunity to renew the foundation timbers at comparatively small expense. A number of large lumber companies, however, have adopted cement as a foundation material. Lumber-storage yards need to be reasonably well drained, or at least the contour of the ground should be such that water will not •stand imder the stacks after a storm. Otherwise decay is apt to get a start and spread throughout the pile. Where the ground offers but poor natural drainage f acihties, some artificial system of drainage usually is employed. Figure 15 shows the system used in the yards of two large lumber companies in the southern hardwood region. This arrangement not only prevents the collection of rain water under the lumber piles, but also gives the required slope to the stack, which on level ground has to be secured by building up the founda- tions/ A top dressing of cinders has been found satisfactory in some storage yards. The following set of rules for piling lumber covers the more im- portant points to be observed in the construction of foundations, shape of stack, arrangement of stickers, etc. : » Humphrey, C. J. Timber Storage Conditions in the Eastern and Southern States, with reference to Decay Problems. U. S. Dept. Agr. Bui. 510. 1917. 20 BULLETIN 552, U. S. DEPARTMENT OF AGRICULTURE^ RULES FOR PILING LUMBER. 1. Foundations (Endwise or Sidewise Piling). (a) The foundations should be strong, solid, and durable. (6) The top of each foundation should be level, and from front to back the top surface of the parallel skids should be in alignment, so that the lumber to be piled will bear equally upon each one. (c) The front foundation should be raised above the second, and the second above the third, to allow a slant in the stack of 1 inch to every foot. {d) The foundations should be spaced not over 4 feet apart, except for heavy planks and timbers. (e) The front foundation should be of sufficient height to provide space for free circulation of air under all parts of the pile. 2. Lumber (Endwise Piling). (a) Skids, preferably 2 by 4 inches, should be laid on top of the foimdations. (6) Boards of equal length should be piled together. (c) The ends of the boards should rest upon the front and rear skids. (rf) A space of approximately three-fourths inch should be left between boards in the same layer. (e) Lumber piled in the open should have the front ends of boards in each layer slightly protruding beyond the end of the layer beneath, to give a forward pitch to the stack. 3. Lumber (Sidewise Piling). (a) Skids, preferably 4 by 6 inches, should be placed across the foundations at about 4-foot intervals. The number of skids depends upon the thickness of the lumber. (6) Boards of equal length should be piled together. (c) The boards should be placed on the skids, with about three-fourths inch between boards in the samo layer. (c?) Lumber piled in the open should have the front board in each layer project slightly beyond the board in the layer beneath, to provide a forward pitch to the stack. 4. Stickers (Endwise or Sidewise Piling), (o) Stickers should be of uniform thickness, preferably seven-eighths inch for 1-inch lumber and li inches for 2-inch lumber. Their length should be a few inches in excess of the width of the pile. (6) Stickers should be placed upon the layer of boards immediately over the skids and kept in alignment parallel to the ftont of the piles. (c) The front and rear stickers should be flush with, or protrude beyond, the ends of the boards. 5. Roof Protection (Endwise or Sidewise Piling). Cover boards, as a roof protection, should be laid on the top of the pile, extending a few inches beyond the front and rear ends of the stack, 6. Spacing Stacks (Endwise or Sidewise Piling). Space between the piles should not be less than 2 feet; 4 or 5 feet is preferred if yardage conditions permit. Bui. 552, U. S. Dept. of Agriculture. Plate V. Fig. 1.— Sun Shields Used to Reduce Checking in Thick Red Oak Timber. Fig. 2.— Lumber Piled so as to Form "Chimney" or Flue near Center of Stack FROM Bottom to Top. Bui. 552, U. S. Dept. of Agriculture. Plate VI. FiQ. 1.— Pole Drying Yellow Poplar Lumber. Fi3. 2.