A; /4 23 SEASONING WOOD ECLR SHIP CONSTRUCTION December 1942 U±Sl No. 1423 UNITED STATES DEPARTMENT OF AGRICULTURE FOREST SERVICE FOREST PRODUCTS LABORATORY Madison, Wisconsin In Cooperation with the University of Wisconsin SEA ■■ ' SHI] I. V. TEZSrALZ. Senior Engineer The resurgence of wood as a prime shipbui] :erial dur • as! year3 has brought thia industry face to face with a diffic bottleneck: a v. of properly seasoned stock. This si1 threal to become more acute as time gees on because ship builders :-r air-seasoned material; and air seasoning is by nature slow and un requiring months and even years for the larger sizes of ship til . 11 can, however, be si some extent by vrc !.liz green lumber. A quicker alternative is kiln drying. Modern kilns c- roduee shipbuilders in weeks instead of the monl s i ars req ton- ing. It is true that the industry has in the pasl ted ral preference for air-dried stock. This preference . ~ ; ly due in lar 9 part to the fart that the kiln-dri I srly Led for shipbuilding. It dees not follow . I wever, I I can- not produce lumber suitable for ship construction; the pr liea er drying of the stock fcr shirs, rather than in the relative nerlta of air and kiln dryi: . It is recognized, nevertheless, that existing kiln ca - fcy ma" not be avail- able for much of the lumber needed fcr Bhij : ucticr. : I n- .ecessity. This article review- I ". ich ^ir :ening can be hastened, together with otiier J co..'- shipbuilders today. Seasoning Requi re :o n t s The 30 fa builder cf ects from these cf most other wood crafl . I but the si boats, use conditl ■ re cue:, thai ' "• I bottom planking, arc under water. t« fit it for the exposure condil ' • ■ pi 1 used for above the watei : 'con- struction should be season" I •- the moisture coi use. Except f r r cabJ and join-"- , all wood -ill have ely higher than 1 t ther com'ricrcial UC . R1U23 -1- Use c^r.iit ims , however, are net bhe r r.ly factors that rr r ,- r | ouire- ments for seasoning cf the variouc3 parte, particularly those use ider water, During construction either in the open cr under ccver, the framework planking are exposed tc the atmosphere, arid any green cr unseasoned lateri; will be subject tn conditions that cause rapid surface dry'. , rally accompanied by serious checking and seme shrinkage. Tc prevent defects that might develop during the construction period, * , -ame preseasoning is desirable even for parts that in service arc alw; 'or If such under-water material is adequately seasoned before use it '-'ill nc t check, split, warp, cr shrink during construction, and the slighl .-'•■ Lli b occurs after launching will help make the seams tight. The o ti moisture content is from 15 to 2C percert. As an alternate, because o J the ■ : of seasoned material, seme shipbuilders or" necessity use terial and attempt to prevent drying luring construction by •■ •"_-'- mcisture-repellent coatings to the parts after they have bee '. Thi methods generally used are only partially effective, and often the parts are subject to considerable checking and seme shrinkage. Soaking treatments with hygroscopic chemicals, en the other hand, offer an effective means of retain- ing mcisture in green wood during construction, and are disc- ssed hen for that purpose. All unseasoned stock received by the boatbuil^er should, of course, be properly piled in the yard or shed so as to -protect it • ainst damage from checking and splitting and, at the same time, to obtain as much seasoning as possible before it is processed. i'- r isture in Wood Wood green from the tree may contain from 30 to 30C percent water, based on - dry weight ^f the wood. This water is held in the wood in twe distinct ways; about 2 s tc 30 percent is imbibed in the cell walls, ?nd the "balance, free water, is in the cell cavities. Shrinkage starts with the va ration of the : lee 1 water from the cell walls, which takes place after T le :>ee t er i ? Di« onet Sir.ce, in seasoning green wood, the surface dries more idly th< bhe interior and reaches the fiber-saturation point first, shrinkage starts in that part even though little or no irking has b; i place in the interior. - is unequal drying is the principal cause of surface and end ch^cki . The greater the difference in moisture content between the surface bhe in- terior below the fiber-saturation point, the greater the stress = jvelooed. Through control of drying conditions, it is possible tc minimise the noisture gradient and, thereby, checking and splitting. When wood is subjected tc a constant temperature and relative humidity, it will in time come to a definite mcisture content calle~ the equilibrium moisture content, which is determined by the prevailing humid.it.Vj This re- lationship between the moisture content of wood ■ ; the surroun -1 ! . atmos- pheric conditions is shown on figure 1. Miraeo. 