Document 2j22yNj66024DbmQ35D0ombY7

N41539 U. S. Department of Agriculture, Forest Service FOREST. PRODUCTS LABORATORY In cooperation with the University of Wisconsin, V;* ' MADISON, WISCONSIN i. \or oio '. OO r . . - -- i O ; V , /'H _ 0 - '. : >: /; f r -- , . , ; ` ; ... . % - vv or* o - 'oiol . , ? r; * -O -i*. '* I or, : -; ^ 1 ' : {\ r*.n .. , ` . . .0 ; / ' ; ! ' c c \i.j. : o V ` t T r? i.vt'tvoi' o!.. r: r, f; only tO* r )! - ' rool r r.-y ! ' ' V/OC- O 0. \.T'l - 'v 0 -V"*;':-': * \ ' *' i . '/. j . * U \ i 1*" ;* " 0 r vi;V:e.n vi" :V'-i'-BTEHAV, I.OR. OF,, H, OUSE .P.A.I.N.T.S it ! '' ^r-V'Crior- o-?.oo hnlx* " -o ':Y- oo'lo K. --oor-oo. i-O-o 1 nk :r j t '! h .."r . ' * l o i '* o 1 By F* L..BROWNE = o Senior Chtmlst TO ! 0 o ; 00-00 0!. i ; ft'.u O' O \ O `O-: , . OOO * 1 SUM-O.-V ? '*'v* *\ U -t. ,"'O3c?0 lo r . I o . ' o.1 ? . 'i '' vo ^ ^ . -0;r'0> i O; ; j`` 0 V r -V- O v r, ; ; '4 . - lot '.0:0 :;.i V ' * O' -.O OO O 0 * `o: ' . - `"O' -M.. / : ' ' ' ' 1: vvVj l 1: . : ? ... hOOO^O a .JO.: .-'O -. ;. y; 4. . ; O 0OyiJv': (Oio*V:^- ` " Ovo. .O', - . , O-..O'.-' .: * ' -or. ' , O. . o.:-.^r -. : ^ . :';'r # ., . -t\ ts^vv'i ic oor;U*: ; . - w- !.? t . ; (ii,. `r}::, : -0.-. ; - ; .. ; ; i- .. . . .: . . . December? 1934 BEHAVIOR OP HOUSE PAINTS ON DIFFERENT WOODS By P. L, Browne, .Senior Chemist, Forest Products Laboratory!:, Forest Service, U, S. Department of Agriculture. At least four factors significantly affect the service ableness of exterior paints on wood, namely, the kind of wood painted, the composition of the paint, the technic of appl"* .-.ation, and the severity of the climatic and local conditions of exposure. This panel? honsiderb only the first of these factors, the kind of wood painted. Even when the best paints and painting pro cedures are used coatings remain serviceable much longer on some woods than on others. Until further research discloses some practicable-means of making paint last at least as long on nil woods as it now does on the best of them the hearing of wood properties on paint behavior should be properly understood if maximum satisfaction Is to be obtained from painted woodwork. Tills discussion is based upon information from two general sources, observation of practices and experience with houses in service and experimentation by means of test fences. Observation of houses in service reveals practical details of paint behavior and particularly of unsatisfactory developments for which the cause and cure should be sought. Real houses, however, are rarely available or suitable for experimentation on a scale adequate enough to develop reliable technical data. Test fences, on the other hand, offer the necessary freedom for systematic variation of woods, paints, technic, and place of ex posure (8, 5)_. . Conditions that lead to unsatisfactory results1 1 "Maintained at Madison, Wisconsin, in cooperation with the University of Wisconsin, O --Numbers In parentheses are references to publications cited at the end of this paper. R 1053 - -1- with paint on test fences likewise lead to dissatisfaction on houses. On the other hand certain forms of paint failure ob served on houses in which moisture gets into the sidewalls (2) are rarely reproduced on test fences but can be reproduced for study by suitable laboratory devices used in conjunction with test fence exposures (8), The principal test fence studies by the Forest Products Laboratory from which data are drawn for thi.s discussion are called for conveniences 1, The 1924 Series; on the painting characteristics of softwoods (3), 2* The 1925 Series, on criming' and soecial priming paints (4). 3, The 192v Thinner Series, on paint thinners and the effect of resins in longleaf pine (7). 4, The 1929 Douglas fir Series, on painting characteristics of Douglas fir (not yet pub lished), , 5, The 1930 Hardwood Series, on painting character istics of hardwoods (11). 6, The 1930 Primer Series, on special priming paints (10). 7, The 1930 Extractive Series, on the effect of extractives in certain woods (not yet pub lished) , These tests were made at 11 stations in different parts of the United States, The test panels were exposed vertically facing south. Supplemental evidence is available from many other series of tests by the Forest Products Laboratory. The results with respect to the painting characteristics of woods are thoroughly in accord with experience as indicated by commercial practices in the selection of woods for building construction in which painting is important and with the writers observations of houses in service. -2- R 1053 Classifications and Properties of Wood Lumber ts cut from trees of many species and the group** ing by species or closely related species Is retained in mer chandising because in general each species has more or less characteristic average properties and finds uses, accordingly. For some uses paintability is a property of much imnortance but as a rule other properties must likewise be considered in choosing lumber ^or a given use. Softwoods and hardwoods A sharp distinction in oractice is made between soft wood lumber, which comes from coniferous or needle-leaved trees, and hardwood lumber, which comes from broad-leaved trees. Soft wood lumber, however, is not necessarily soft wood; both softwoods and hardwoods vary widely in hardness and softwoods like southern yellow nine are much harder than hardwoods like yellow poplar, In general the higher the specific gravity of the wood the harder it is. As a class the softwoods are more abundant, cheaper, more easily worked with tools, and better suited for the framework and exterior surfaces of buildings than the hardwoods. As a result the softwoods are very much more important than the hardwoods from the point of view of exterior painting. S'ding, shingles, exterior trim, and millwork are made predominantly from softwoods although a few of the light-weight hardwoods, such as poplar and basswood, are used for such purposes to a minor extent. Heavier hardwoods such as oak are sometimes used for front doors, door sills, and half-timbering but are more often varnished or stained and varnished rather than painted. Microscopic structure of wood Most of the prooerties of wood are profoundly affected by its characteristic cellular structure. In the softwoods the principal wood cells are the tracheids, which are long hollow cells (fibers) with celluloslc walls, tapered and closed at the ends, and cemented together in a matrix of lignin. The tracheids are roughly 2 to 4 mm, long by less than 0,03 mm, wide and run vertically in the trunk of the tree. There are also ray cells running radially in the trunk which are shorter, and blunter than, the tracheids. The hollow centers of the tracheids and ray cells R 1053 -3- Figure 1. Microscopic structure of a softwood (left) and of a hardwood with large pores (right). The top surface, TT, is transverse (end-grain), TG tangential (flat-grain or plain-sawed), RR radial (edge-grain or quarter-sawed), AR annual growth ring, S springwood, SM summerwood, MR medullary ray, ML middle lamella (lignin) which cements the cellulosic cell walls, SW together. In the softwood TR tracheid, MRT medullary ray trache'd, FMR fusiform medullary ray containing horizontal resin passage HRD, VRD vertical resin passage, BP bordered pit, SP simple pit. In the hardwood F fiber, P pore or vessel, SC open grid at spliced joint in a large pore, K pits. R 1053 Z M 34953 1 are pot completely Isolated from each other because the cell walla