— Pole Framework Used to Dry Yellow Poplar, Basswood, AND Cottonwood before Placing It in a Stuck Pile. Bui. 552, U. S. Deot. of Agriculture. Plate VII. H a 2 2 5 a > '^ > '3 < ^ CO ft Bui. 552, U. S. Dept. of Agriculture. Plate VIII. THE SEASONING OF WOOD. 21 7. Dimensions op Stack (Endwise or Sidewise Piling). The customary width of stacks is from 8 to 16 feet. The height is governed by the size and character of the lumber and by the methods of moving it. 8. Treated Ends (Endwise or Sidewise Piling). The ends of lumber 2i inches thick or over, unless of the lower grades, should receive a brush treatment of paint or some liquid filler. The rules just given are based on information obtained through field investigations and from lumber manufacturers and wholesale and retail dealers, and accord with the best lumber-pihng practice in general commercial use. Certain species of wood, however, re- quire particular care in air-drying, and in this case shght variations from the rules are necessary in order to secure the best results. Some lumbermen in the South, for example, find that thick red oak checks badly on the ends, and in au--drying such stock have adopted the scheme of protecting it with sun shields, as shown in Plate V, figure 1, which they claim reduces end-checking to a minimum. Mills cutting red gum formerly experienced diflSculty in drymg the lumber, on account of its tendency to warp. This objection, however, has been largely overcome by the exercise of care in sea- sonmg. In erecting a pHe of gum lumber, stickers are placed every 2 feet apart, some lumbermen claiming that 18 inches in none too close to obtain the best results. Another scheme in more or less general use among gum-lumber manufacturers is to construct the pHe so as to have a flue or "chimney" in its center, thus providing ample air circulation vertically through the stack, as shown in Plate V, figure 2. Green cottonwood, basswood, and yellow poplar lumber is hkely to stam badly when piled. Accordingly, a number of lumbermen either end-dry the material or pole-dry it for a week or two and then place It in a 'stuck" pile. In end^rying, the boards are stood up on end edge to edge, under a speciaUy built shed, with stickers arranged horizontally one above the other at specified distances. Such a pile presents exactly the appearance of a regular lengthwise pile of lumber set up on end. Plate VI,figure 1, shows a quantity of yellowpoplar lumber beingpole- dned, while Plate VI, figure 2, shows the frame used for the purpose Hickory and ash lumber frequently check badly when air-dried Lumbermen m the southern hardwood region have found that these checks wiU close up entirely if the lumber is first stuck-piled for 6 to 8 months and then bulk-piled and protected by good covering preferably sheds. ^' Plates yil and VIII show lumber piles in yards where careful attention has been given to the matter of pihng and yard arrange- 22 BULLETIN 552^ U. S. DEPARTMENT OF AGEICULTURE. KILN-DRYING. Lumber is kiln-dried when there is need for seasoning it quickly, or when the manufacturer does not wish to carry large stocks in his yard. A kiln is used also to further dry partially au'-seasoned or even fully air-seasoned material, for special uses. The main problem in kihi-drying lumber is to prevent the moisture from evaporating from the surface of the pieces faster than it is brought to the surface from the interior. When this happens the sm'face becomes considerably drier than the interior and begins to shrink. If the difference in moisture content is sufficient, the sur- face portion opens up in checks. The evaporation from the surface of wood in a kiln can be controlled to a large degree by regulating the humidity, temperature, and amount of air passing over the wood; and a correctly designed kiln, especially one for drying the more difficult woods, should be con- structed and equipped in a way to insure this regulation. A dry kihi may consist simply of a box in w^hich lumber can be heated, or of a good-sized building or group of buildings (battery) containing steam pipes, condensers, sprays, and various air pas- sages capable of adjustment to regulate the amount of ventilation. The elaborateness of the kiln depends, of course, mainly upon the value of the lumber that is to be dried. For lumber worth SI 00 per 1,000 board feet, it obviously will pay to use more careful dry- ing methods than in handling material valued at $20 or $25 per 1,000 board feet. TYPES OF KILNS. Kilns for drying lumber may be divided into two general classes (fig. 16): (a) compartment kilns, and (6) progressive kihis. In compartment kihis the conditions are changed during the drying process, and all lumber in the kiln is dried at one time. The condi- tions at any time diuing drying are uniform throughout the whole kiln. In a progressive kihi conditions at one end differ from those at the other, and the lumber is dried progressively by being passed through the kihi. Compartment kilns are used w^hen it is desired to dry lumber of various sizes and species, while progressive kilns arc used where uniform stock is handled. The methods of operation generally used in lumber kilns are: (a) natural ventilation, (h) condensing, and (c) superheated steam. In kilns operating by natural ventilation, the humidity or dampness is controlled by the use of escaping steam and evaporated moisture. Circulation in progressive kilns is largely longitudinal and in compart- ment kilns transveree. Moist air is allowed to escape from the kiln. In condensing kilns the humidity is controlled by recirculating the air, which has taken up water from the lumber, across water pipes THE SEASONING OF WOOD. 23 Fig. 16.