111423 -2- Air Drying ^f Lumber h control lie relative humidity, it Ls isiblt - wood. . rate of ti fua I : .. - fasl r at jh than at low t eurocrat' res. In air i It U sible tc cent re I the temper I . rogh r^ er Lt >s sible to obtain some -.-.e humidity within a pi"- >er andi thus, tc Lncr sc or decreas drying as desir Some ik, are liable I - and 3t be dried 3lowly tc minimize checking. is can t> re rapidly wj •-. Ar the thickness inc icy bo c eck be: ir. all species. In air dryin . 3 itrol are -. ' iling or covering of 1 that In- crease cr deer. the vclume cf air moving through the pil . -be * • . " • - sed by lumbermen, applies to materi- L to the air for ai i of time. If exposed for i suffi - •' loisture content ranging from 6 percent , as in i arid Sc - w 3t, tc 2U percc b, aa in winter ir. the Pacifi ." F r the ' States as ( minimum moisture content ougj 1-inch r is 12 to 15 percent in t v aver- ge is 3r. Piling fcr air 3 ease euld accr ber of It sh aid rovlde f r roper air circulation, it s • suitab] rotecti n fi sun a: Ln, and it should keep - re If these things are accomplish I e best ir; li will r lefocts will be at • minim m. Amon - 3h defects are st I surface check! , and . ."' ^r.<~ rule will ■ ly t all weather to all cl stock; rrecies, particul " ocd must bo ^pen piled to hasten i I vert : .lie 0*' e 3 the it be close piled tc 1 m y cause checki I he ( - : ' :iples will i b season^ : Stain Prevention .- If i1 bhat s1 likely to ncc r, sut 3apwood si tion for protectic I I bac ; -r- b eas onin d . Ly inf( 3 1th ** -. 3dia1 ■ ' bat ions are . ractical me: r Ming Ln is br t. treat me Lth tould r. ■ • bh logs wit! lly retard the occ s1 tor aa -23 -3- much as 3 months. The same treatments are suitable for softwood logs, hut may not rive adequate protection for so long a period. Information as to where such chemical treatments can be obtained can "be found in any current lumber trade journal. All chemicals and chemical mixtures used for this purpose are more r r less poisonous, 'Caro must be taken to fellow the directions of the manufacturers closely and to protect the skin and eyes. Vessels used in mixin and apply- ing chemicals should "be thoroughly cleaned, and left-over chemicals either destroyed in a safe manner or plainly marked and stored where they will not be readily accessible to children or animals. Simply-constructed dipping tanks can be used for treating freshly-cut lumber. For maximum effectiveness and safety in handling, manufacturers' directions should be followed. Fcr hand dipping and handling cf treated lumber, water- proof aprons ana rubber gloves protected by canvas or leather pads to reduce wear, are advised. Foundations . — The pile foundations (pile bottoms) should be constructed as follows : (a) The foundations should be rigid and properly leveled. (b) The foundations should be high enough from the ground to allow good cir- culation. The distance from the ground to the under side cf the lumber at the rear cf the pile should be not less than IS inches. (c) Foundations should slooe from front to rear about 1 inch to the foot. (d) Material for piers: may be of concrete, masonry, pressure-creosoted blocks of any species, or the heartwood of baldcypress, redwood, or cedar. ('Then untreated woods are used, all points of contact should be given twe coats cf hot creosote.) (e) Beams and stringers should preferably be pressure-creosoted timbers, or steel if available. Untreated heartwood of durable woods, painted with two coats cf hot creosote at points of contact, may be used when the first two are not available. (f) If existing pile betters are to be used, they should be inspected to see t they comply with required height levels and drainage conditions. All vegetation, debris, and decayed wood should be cleared away. Any part of the pile bottom containing ^.ec^i r should be removed or the decayed area, cut out. All wood parts should be painted with two coats of hot creosote. Air Flues . --~he following suggestions are offered as minimum requirements: (a. Stock 10 inches and wider should have spaces between the boards or planks not less than 20 percent of the width of the board. The boards in- each succeeding layer should be placed directly over the boards below so that the spaces between boards will form uninterrupted vertical flues. ?vo cr more narrow boards c?