contain small pits covered by membranes some of which are known to have tiny openings about 0.00002 mm. in average diameter for intercommunication. Air and liquids can pass through such small openings but the oartlcles of nearly all paint pigments are far too large. In the living tree the tracheids and ray cells pro-? vide both mechanical strength and channels for movement of aqueous sap. In air-dry wood the hollows are empty except for air, which can move through the wood freely, though slowly. In some softwoods, such as the pines, spruces, larches, and. Douglas fir, there are scattered through the structure certain openings between the cells called resin passages, which run both vertically and radially. The true .firs, hemlocks, cedars, cypress, and red wood do not normally have resin passages. Growth in the living tree trunk takes place at the junction between wood and bark and in temperate climates i3 con fined to the warmer part of the year. Thus each year a new layer of wood known as an annual growth ring is deposited over tho wood of previous years. During the early part of the growing season sor.ingwood is formed, which has tracheids with thin walls and large cavities so that it is light in weight and in color, soft, and as much as 80 percent of \ts total volume when dry may be air space. Later in the growing season summerwood is formed, which has radially flattened, tracheids with thick walls so that in is heavy, darker in color, hard, and as little as 40 percent of its volume may be.air-space, Table 1 presents the writer's determina tions of the specific gravity of springwood and summerwood whittled from the same annual growth rings in a few samples of some typical softwoods. Although summerwood has only half the capacity of springwood for holding liquids, liquids generally move through summerwood much more readily than through springwood and paint oils penetrate deeper into the summerwood. Pieces of wood differ widely in the width of their annual growth rings, usually expressed as number of rings per radial inch, and In the proportion of springwood and summerwood in each ring. If the tree grew rapidly the annual rings are wide. The number of growth rings may vary from less than Z to more than 100 per radial inch, A high ratio of summerwood ;.:o soringwood makes wood heavy and strong. The summerwood may vary from more than one-half of the annual growth ring to an almost negligible proportion of it. In hardwoods the function of the tracheids are re placed by two types of wood cells, '"he fibers, which chiefly R 1053 -4- CSOT H T able 1 .. S p e c ific g r a v ity o f sam ples o f springw ood and suam eryood o f tiie same a n n u a l g ro w th r in g in ty p ic a l softw oods 11 11 1 1 nd I I O 1P to1 d 1 Sr 1 <D 1dfo ! 0Id 1 10 13 Ph I 1 CO f 1 O9 a 99 1 rd f 1 o tP CtJ 0} \ p I d 1 2b 1 O S 3) J O !d1d 1P 1 0 1 d 1 P I P, 1 1 CO ! 11 9 0 o a 1 rd 1 .1 o I 1 1 ofTn*. 1 I 1d 10 H I I n3 1 I 51 3 i 1 CO 1 { 99 8 I <d 1 IoI 1oI P iv 1 P | to 1 P(d1 {P 1 td1 f Qa I I CO a I 0 9 1 ! 1 1 | | i! I 1 PI o1 oI $3 1 1 t 1 1 1 S f I 1 1 1 I t- o '-O ON o o O' k n H- d* KN * a 9 0 0 9 9 9 a e a * 9 00 P ON CM a ON o o o* 00 CO CO VO 00 00 e 9 9 9 9 9 9 e a * o 90 * BO C A 9 9 9 9 P to H CO rH H KN KN H" o KN AO O o UN ON o n CO 9 '0 On ON cO ON 0 ** * o * a e 90 0 9 9 9 9 <9 0 & o <# 00 60 09 09 OJ CM k n u n ON 0~ H NN 00 rH VO ON NO o ON k\ CMo KN CM CM 0 KN CM o 0 9 * 0 $ * $ 9 0 0 9 * CD 0 i--1 P P< PU & c3 a d CO CO p rH to ^d 0 0 P P CM rH iH O P-. P< d to CO 3 3 0 to CO cd ctf a 0) d 0 d 0 0 o3 P< P o 0 d u 0 a H CM P dP da <s c*\ rl d 0 si d 0 ,d rd Oo rod o P *0 ctf 0H p p ff. fa P to d d d *d *d P o 0 0 rd od co CO fd nd n 0& && 09 99 0 a H P. frj 0o a rH H rH 0 >5 ct5 0d od d0 0 rd P dd oa CO 'd do d -h 0 <H OO d0 3 p, p0 0 fik dO 0 0 rd 0 oo 9Q Cu P Oh 0 o ,o d 0 P *H 0 >d do U0 to d at oU H P fH X H0 O 0P Pt o 0 P 0d ,d 0 pp a do oo P 0 UNr+d3 . rH in .d * P O rH OP ^6 0do H o d 0PP Cd P P o 'cO 0a 0 d *h pdd PO0 OP dp O to P d rd Pp 0 *P POP d 0p P< rH > a to cd O0d o d bO Pi provide mechanical strength, and the cores or vessels, wh^ch serve as the main channels for movement of liquids in the living tree. In general shape the fibers resemble the softvrood tracheids, that is, they are long cells, tapered and closed at the ends but as a rule are much smaller in size than tracheids. Interspersed among the hardwood fibers are the vessels or pores, which may be de scribed as long pipe-lines extending longitudinally through the wood. The pore cells are approximately cylinders open and spliced together at the ends. The pores are considerably larger in diameter than the fibers and are usually the elements that give characteristic aonearance or ''grai nu to the hardwoods. It is convenient to classify the hardwoods on the basis of size and distribution of pores into* 1, Hardwoods with relatively large pores, a. Ring-porous hardwoods b. Diffuse-porous hardwoods. 2, Diffuse-porous hardwoods with relatively small pores. In ring-porous hardwoods, such as oak, chestnut, and ash, the pores are larger in the early sprlngwood and compara tively small in the sumnerwood. In diffuse-porous hardwoods, such as walnut, birch, and gum, the pores are more nearly uniform in size and distribution throughout the annual growth ring. The hardwoods with relatively large pores have pores larger than those in birch. In ring-porous hardwoods the variation in size of the pores makes the summerwood denser1 and harder than the sprlngwood but in diffuse-porous hardwoods there is usually far less contrast in density between summerwood and suringwood than there is among softwoods. In many species both of softwoods and of hardwoods the older wood nearer the center of the tree differs in color and in content of certain chemicals from the wood nearer the bark of the tree. The former is the heartwood, the latter the sapwood, Heartwood is usually less readily permeable to liquids than sapwood, often much more durable against decay, and is. usually immune from blue stain fungi. R 1053 Commercial grading of lumber In commerce lumber is first classified by species and then the lumber of each species is divided systematically into grades according to the number and size of defects appearing on the surface of the boards. Writers about painting often confuse classification by species with grading for quality. It is never correct to speak of species A as lumber of higher grade than soecies B, For certain uses species A may be preferred to soecies B but as a rule there are other uses for which species B is superior to aneciea A. Even If species A is one that generally holds paint better than soecies B, high grade lumber of species B Is probably better for exterior painting than low grade lumber of species A. Some of the defects considered in grading lumber and objectionable for painting are knots, knot-holes, pitch pockets, pitch streaks, pitch, and loose grain. Lumber of highest grade admits only boards with minor defects and blemishes. Among the' softwoods the four highest grades are known collectively as the select grades. They contain only ''lumber of good appearance and finishing qualities'* and are the ones most likely to be used where painting is important. The select grades are identified by letters A, B, C, D, In redwood and southern cypress the highest grade is known as "clear heart" and admits no sapyvodd on the face to be exposed to the weather. Limber containing defects that Impair painting falls into the several common