— TypesofkUns, 24 BULLETIN 552, U. S. DEPARTMENT OF AGEICULTUEE. INCLINED FILES £DGC fILCS Fig. 17.— Methods of piling lumber for kiln drying. THE SEASONING OF WOOD. 25 or through water sprays. The temperature of the pipes or sprays governs the amount of water that condenses from the air, and con- sequently regulates the humidity of the air when reheated before being passed over the lumber again. The circulation of air may be either natural or forced. Condensing kilns are generally of the com- partment type. Kilns operating with superheated steam are used only where the species to be dried are not injured by high temperatures, and where fast drymg is essential. Lumber may be piled on the trucks which carry it into the kiln in any one of three ways (fig. 17): (a) flat or horizontal, (h) edge or vertical, and (a) incHned. Flat piUng is best for longitudinal circu- lation. It is not so well adapted for transverse circulation, and is not economical for downward circulation. Vertical piluig mcreases the truck capacity, as there are no vertical spaces between the boards. Probably it is the best method for downward or any fast circulation. Provision has to be made, however, for keeping the boards in place in the stack. Inchned piling allows for a definite movement of air either downward or upward (forced draft). It is an improvement, as regards circulation, over horizontal or flat piling. Some kiln operators usmg the flat or horizontal method of piling report excellent results from the construction of a V-shaped, openmg in the center «f the truck pile. Such openings are from 2^ to 3 feet wide at their base, and from 3^ to 4 feet high. Wliere this practice is followed it is customary also to place the boards in the layers closer together as the top of the stack is reached, to force greater lateral circulation. In loading lumber on kiln trucks by any one of the three methods mentioned, the stickers should be of a uniform thickness and arranged in the piles in ahgnment. It is advisable also not to attempt to dry various thicknesses of lumber together. Thick lumber takes longer to dry than thin lumber, and when different thicknesses are mixed the operation has to be governed by the thick stock, to the possible detriment, or at least the unnecessarily long drying, of the thin stock. PRELIMINARY TREATMENTS. Lumber to be kiln-dried is sometunes steamed in a separate com- partment before being placed in the kihi proper, especially where the kiln is not designed for moist-air treatment. In transferring lumber from a compartment for prelimmary treatment to the kihi proper, every care must be used to avoid a sudden change in humidity. The object of the steaming is to heat the lumber and thus make easier the transmission of moisture from the interior to the surface, and also to moisten the surface in case it has become casehardened or "'set" 26 BULLETIN 552, U. S. DEPARTMENT OF AGRICULTURE. during partial air drying. Other effects, also, are produced, which to a greater or less extent change the properties of the wood. The ' ' sap ' in the wood is changed by ' ' cooking," as indicated by a darken- ing of the wood, the degree of coloring depending upon the tempera- ture and duration of the process. Other changes of a chemical nature apparently also take place m the wood durmg steaming. The pressure and duration of steaming desirable in kiln-drying have not yet been thoroughly worked out. Durations of from 5 mmutes to 24 hours or longer, and pressures rangmg from atmospheric to 50 pounds gauge, have been used in practice. The higher the pressure the greater is the effect produced, and the longer the time the more thoroughly the treatment penetrates the wood. Experi- ments have shown that a pressure slightly above atmospheric for 24 hours will shghtly darken 2-inch maple clear through, and a pressure of 40 pounds will turn oak and probably other hardwoods almost black. Even where the strength of the wood is not the primary consideration, it probably is not safe to exceed 15 pounds gauge pressure (250° F.), except for special purposes. THE PROCESS OF DRYING. After the wood has been heated thoroughly in a humid atmos- phere, either in the kiln proper or in a separate compartment, it is ready to have the moisture removed by evaporation from the sur- face. In kdn-drying uniform circulation apparently is the most important thing to be secured. The fact that air when it enters the drying chamber wiU be cooled, and therefore will tend to fall, should govern the method of pihng and the direction of circulation.