~. be placed edge to edge between flues. The distance between flues should not exceed l6 inches. Miaeo. B.1H23 _L_ (b) Heavy plank and t: iculd be piled bo thai but not ov inches wide are formed bet" r r. adjacent pieces, o1 ' -' • as described for boards. Stickers . — The follow^ ions are offered <• Lmum re ts: All stickers must be sound, thoroughly iry, free froi st»in, -on (b) Each tier r ^ stickers should h<^ alignc rest (c) Stickers for U/U-inch lumber should be of nominal Ln r : not more than U inch e wide. For thicker lumber I , stickers should be at least 1-1/2 inches thick • ■ ■ (d) Stickers should extend slightly beyond tb ice i .a check!. . (e) Stick Ld not bo over 2 feet apart for es3. For thicker har w Lsand all softwoc s, the I f I tiers of strikers for lc-foot stock should be used. (f) Stock should never be self-stickered. Placing ^f lumber . — The following suggestions ^ro c Lre- " b : (a) Piles should be erected of beards cf equal lengl (b) Box piling should be used for mixed lengths. " ing- est stock is piled in the outer tiers and short ler. rths with one end of a board at one end of the pile and r board at the opposite end r f the pile. In each 3uccee -it- side ends of boards should be kept immediately ever the - below. (c) Each layer should be con : of boards of the same thic a . (d) The pile should have a forward slope cr. pitch to 1 tenl ch each foot of height. (e) ETarroi a are iesirable for stock that will subject • tain, such as I srial containing ~- . g ... siral for stock sue of which must b^ r :d to prevent chec '. (f) Space I be 2 feet for ' : ~t- wood 2nd Coatings .— Eno " '" end check: Lied to freshly c . After : tecks devolcr , coati value, but I - 3an bp tri ted to pre-- I I checks fro deeper into the piece • . RIU23 be trim icd off and the fresl i coated to prevent the checks from exte ieeper into the piece during subsequent seasoning. The coatings ordinarily used pre cf two classes. The first are liquid at Linary temperatures and can be applied cold. The second are solid at ordinary temperatures and must be applied hot. Ccld coatings have the advan- tage that they may be used easily en logs and lumber; hct coatings, because ' 'he nethod of application (end dipping), are net easy to use on large st ock. Melted paraffine has proved verv satisfactory as an end coating for stock during air seasoning. A cold coating developed at the Forest Products Laboratory is hardened gloss oil thickened with barytes and magnesium silicate. 11. e manufacture of hardened -loss oil involves technical operations and should not be attem; bed by the novice. Because gloss oil is made commercially in a number of wa~ r :. and because some C"^ the products are unsuited for ciid. coatings, a gloss oil should be specified which is made in accordance with t he folic w ing f o rmul a : 8 parts by weight quick lime 100 " " " rosin 07.5 " " " spirit 10 rts cf ~" this loss oil add 25 parts barytes and 25 parts magnesium silicate of a grade commonly usee 1 as a pigment for paints. One or two parts cf lampblack may also be added if a black coating is desired. The magnesium silicate helps to keep the pigment in suspension. In time, hevrever, it will settle, and the spirit will evaporate. As a result of these two actions the filled, 'dened gloss oil tends to beco . p pasty if allowed to stand any considerable It is suggested, therefore, that the user ^rctect his gloss cil from 'ation and mix relatively small quantities of it with the barytes and slum silicate as needed. Covering . — All material should be und.er cover, either in an open shed cr with covers for individual piles. One satisfactory type of pile roof consists of two layers of lotir-grade boards, these in the upper layer being staggered with respect to those in the lower layer. (a) A minimum front height of 6 inches abo^e the lumber, with a slope of at least 1 inch