grades identified by numbers 1, 2, 3, 4, and 5. Gra In Boards cut with their principal surfaces roughly tangent to the annual growth rings of the tree are known as flat-grain or slash-grain boards In the softwood industries and as plain-sawed boards in the hardwood industries. Boards cut with the principal surfaces roughly radial to the annual rings are known as edge-grain or vertlcal-g'cRin boards among softwoods and as quarter-sawed boards among hardwoods. On flat-grain or plain-sawed boards the bands of summerwood flare out Into much larger areas than they do on edge-grain or quarter-sawed boards. The two principal faces of an edge-grain board as a rule are practically alike but those of a flat-grain board are not be cause the annual growth rings are convex toward the face nearer R 1053 -6- the bark of the tree, the bark-aide, and concave toward the face nearer the pith of the tree, the pith-side. In many softwoods the plth-rside is distinctly inferior for exterior paint?.ng becayj!Q of a tendency to develop during weathering a form of cracking ' known as loose grain# Lumber cut mostly from relatively small softwood trees , consists predominantly of flat-grain boards. In cutting lumber from large trees that yield a high proportion of clear lumber it is possible to turn out a predominance of edge-grain boards and to sell edge-grain lumber separately from ndxed edge- and flatgrain. Chemical nature of wood The walls of the wood cells .consis,t chiefly of cell-ulosp and closely related carbohydrates' fb.e pells .are cemented to gether with lignin. The ppLluiosp .of .aijl wpods i:s chemically mi^ph the same, perhaps identical, The jLigni,n is likewise very similar in all woods. Together the pellvilpse^ itp plosely related carbo hydrates, and the lignin r,e .calledfor pon-yenien'ce .the wood sub stance. Wood substanc.e la a hygr-.p.^c.p.p^.c ^weCiing material whose moisture content app yoilurae ,cpmp .9'brtpja:.wl'jbh ytehp relative humidity of the -wpod .substance contains roughly 3$ .percept |by' iseigih.t ;of water and is fully, swollen Green wood apLwayp'contlaipe mpr,e then enough water to saturate the wood substance, the expesp r'emeih^h.ifj h0 liquid water or water j vapor Ip." hw'ood, On drying the free water Is removed'''f lr.s'tV'' '^he moisture p.gptepf; when free water is absent but 'jbhe'Woph^suhstance is satupgifepd is palled the fiber-saturation go|ny*0.. ''&ry)ip causes w,o.o.d .substance to shrlpk but ;of: `thg shrinkage ffppear.s ip .changes in external dimen-. sionsv,of the b.oa?'^ because part ijs jfcajken up by change in the volume'of the cavities in the w.oo.d pqlls.. External shrinkage is greatest' tangentially q `the anmial growth rings, somewhat less radially, and very s^ght longitjuihihally. Thus edgergnpin boards shrink less in width"'tfia'n fiat-gbain'boards arid neither kind, changes much1|n'lehb^h'hUrihgdryipg Unless the boards contaip an abnormal tyHe of'hood'known as ^cpaipre'sston wpod". When in the air-dry condition wood is in a partly shrunken condition with tt.p' moisture content1 and dimensions `subject' tp change in response tp changes in relative humidity of ihe'fslr# In their effect op thp dimensions of wood, changes ip'humidity 'are usually much more, important than changes in temperature. The common naint liquids are taken up by wood only as free liquids in the air-space apd not cause any change, p dimensions pf the wood, Alcohol, howeypp^ causes wood to swell slightly.` R 1053 r7T In addition to wood substance, wood contains extraneous constituents that can be removed fairly easily by extraction with suitable solvents and are very variable in nature and in amount, differing not only between species but often between heartwood and sapwood and. in the heartwood in different parts of the same tree. The extraneous constituents are conveniently classified into those soluble in organic liquids like ether and alcohol and those soluble in water. The ether soluble extractive of longleaf and slash pines is a resin containing turpentine and rosin. Rosin is a substance of acid reaction capable of forming soans by reaction with basic paint pigments. The other pines, both white pines and yellow pines, contain similar resins in material amounts. Western red cedar, redwood, the true firs, Douglas fir, hemlocks, spruces, and larches ordinariljr contain only small amounts of ether soluble extractives except for resins that occur in local deposits in the tree like pitch pockets. Hardwoods do not contain resins like those in the pines. Southern cypress and some of the cedars con tain characteristic oils soluble in ether and some of them soluble in linseed oil.. These oils are quite different chemically from the piny resins and contain very little material of acid re action. Alcohpl.ic extracts from wood usually contain most of the ether soluble substances together with some of the water soluble ones. . The water soluble extractives include tannins and other complex organic substances often of an aromatic nature, some of them highly colored carbohydrates, organic acids like formic, acetic and their homologs, traces of organic nitrogen compounds, and mineral constituents of wood ash. Woods in which the heartwood is strongly colored, such as redwood, red cedar, and walnut, owe their color chiefly to extractives of this class. The boards used for the 1924 Series of tests were analyzed for their content of extractives with the results pre sented in Table 2. -8- R 1053 Average content of extractives in the boards of softwoods used, for the 1924 Series of paint tests. Wood Percent of the dry wood iilligrams bf~ po soluble in-- sts ssi\im hydroxide ----------------------------------------srequ^red. to neutraCold Hot s Ethyl ;Ethyl;lize extract from water via ter; a lcohol; e ther; 1 gram of drjr wood ; by-- 4 4 4 -a 4 4 04 0 * 4 4 Ethyl 4 Ethyl 04 ft 9 9 4 alcohol 4 4 ether Alaska cedar 2.8 3.9 4 a 4.0 Port Orford cedar ; 2.5 4.0 > 4 3,8 Western red cedar ; 8.0 - 11.5 0 13.2 Southern cypress ; 1.6 2,0 ft 9.0 Douglas fir ; 3.7 c 5.8 A 4.3 White fir (commercial): 2.0 0 0 3.1 0 ft 2.7 Eastern hemlock 2 9 4.2; o e o ft fi 5,0 Western hemlock ; 2.7 0 0 3.8 o 0 3,,4 Western larch ; 14 ,, 9 ft 0 17.0 ft 0 4,0 Pjnderosa pine 2.4 5.0; ft ft 0 0 5,,7 Sugar pine ; 4.5 0 0 7.2 0 9.0 Northern white pine : 2.9 9 4 5.1 0ft 8.0 Western white pine ; 2.8 ft 0 4.3 0 0 6.5 Southern yellow pine ; 3.5 0 6.2 ft 0 8o5 Redwood ; 6*4 0 9 10.9 0 ft 11.8 Sitka spruce 4.62b7 0 ;9 ft 0 3.6 White spruce (com.) ; 1.3 4 0 2.4 0 0 4 C ft J0 0 2.2 00 1.30 o ft ft 0.83 * ft 0 1.40 ft 0 5.5ft 0 ft 1.0ft ft 0 0.6O 0 o0 1.2ft 4 ft 0 0.40 ft 0 ft 0 1.1 0 3.3 0 00 4.30 ft 00 0 0 4.0 ft 3.34 0 00 5.2ft ft ft 0.7ft ft 04 0.60 ft 04 0.7ft 4 00 fftt 0 0 2.9 2.7 7,5 2.3 5,5 1c4 3.5 4.7 4.6 6.9 8.5 6.7 5.6 8.2 0.6 2.7 1.6 4 0 0,8 4 s 0,4 j- o 9 0 C ,*1 a. c C7 c4. 0 ,, 3 . 9 0 1,0 0 0,4 l,0 ft o ft 0 3,9 5 90 ft , 0ft 3.7 ft 0 3.0 0 ft 7.1 0 50 0. ft 0 0.8 4 0 0.7 ft 0 Painters often call the bands of summerwood in softwoods "pitch streaks" because they attribute their darker color and hardness to the presence of resin. True pitch streaks, however, do not follow the annual growth rings and affect both springwood and summerwood. Moreover the summerwood of softwoods relatively free from resin is often just as hard and dark in color as that of resinous species (Table 1), Summerwood and springwood of the same growth rings were whittled out of boards of ponderosa pine and southern yellow pine and their content of extractives deter mined with the resxj.lt s shown in Table 3. Boards of high resin content had more extractives in the springwood than in the summerwood while those of low resin content bad a fairly uniform distribution of resin between springwood and summerwood. R 1053 -9- s ; 5 ? ? ; ! .j ii ? ) i j [ Table 3, Content, of extractives found in springwood 4 i and summerwood of the same annual growth ring in ponderosa nine and southern yellow pine. Description of wood ;Percent of the dry wood soluble ;in~- 9 9 Cold. a 9 Hot 9 Ethyl 9 water 9 6 water 9 9 alcohol Ponderosa pine. springwood u u Summerwood. 