^ This means that the air should be allowed and assisted to pass downward through the pile, either by entering at the top of the pile or by an adaptation of this principle to other methods of piling. The rate of evaporation may be controlled best by regulating the amount of moisture in the air (relative humidity) circulating about the lumber in the kiln; it should not be controlled by reducing the air circula- tion, since a large circulation is needed at all times to supply the necessary heat. Air at 100 per cent relative humidity contains aU the water it can carry and has no effect in drying wood. If, how- ever, the humidity is reduced to 90 per cent and the air then passed through a pile of wet lumber, the air can take up a certain amount of moisture. If drying does not progress rapidly enough with the cir- culating air at 90 per cent humidity, it may be reduced still further. This may be accompUshed by ventilation, by condensers, by water or steam sprays, or in a number of other ways. Any well-made kiln which will allow the control of the humidity, temperature, and cir- 1 See " The Circulation in Dry Kilns," by H. D. Tiemann, Lumber World Review, May 10 and June 10, 1910. THE SEASONING OF WOOD. 27 culation of the air passing over the lumber should give satisfactory- results. If checking begins during the drying process, the humidity should be increased until it stops. Steam jets in a kiln are often useful for this purpose. In changing the humidity the circulation should not be reduced. A large body of moving air is necessary in order to keep a uniform temperature clear through to the center of each piece of wood in the pile and at the same time supply the heat required for evaporation. If suf- ficient circulation is not secured, the supply of heat for both pur- poses will be lacking and J;he material will not dry uniformly. Figure 18 shows the conditions in a kiln during a run with refer- ence to temperature, humidity, and moisture in the wood. It will be noted that the humidity is kept high at first and lowered gradually. The temperature is held at a certain level for some time and then raised. The mois- ture is lowered gradually to a final condition of less than 5 per cent. The maximum rate of drying at a given temperature is reached when moisture is evaporated from the surface of the wood just as fast as it is transmitted from the interior. This rate is fixed by the rate of transmission of moisture within the wood and varies with different woods. The temperature of drying ap- parently influences the rate of transmission of moisture within the wood. The higher the temperature of the wood the more rapid is the rate of transmission of the moisture, and hence the rate at which the moisture may be evaporated safely. This, of course, apphes only to temperatures below those which might result in injury to the wood. |l . -. r" _4 ~ T r r / ^ " " r"; V " ~ r r _______ ( ; c -1 - 7 r - i -_ _l^ ) / ( '-^r-t— -.J--— n _4. -^ l-l 1 » 1 '. t -i - i \ \, "Vi 1 1 ___::3 ) • \ 1 ~ r- i I 1 > 1 < r '~ > 1 ^ r ; / \ \ \/ § " ? 9 ? s ? i s — 1 — s I^HN^UMfJ $3311930 - 3afu ra3dt<3j 28 BULLETIN 552, U. S. DEPAETMENT OP AGRICULTUEE. Drying tends to render wood more or less brittle. Although the strength of wood increases with its degree of dryness, yet wood which has been dried and resoaked is less resilient than when green. There- fore, where strength is the prime consideration, it is preferable not to dry the wood beyond the degree at which it is to be used. The final stage of kiln-drying is generally conducted at a humidity some- what below the actual humidity that on long exposure would pro- duce the same average moisture condition. This is done in order to hasten the drying and to make it uniform throughout each piece. ADDITIONAL COPIES OF THIS PUBLICATION MAY BE PROCURED FROM THE SUPERINTENDENT OF DOCUMENTS GOVERNMENT PRINTING OFFICE WASHINGTON, D. C. AT 10 CENTS PER COPY A WASHINCTOX : OnvERXMENT PRINTINf! OFFK'B ; 1917 wr v^unoKCOO liiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiinii 017 111 514