to the foot, should be required. (b) The ends and sides of the cover should project sufficiently to prevent snow and rain from beating into the pile. (c) The roof should be securely fastened. Site . --The yard she Id be well drained and keut free of weeds and debris. '.im.ee. RIU23 _6- Kiln Drying 'i Lumber : drying of green lumber tc a moisture content suited to "beat ccn- tiould produce Tutorial fully equal, if not superior tv , air-dried 3t in very much less time. Since "beat stock should have a moisture tent materially higher than 3tcck kiln dried fcr most commercial pur] lit ions that will prevent overdrying must be controlled and maintained. This means that the drying schedules ordinarily used for commercial kiln ing must be modified to suit boat material. Srying schedules have been established fcr most commercial species, and the initial temperatures for green stock should be satisfact r beat material. The relative humidity, however, should not be lowered below abrut 35 percent. Somewhat higher humidities maybe required in some cases, particularly fcr oak mere than 2 inches thick. Thus, through control of the relative humidity, the surface portions of the stock will not be dried below about 17 percent during the interval required to dry the core. Most commercial kiln drying applies to U/U_inch strok, some S/^— , and a very limited amount of 12/U-inch and thicker. While it is comparatively easy I season U/.h-inch stock with very little degrade, difficulties mount rapidly as the thickness increases. Since boat lumber is not dried to so lew a moisture content as commercial stock, less degrade should develop in a given thickness and, for the same reason, it should be possible to dry relatively thick material for boat lumber. Surface checking is more common in boards 8 inches or mere in width than in narrow stock, regardless of thickness. Hence, less surface checking would be ected in U by 6-inch decking than in 3 by 10-inch planking. Some spec: are more susceptible to drying defects than others, and such characteristics . the maximum thickness that can be dried. Generally, the softwoods less difficulty tc season than the r woods. The practical limits of kness for stock dried to 20 percent moisture content are about 2 tc 2-1/2 inches for oak and about *+ to 5 inches for softwoods. Essentials of Sood Kiln Dryi M For the drying of stock from the green condition, the dry kiln ~. re both temperature and humidity automatically controlled so that optimum conditions ing can be maintained. Circulation of air adequate both In uniform:- is nec( 3 ccntr- 1 peratures and hu ty, hence ^hanical moans of creating circulation are nec- essary. 3 of piling must be suited t" the circulation system of the kiln. All lumber should be flat stacked, with edges of th Load in v n- • lumber rs, particularly those cut*. f1 — 1 of type required and skillful 0] I rs. Such companies c .rtake the drying of boat luml r for shipbuilder , • -^23 -7- Water Storage It is a practice of some boatbuilders to store under water endir I ir i in construction large timbers, natural crsoks, knees, i thi L] . Si-nce : Irying takes place in the water, they do net check or split. As l r .. they are submerged they will net decay. Seme builders are of the opinion t long exposure under water hastens the ultimate seasoning after removal from the water and minimizes shrinkage and checking. There may b< bg.sis f^r this belief, though the advantages of water seasoning \ ear to be over- Cne definite advantage which develops when . i ce is taj from the water and worked iown for the part it is to serve bef re ■? ssoning rts is that the reduced volume and size of the finished p.irt compared to it of the original piece will reduce checking and splitting. This ndvant ir counterbalanced, however, by the fact that the seasoning takes 1; :e -'tor i and while in position in the framework of the boat, and th< s rink* will thus cause seme loosening of joints and fastenings. Dccasicr Lly, seasoning defects are serious enough to cause a timber or other member to be rejected, and it is better if the rejection is made before the -niece is worked up than after it becomes a mart of a beat frame. C oatings Moisture repellent coatings, such as oils, paints, or tarnishes, are seme- times applied to wood surfaces after they are ~ - - q in an attempt to retard surface