999 ; 1.8 .9 9 3.0 9 9 3.53 j 1i6 9 2.8 9 9 3.08 a1% Southern yellow pine. n nu springwood summerwood : ; 1.6 1.2 9 9 9 5.3 3.3 9 9 4.76 9 9 4,36 6% Southft ern yellow tt pine, ii springwood summerwood ; j 2.0 1.1 9 9 9 9 6.3 3.9 9 9 25.42 9 9 17.71 %9 9 Southern yellow pine in which the; a 9 99 resin was evident to visual in- ; 9 9 9 9 spection* springwood ; 1.3 9 9 6.3 9 9 37.10 summerwood ; 1.2 9 a 4.1 9 9 24.42 99 99 9 Behavlor of Paint on Woods During Application The first coat of oaint applied to new wood Vs very largely consumed in filling the cavities of those wood cells that were cut ooen in planing the surface of the boards. Much of the liquid of the priming-coat paint penetrates deeper into the wood because it can pass through the pit membranes in the cell walls and reach the cavities in wood cells beneath the surface. Since woods vary in.the volume of cavities presented at the surface and in absorptiveness for liquids they likewise vary somewhat in consumption of priming-coat mint. In comparison, however, with porous materials like plaster, concrete, and stucco, wood con sumes relatively Itttle paint because its structure Vs cellular rather than labyrinthine or sponge-like. R 1053 -10- Consumption of paint by different softwoods During the exposure of the tests of the 1924 Series careful record was keot of the amount of paint consumed (1). There were 44 oanels of each wood. Sets of nanels were painted by each of 11 painters of whom 4 were professional painters and 7 were technical men or laboratory aids in paint testing laboratories. The average spreading rates for priming-coat paint (square feet covered by 1 gallon of paint) on the differ ent woods and the average specific gravities of the woods are given in Table 4,, In general the specific gravity varied directly with the spreading rate, that is, the lower the specific gravity the lower the spreading rate. how specific gravity, of course, indicates a relatively large proportion of sprlngwood with its larger cavities in the tracheids. There ore, however, other factors affecting spreading rates because redwood, Sitka soruce, western hemlock, and Doug],as fir bad somewhat lower spreading rotes than the specific gravity indicates. Point seems to wet these woods very quickly and the oil is absorbed while the brush sweeps over it, thus encouraging the naintei1 to apply more paint. It should he emphasized that the extent of the varia tion in paint consumed is too small to be of much practical im portance. Redwood took about 11 percent more and southern pine about IS oercent less priming oaint than the average wood. The spreading rates of second-coat and third-coat oaint were inde pendent of the nature of the wood so that the variations in total paint consumed amounted to roughly plus or mini's 4 percent of that required for the average wood. By mixing the priming paint for redwood with slightly more and that for southern pine with slightly less liquid the variations could he offset entirely. Personal variations among painters exert far more effect on spreading rates than variations between woods. In the experi ments of the 1924 ` Seres the average spreading rate at which the different painter's applied priming-coat paint varied from 561 to 869 square feet oer gallon. The experienced painters applied paint more generorisly than the technical men and laboratory aids becatrse the former had an average spreading rate of 604 and the Intter 751 square feet per gallon. The writer believes that as a rvid priming-coat paint should be applied at 550 to 600 square feet nor gallon and subsequent coats at 600 to 650 square feet ner gallon for best results. All of the spreading rates for new wood in Table 4 should he regarded as too high for best practice, though they are probably representative of much practical painting and reveal the extent of variation between species reasonably well. R 1053 -11- Table 4. - Specific gravlH.es and a ore ad 5 ng rates found for prtmiup-cd.vt nalrt on softwoods '-hen new and after we a. therlnp for one year. Wood S oec1fic A t> pr o x i ma t e : A vo r a pe s n reading gravi ty volume of jrate of oriming- of wood a 1 r - s a c e c oa t p a i ri t on in wood-- ;--------------------------- -------- ; K'ev' ; Wood i ;weathered i : f or 1 ye a r Percent ; Square feet 4i%3 ; per gallon Woods with spreading rates Wes torn red cedar Wh i i e f 5 r ( c omne r c i a 1) Sugar pine Redwood Eastern hemlock Sitka spruce Western hemlock below 640 square * 0.330 7<s 0 .362 0 0 76 0 .362 a o 76 C 0 < 370 a 75 .375 75 < 0 .395 0 74 * 0 .473 68 feet oer (-a 11 on i r V die i 0 0 612 0 0 614 c & 508 * 0 601-- 0 0 620 : a 0 625 : 396 87 9 283 8.^1 dll 390' 400 Woods with spreading rates Northern white pine Port Orford cedar Douglas fir bet ween 6*q_ and . 174 a 0 76 .444 r 70 s 9 .495 0. 0 67 700 square feet 68.8 6 673*1 : 9 9 666 ; oer gal,, 36,; 42 4 319 Woods with spreading rates above 700 square Eastern spruce(commoroia1); .380 j 74. Western white nine ; .4 02 73 Southern cypress : .4 08 i 73 Ponderosa pine : .421 72 Alaska cedar ; .432 ; 71 Vvestern larch : .520 j 65 Southern yellow pine : .531 : 65 feet ner gallon * 7 7 O'15 - 394 : 742 349 : 738 : 410 768 : 41o ; 766 ; 457 *. 752 : 360 ; 790 : 393 A venire for all woods tested 685 * 380 ; ' ii "Computed by considering that the entire weight of the oven-dry wood is wood substance of specific gravity 1.5. This int* o- ducos an error, usually small, in that the extractives in the $ % wood are assumed to have the same specific gravity as wood sub stance. si: J\\ --Id Eastern hemlock. Port Orford cedar, and eastern spruce were not A s represented by full sets of test panels so that the data for I ? these woods are not fully comparable with those for the other woods. f. R 1053 c 0 p q p cd P rd cd 0P p cd id 0) I M,- 0 0 0q q P O' P q 0 0 > cd tu cd ^-1 p 0 q a <+H P a ,q p q 0 'p p 0 0 T3 0 **S. co 0 O ,q 6 b rH b 3 q p p ,c| i--1 p p ro P00 q 0) 0 P .q 0 1--1 0 O P ,b 0 p p ro P O p 0 cd 0 0 +0 q to q p q> P *d q p p p >0 q cd q 0 a' 0 0 cd 0 * cd 0 0 0 00p 0 b q ,G o c C rd H P ,-cj *rJ ;.. oj O P .h O P p PM p 1- 0 -H po (D rq o o cd >j n cue P d> rH P tQ .Cj (ii 0 CM P (0 ,q d -H ^ p p <H OP O p, CD P ,q a ^ co .cj o p * O.Q tO q M P 0) P P Q) p C Sd -rH .-.li o Pd o c P P q P cd (d O P q P P p a; P q U p CO cd pq cl P o o >> fcO Oa a) c d q P cd i cd p co o CD a CD p, P P q P P, o V) p q p h cd o (D cd q cd 0 d > O >j Q) :-j P nj ^ --- P cd cj) P fd O d ^ P0 d o rQ <rl Tj p P O P CD P, > U U) co CD CD cd cd ;.0 tf) co p, P q 0 ,q oop q *d .h CD ,p P cd -H CD P-, .< t> Eh P CD c.o (--} P P cd O 0 O p Id <D H :d P ^P l;0 P P P P O P PP cd q oj cd P cd o O <d U P P S q O, co U ;>> P O P O P q q o *r c q 7: q 0 -d p ,q p co c d q 0 0 CD crt cd p c d P p q o ..c| o O Pq CD co P 0 co 0 cd P 0 p< p -ri 0 q aj co 0 aP q P p. cd -.a o to rH 0 n, q p ,.q {X| O co 1 CD cd q 0) 0 q 0 Q< *r i q 0 ,r;| o t> <d ,Q Cd > p q< p p 0 o q P r P r.O qqpoooHoo OJ 0 P q P .:D^|P O p 1 0 pp..pto pq cd p q cd o ^o - q ,q q q q p. p p q > 0 p, q o q qq cO 0 *--I 0 c do p o q a> < 1 .q ,q p, o ai o 3 rd Pn Eh cd Eh O P pi, 0 0 P R.1033 The discussion so far refers to smoothly planed wood such as the planed side of house siding, Rough surfaces such as the sawed side of bevel siding take nrnch more point, Weather-beaten wood becomes rough and consumes much paint if subsequently pointed. The last column of Table 4 presents data for smoothly pinned woods exposed vertically facing south at Madison, . V/is, in 1924 and painted one year later after they had become greatly roughened. After weathering all woods seem to be greedy for paint and no relation can be discovered between spreading rate and specific gravity of the wood, Hardening of paint on different woods. Ordinarily the time of hardening of paint depends almost entirely on the composition of the paint, the intensity of sunlight, and the temperature and relative humidity of the air. Extractives present in certain woods, however, may retard hardening under cer tain circumstances (14), In full sunshine and at temperatures above 70P. paints made with sufficient drier should harden promptly oh all native woods provided that the wood is reasonably dry and the relative humidity of the air is not unusually high. Exceptionally oily boards of southern cypress, however, may retard the hardening of linseed oil paints that contain no white lead or zinc sulfide pigments. On redwood and southern cypress containing much more than 20 percent moisture, priming-coat paint may remain liquid for days, especially in the absence of sunshine, at temperatures well below 70F,,, and with paints that contain no white iead or zinc sulfide pigment. Unverified reports from industrial sources point to difficulty with hardening of paint under similar conditions on some, of the hardwoods that are relatively rich in tannins, such as the oaks, chestnut, and. walnut. The identity of the extractives in woods that ars responsible for retarded hardening of paint is not yet known. It has been proved that the responsibility lies with some of the extractives because water soluble extract of red wood and alcohol extract of southern cypress have been transferred to white pine and eastern hemlock and imparted to those woods the property of retarding hardening under the circumstances described. ;] Filler necessary on hardwoods with large pores. Ordinary painting by hand'brushing or spray application does not succeed in filling the large pores in hardwoods that have pores larger than those in'birch. The largest pores remain as ob servable depressions or even as holes in the cca ting even after three substantial coats of paint have been applied. As time passes R 1055 -12- Figure 3. - Large pores in hardwoods are not filled by conventional painting., Upoer view - Chestnut painted with three coats of white lead paint with the large pores still showing as deoression3 in the coating. Lower view - Chestnut painted with three coats of lead and 2inc paint showing failure of the coating beginning over the large pores , Z M 34370 S' the disintegration of the coating sets in abnormally early at these points of weakness and shortens the life of the coating. Apparently paint that is fluid enough for application by brush or spray gun is too fluid to plug or bridge large pores adequately. If the large-pored hardwoods must be used under paint coatings they should first receive a wood filler, that is, a pigment paste thinned with volatile thinner for convenient application and then rubbed into the pores shortly after the volatile thinner has evaporated. A smooth, level'surface for the reception of subsequent coats of paint can be produced in that way but the extra cost of the filling operation makes the proner paint ing of large-pored hardwoods relatively expensive. Defects in lumber of low grade. When lumber is to be cut into smaller pieces for use it is sometimes economical to buy lumber of a common grade, cut out the defects, and obtain clear lumber for the Important surfaces. Sven then the wood may be somewhat Inferior for painting to lumber of the same species in a select grade because the common grades come in considerable, proportion from parts of the tree that grew rapidly, have v>i der growth rings, and do not hold paint quite so well as the wood of somewhat later growth. In building con struction, however, where nearly all of the lumber is used with as little cutting as possible, defects in common grades mus.t be covered with point. Very small knots may be concealed satisfactorily and may retain paint about as well as the clear wood. Large knots, however, can rarely be concealed because they do not follow the clear wood in swelling and shrinking with changes in moisture content and on weathering they often crack open, breaking the coating with them. Over knots In the pines, both white pines and yellow pines, point often becomes promptly discolored with a yellow substance that appears to be soluble in linseed oil and the coating, over the knot soon becomes brittle and scales, off. For that reason it Is common practice to shellac knots but the shellac should be applied after putting on the priming-coat naint, not before it. Unfortunate ly shellac is not so durable .for exterior exposure as good paint but somewhat longer life over the knot can be obtained by adding 5 or 6 ounces of castor oil to a gallon of the shellac varnish. When aluminum priming paint is used an extra application over the knots may prove more satisfactory than shellac. It is never desirable to shellac knots in any wood other than a pine because knots In such woods do not discolor or embrittle paint and often bold paint longer than the clear wood. R 1053 -13- Figure 4. - Premature failures of paint over defects in lumber of low grade. A - Cracking and flaking of the coating over a pitch streak while the coating is still reasonably s o u v j over the clear wood below the pitch streak. B - Unsatisfactory appearance over knots and raised grain even though the coating remains sound. Note that on this wood, which is not a pine, the paint remains intact over the knots although the knots themselves crack badly. C - Discoloration and flaking of paint over a knot in one of the pines long before the coating over the clear wood has begun to disintegrate. Pitch pockets are best pointed by cutting them out with a chisel and filling them with good nutty after the priming-coat ooint has been applied. Knot holes aiiouid be puttied in the same way. Pitch streaks may mar the painted surface by exuding resin and embrittling the coating but the writer knows of no method of dealing with them satisfactorily except by rejecting the hoard or at least that part of it containing tho pitch streak,' Loose grain likewise is beyond remedy. Wood checks or cracks should be puttied after priming to keep them from acting as centers of early dis integration of the coating. By the time that proper attention has been given to the defects in common grades of lumber the extra costs for painting it usually make it more expensive than the select grades that are better suited, to uses calling for painting. For uses in vh oh good paint service is important select grades of the wools of gj'-'-uos 1 and 2 of Table 6 are best but If cheaper woods ma-w. be nht 3en it will often bo better to buy a select gra.de of a cheaper suedes than a common grade of a wood of groun 1 or 2, Effect of Woods During