drying. This method, though a worth while expedient, is only par- tially effective, and the parts are often subject to considerable checking and some shrinkage before the ship is launched. Moreover, some drying and checking may take place before the part reaches the stage where the coatings are applied. These ccatin s vary in degree of effectiveness from very low in the case of boiled linseed oil to fairly high in the case of two or more coats rf lead and oil neir.t. Chemical Soaking of G-rc-en Stock Chemical treatments offer an effective means of retaining moisture ir. wood, and thus reducing its tendency to shrink. That certain chemicals hnve hygro- scopic properties, or the power to attract moisture from the atmosphere, is ^ r ion knowledge, Tabic salt becomes wet and sticky in damp weather. Chemicals or salts in solution have the same property. Conseque tly, if a roscopic chemical is applied to green wood, it is diss lved in the con- tai ' water \ .'. the cuter parts of the wood absorb some of the chemical. This impregnated sone partakes of the properties of the chemical solution, attracting moisture from the atmosphere when the relative humidity reaches a certain point. It will also maintain a higher moisture content than will natural wood under a definite atmospheric relative vapor pressure or humidity (7i:;. 2). This phenomenon has been utilized by lumbermen in the s; lting of surface checked lumber t close the checks, and by shipbuilders in salting ski; s . Miueo. EIU23 _g_ Whenever it is desirable *- restrain the drying of ship members they may I treated with a chemical "before installation. Such treatmer. • not assure that all checking and shrinkage can te eliminated, "but they will materially rpduce the extent cf these defects. They are mere effective when used on Dc . ;li s-fir than on oak hut are worth while for oak. Further, chemically trc- • later ial will check lesi under proper kiln drying conditions *he same material wculd under normal air drying conditions. Common - Lt La cheap and effective, hut corrosive. Urea, which is scmewhat mere cost. both effective and noncorrosive. The timbers may be s~~ked in an aqueous solution (35 parts by weight common salt to 100 parts of water; or 100 parts cf urea to 100 parts of water) for a week ^r :r.cro, or they may be sprinkled with the dry chemical. In the latter instance, green wood is piled with alternate layers of the chemical (the layer of chemical shculd be ab^ut l/ 1 ^ inch thick) and left undisturbed until the cnemical has disappeared, but for net less than a week. The treat- .t should be appliel as soon as possible after cutting the timbers. If this is done at the mill, the members will be protected daring transit. If the surfaces of the 1 . e become dry, they must be wet with water before the rile is cc icted. Where the dry method of application is used, the e - f planks and timbers ild be BTirayed nr cabbed once daily for 5 days with a solution of t) chemical, saturated at the temperature of application, to minimize ei checking and splitting. Whether the treatment is applied by soaking or by the dry method, the chemical contained in the outer layers cf the wood will reduce the amount of drying that takes place before launching and, theret decrease the amount of damage caused by dryir -. The chemical will penetrate into the wood for about l/ 1 - inch, with the b concentration at the surface. Any shaping or cutting that removes the ] : od exposes untreated wood, and normal moisture changes may be expected in that ar . Stem ■ : . ceo r v rimilar parts that arc worked deeply bit be treated after v * bside surfaces of planking should be paint- ed j - ssible after the smoothing operation. Other members that arc worked ~r iressed deeply enough tc cut off the treated zone should be painted or otherwise protected - -ure change. chemical treatment could be used tc " advantage on all jx rts 1 except the decki. Lcl i be of seasoned material. f unseasoned wood in shij jonstruction increases the probleo r.g and paint maintenance, particularly above the boot top] ox- ter. - . presence <^f the c 5al adds to tl extent that it increases the a Lstur ; I at. - • • Lth there salts will r ' ; . t o the . I fact, the "salting" of wc Lps as tective mear re • as been a rec bice arr ipbuil I sars. . 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