Early Life of Paint As a rule the nature of the wood has little effect on the behavior of Paint during its early life, say during the first year or two after truncation. The composition of the paint, the adequacy of the technic of application, and local and climatic conditions are us via 11 y much more important than the kind of wood t hr ough th l s ne r i od . Undesirable early development.s over defects in lumber of low grade have already been discussed. Exudation of resin through paint coatings over clear wood some times occurs early without im pairing the integrity of the coating. It is observed on the nines more often than on other softwoods but has been found occasionally even on softwoods that contain relatively little piny resin. Such exudation is most likely to occur when the lumber has been seasoned, at low temoeratures and is used soon after manufacturing. Lumber kiln dried at higher temperatures or stored for some tine before use is less likely to exude resin because less of the turpentine is lex l In the resin to make it soft enough to move freely. The? oils in Alaska cedar, fort Orford cedar, eastern red cedar, and southern cypress may come through point and collect in a sticky mass on the surface if there is no circulation of air over the surfaces, as when boards ore stacked on top of one another. There is no such occurrence if air circulates over the surfaces because the oils are R 1053 -14- slowly volatile. Colored writer soluble extra at Ives In western red cedar, redwood, and some of the hardwoods do not come through sound oalnt costings unless free water In some way gets Into the wood behind the coating. If cracks appear in the coating the coating at the edges of the cracks may be discolored for a short time im mediately after n rain storm but the writer lias never seen enough discoloration so produced to affect materially the good appearance of the coating. Occasionally flat-grain hoards of softv/oods exposed to the weather and painted on the pith.-side will develop loose grain and crack the coating vd thin a few months even though there was no evidence of loose grain when the paint was applied. The bark-sides of such boards rarely. If ever, develop loose grain. The pines seldom show loose grain even on the pith side except on floors or other places of severe mechanicul wear. If long life of paint coatings is to be assured the moisture content of lumber must remain always well below the fibersaturation point. khen free water accumulates within the wood early failures of paint may occur that the writer has discussed in detail elsewhere (2). Effeet of '.ood Properties on Disintegration of Paint* 1 The deterioration of paint coatings proceeds in sticcesslve but overlapping stages as follows (12); 1. The soiling stage. The coating, gradually becomes dirty. 2. The flatting . tare. The coating loses its gloss. 3. The chalking stage. Dirt may be thrown off more or leas comnletely but colors a mear to fade. 4. The fissure stage. Fissures are of two general types, depending upon the nature of the oaint; a. Checking. The fissures are at first superficial but later may penetrate entirely through the coating. R 1053 -15- I b. Cracking, The fissures pass entirely through the coating when first observed. The coating at the edges of the cracks sooner or lGter copies loose from the wood and curls outward. As a rule crack ing comes at a more advanced staple of deterioration than checking. 5, Hie stage of disintegration. Behavior in the fissure stage determines the form in which disintegration takes pla ce % a. Crumbling, which develops from checking, consists in the failing away of tiny fragments of coating cut off when the fissures become interwoven (reticulate) and oenetrate entirely through the c ou 11 n g, b. Pinking, which develops from cracking and curling. Tire loosened edges of coating that curl outward finally fall off, after which the newly formed edges curl and the orocess continues progressively. Flaking is usually a more rupid form of disintegra tion than crumbling. The soiling, flatting, and chalking stages may be con sidered the early life of the coating, the fissure stage the later ;..JPe, and the stage of disintegration the oeriocl of paint neglect. Hood practice in paint maintenance calls for reoainting before the coating has passed far into the oeriod of neglect but unfortunately many buildings are not reouinted until the coatings have been neg lected for a long: time. The effect of the wood usually aonears first after the fissure stage is reached and dominates further developments by the time the stage of disintegration sets in. Uo to the fissure c r.age the coating seems to remain tough and adherent enough to a (.ay intact and follow slight swelling and shrinking of the 'wood with changing: atmospheric conditions. Fissures appear when these oroperties are lost and the coating becomes brittle, loses its ad herence to tlie wood, and remains clinging to the surface chiefly through mechanical anchorage in the cavities of the wood cells at the surfaces of the boards (6). The situation might be expressed by saying that in its early life the coating sticks to the wood but when the paint has become embrittled with age the rood holds the coating as best it can. R 1053 -16- In softwoods the snringwood holds the aged coating to better advantage than the summerwood. because of the larger cavities in the wood cells (Table 1), not because of a difference in resin content (Table 3). At any rate disintegration proceeds much, more raoidly over the summerwood than it does over the springv/ood and the amount and distribution of summerwood in softwood boards very largely determine how long the coating lasts. On hardwoods the orincipal factors seem to be the size and distribution of the pores together with the specific gravity of the wood. Repainted coat ings behave in much the same way as initial coatings except that the fragments of coating that fall off may be somewhat larger and may not begin to fall off quite so soon. Specific gravity of wood Since the proportions of summerwood and springrood very largely govern both the specific gravity and the rate of dis integration of aged oaint on softwood boards a definite relation she tween specific gravity and paint durability on softwoods is to -be expected. Such a relation Is shown in Figure.5, in which the overage durability of the oaint coating's on the softwoods tested in the 1924 Series Is plotted, against the average specific gravity of the hoards of those species used in the tests. in general the woods that held aged paint well were the woods of low specif!c gravity and those that held paint poorly were high in specific gravity. The relation is by no means exact, however, because redwood, southern cypress, Alaska cedar, and Port Orford cedar held point better and white fir poorer than the specific gravity indi cated. Reasons f*or these discrepancies are suggested in subsequent paragraphs. Figure 6 presents similar data obtained in the 1930 hard wood Series, If hardwoods with large pores, hardwoods with small pores, and softwoods are considered separately trio general relation between specific gravity and rate of di a Integra t.ion of coatinpjs applies for the woods within each group but the relation differs for the throe groups. On the hardwoods with small pores the coat ings lasted longer than they did on 'wood of equal specific gravity In the other two groups. The end-point of the useful life of a coating os the writer judges It occurs very early In the stage of disintegration, however, and if dIsintogratlon is allowed to proceed very far before repainting the comparison between softwoods and hardwoods with small pores changes in favor of the softwoods. On R 1053 -17- FI. pa're 5. Relation between specific gravity of softwoods and durability of c.oa tings of house paint* R 1053 K DURABILITY OF COATING IN MONTHS 0.35 SPECIFIC PAINTED WITH WHITE LEAD PAINT PAINTED WITH LEAD AND ZINC PAINT 0.40 GRAVITY OF Q45 WOOD 050 Figure 6. - Relation between specific gravity of wood and durability of paint for (a) softwoods, (b) hardwoods with snail pores, and (c) .hardwoods with large pores* fhe enamel finish on hardwoods with large pores failed prematurely because of a faulty combination of pigments in a wood filler used on those woods only, consequently the normal relation of durability to specific gravity was not realized on those woods. R 1053 DURABILITY O F COATING IN MONTHS A REDWOOD POPLAR 40 35 30 25 SPECIFIC GRAVITY OF WOOD Figure 7. - Effect of specific gravity of softwoods on durability of paint and of soecial priming paints for the heavier softwoods. Parts A and B are adjacent areas of the same boards painted with the same kind of paint. The too board, is redwood, the second board northern white cine, the third board Douglas fir, and the bottom board southern yellow pine. Part A was oainted conventionally, using the white paint prooerly thinned for the priming coat. Part B was primed with a special primer cons'sting of flake graphite in long-oil sour varnish. Both parts then received a second and a third coat of white paint i| the softwoods disintegration of the coating remains for a long time confined to the bands of summerwood but on the hardwoods with small pores the relatively uniform texture throughout the annual growth ring oermits disintegration to spread fairly rapidly over the entire board. For that reason only the lightest hardwoods with small pores should be considered as acceptable as the softwoods for conventional house painting. None of the hardwoods with large pores can be regarded as satisfactory for such purposes because conventional painting leaves the pores unfilled. Width of annual growth rings in wood Among softwoods the distribution as well as the amount of summerwood affects the disintegration of ouint. For the seme ratio of summerwood to springwood the individual bands of summerwood are narrower the greater the number of annual growth rings per radial inch, that is, the less rapidly the vfood grew. In Figure 1 the depactures from the general relation between specific gravity and durability are largely attributable to such differences in width of annual rings. Many of the boards of white fir used in the 1924 Series of tests contained "compression wood", which is an abnormal type of wood in which there is a high proportion of surnmerwood but it is lighter In weight than normal summerwood. The white fir likewise tended to have few annual rings per inch. These facts account adequately for earlier disintegration of the coatings on the white fir than the specific gravity indicated. On the other hand all of the boards of Alaska cedar and port Orford cedar had very many annual rings per Inch, as did many of the boards of redwood and southern cypress. These woods therefore would be expected to hold paint longer than woods of similar specific gravity but fewer rings per inch. Among the diffuse-porous hardwoods the width of the annual growth rings seems to have very little bearing UDon the rate of dis integration of coatings as would be expected from the fact that there is little difference In textxare between springwood and summerwood. On the ring-porous hardwoods, all of which have large pores, the width of the growth, rings likewise seems to be of minor importance. R 1053 -18- Figure 8. - Characteristic forms of paint disintegration on softwoods and on hardwoods with small pores. Upper view - Badly neglected coating of lead and zinc paint on southern yellow pine. Disintegra tion Is still largely limited to the bonds of summerwood. Lower view - Similarly neglected coating of lead and zinc point on yellow birch. Disintegration of the coating has laid bare much larger areas of the wood. I 11 i! A r i. 1 i T Z U 24366 r Grain of wood Edge-grain boards of softwoods present much narrower bands of surnmerwood on the painted surface than flat-grain boards of similur specific gravity and rings per radial inch. For that reason coatings last longer on edge-grain boards of softwoods. Among the hardwoods, however, there seems to be less difference in the rate of disintegration of uaint on quarter-sawed and plain-sawed boards, especially among the hardwoods with small cores. Variation Within soocies Different boards of the same species, even different boards from the same tree, may vary widely in specific gravity and in number of annual growth rings per inch. The durability cf paint varies correspondingly. Account is not taken of'such variations in the combercial grading of lumber, except in structural timbers of some species \vhere mechanical strength rather than oaintnbil 1 t.y' is the object in view, Whenever it is practicable to buy lumber on special specifications or to cull over a large supply of boards for those best suited for painting it is possible to take advantage of the variation in specific gravity, grain, and rings per inch to select boards materially better for nainting than the general aver age for the species. . Results of the 1929 Douglas fir Series of tests illustrate the possibilities in selection of lumber for nainting. Five ship ments of Douglas fir siding wore obtained, four representing socalled "red" and "yellow" Douglas fir each sub-divided into "soft" and "dense" types, and the fifth representing a special selection of Douglas fir for good painting characteristics. "Red" and "yellow" Douglas fir in "soft" and "dense" types are traditional classifications by lumbermen,but they are not recognized In the present methods of grading; for sale. When the specific gravity and ring count of the boards were determined the average values and ranges in these properties were much the same for the four groups and the average curability of the coatings was likewise much the same. On the group specially selected for painting, however, the durability of the coatings was better than the average for the other four shipments, namely 44 months as compared with .'-58 months. When the hoards of all shipments were reclassified according to the measured specific gravity and further subdivided Into flat-grain and edge-groin boards the data of Table 5 were obtained. Edge-grain R 1053 -19- A Figure 9. - Variation in properties within a species. These four boards of ponderosa pine were taken from test-panels after the paint coatings had worn out and were resurfaced to reveal the wood. Boards A and B held paint well, boards 0 and D poorly. Board A had 36 annual rings per inch, specific gravity 0.40; B had 27 rings, specific gravity 0.40; 0 had 6 rings, specific gravity 0.48, and. D had 15 rings, 0.73. I il l :H i rZ U 12215 boards held paint longer than flat-grain boards of similar specific gravity and boards of low specific gravity held paint longer than boards of the same grain but high specific gravity. Table 5. Variation in durability paint coatings on Douglas fir classified by grain and by specific gravity of ;| If !:l ]!; Specific gravity of wood Below O.b 0.5 to 0.6 Above 0.6 Average durability of coatings on Flat-grain boards Edge-grain boards Months 41 38 29 Months 44 39 37 Extractives in wood Certain extractives in wood of some species undoubted ly affect the durability of coatings but in general the effect of such extractives is small in comparison with the effects of Such physical characteristics as specific gravity, ring count, grain^ and grade. This fact should be emphasized because the importance of extractives in wood is grossly exaggerated in much of the literature on painting. It is difficult to account for the position of redwood, southern cypress, Alaska cedar, and Port Orford cedar in Figure 5 and of redwood, in Figure 6 wholly on the basis of the high ring count in the lumber of those species used in the 1924 Series and the 1930 Hardwood Series of tests. Some of the boards of redwood and. southern cypress had relatively few annual rings per inch and yet held paint longer than the specific gravity indicated. In the 1930 Extractive Series the cold water extract of redwood and the alcohol extract of southern cypress were transferred to boards of eastern hemlock before painting. Disintegration of the coatings was definitely retarded by the redwood extract but the results with the cypress extract were not so conclusive. It is possible that the ingredient in redwood responsible for the good effect on paint retention is the same one that retards hardening of paint under poor drying conditions. If so the close R 1053 -20- botanical relation between redwood and cypress suggests that the same substance or one similar to it may occur in cypress among it water soluble extractives rather than its alcohol soluble ones and that more significant results might have been obtained with the water rather than the alcohol extract of cypress. Figure 5 reveals a difference between the pines and the softwoods other than the pines in the relative durability of pure white lead paint and of lead and zinc paint. On each one of the pines the pure white lead paint outlasted the lead and zinc paint but the lead and zinc paint outlasted the white lead paint on all other softwoods. The same tendency appears in Figure 8 in that redwood and white pine held white lead paint about equally long but redwood held lead and zinc paint much longer than white pine, but the tendency is not so clearly shown for southern yellow pine and western larch. With the two latter woods the high proportion of summerwood. dominates paint disintegration so much that minor effects of extractives are often obscured, but the tendency for paints containing zinc oxide to disintegrate, faster on the pines of lighter weight than on other softwoods of similar specific gravity has run consistently through many series of tests by the writer. The data of Table 3 strongly suggest a connection between the ether soluble extractives of acid reaction present in the pines and the relative rate of disintegration of paints that do and. do not contain zinc oxide. The suggestion was confirmed in the 1930 Extractive Series of tests when it was found that transfer of the alcohol extract of ponderosa pine to eastern hemlock impaired the durability of lead and zinc paint but did not affect that of white lead paint. It is further in line with an opinion prevailing among paint experts that paints containing zinc oxide should be made with linseed oil in which there is no more than a very moderate amount of free acids al though linseed oil of materially higher acid number is considered suitable for ma.king white lead paste paint. The effect of the extractives in the pines seems to be due chiefly to the nature rather than to the amount of extract present in the wood. Since a very small volume of paint is placed on a relatively large volume of wood it may well be that the least resinous boards of a pine contain enough of the reac tive substance to complete whatever reaction takes place in the paint, in which case boards containing still more resin would act no differently. In the 1927 Thinner Series of tests boards of southern yellow pine were sampled for determination of their content of ether soluble extractive and then painted for obser vation. The variations in durability of white lead and of lead R 1053 -21- and zino paint followed closely the variations in specific gravity and ring count of the boards but were independent of the content of ether extract, which varied from about 1 to nearly 20 percent by weight of the wood. Glassification of Woods for Painting: Table 6 presents a classification of native woods ac cording to their desirability for exterior house painting by con ventional methods. Most of the woods are classified on the basis of the 1924 and the 1930 Hardwood Series of tests together with observations of the woods in use on houses or other practical service. A few less important woods that were not tested are classified according to their physical and chemical properties and their similarity to some of the woods tested. Average ship ments of lumber of a select grade in mixed flat- and edge-grain are assumed. Lumber specially selected for superior painting characteristics as pointed out in the preceding discussion may . be entitled to a higher rating than that given the species in Table 6. It was found in the 1925 Series of tests and in other tests (13) that the differences between woods of groups 1 and 2 on the one hand and groups 3 and 4 on the other in their effect on durability of paint cannot be reduced materially by altering the proportions of pigments, linseed oil, and turpentine in the conventional priming-coat paint or by substituting special vola tile thinners for the turpentine. The generous use of volatile thinners in priming coats for woods like southern yellow pine and Douglas fir, though often recommended* is distinctly bad practice. On the other hand, numerous tests (4, 9, 10) have shown that certain special priming paints used in place of the conven tional priming paint add materially to the durability of paint on woods of groups 3 and 4. When such woods are to be painted results more nearly comparable to those obtainable on woods of groups 1 and 2 will be obtained if one of these special priming paints is used. The most satisfactory special priming paint so far discovered for this purpose is aluminum priming paint. It con sists of 1-3/4 to 2 pounds of dry aluminum powder of standard varnish grade in 1 gallon of very long-oil spar varnish. If de sired 2 pounds of commercial paste aluminum of 1-1/2 pounds of dry aluminum powder of standard lining grade may be used in place of the standard varnish powder. Aluminum priming paint, if properly applied so that the grain of the wood is entirely R 1053 -22- T a b le b . C la s s ific a tio n o f IT a tiv e Woods fo r A b ilit y to H e ld Aged C o a tin g s o f H ouse P a in t egr th e (ta b le c o n tin u e d on n e x t page) 0 43 q 0 Ph *h o Ph 0P 0Q rcdd O 43~r-Pp Pi *H d 12 Ph nd 0 nd K cd *H a> a r-1 0 Q) d +3 H H 'I 8 o 0q H tSl f\0 0 cd H q cd 0F* *P q 0 0 q H 0 43 rH q rQ rH cd P Pi O0 d d q cP cd 0 q 0 0 q Jj 0 0q 0 rP 1H O 1--l ,Ur j fd rH q 3r4 cd Ph r UQ 43 q d *H rT-P- rH Ph qG /: O ,q W) 43 q H 0 14 pG cd cd P <4 G 0 43 1--l rH p4 cd O Ph 0 43 q0 *H f1h cd K Ph 43 t 43 0 H t> 5 43 cd 4-3 qa 0 0 0 H Ph O Ph 0 0 Ph 0Ph 4C* M^0in1O4^*OHHOJH cd o H 3 43 o pq h Eh co 1 rQl 1 Ph i cd 1 d 1 0p p 1 0 q 1 1 CD i q1 1 q1 P cd d * d rQ nd P 00p 0 Ph fd Ph cd d -p H ,q 0 ^d 0 rd 0 O fd 'd a) $3 P 0 0 0 0 G) P Ke-j O 0j 00 0 p q q { 1 O q <44 q q 0 0 p P Pc q p nd 0 1 rd cd 0 0 O 0 p 0 0 0 1 l O 0 0 q ,q 0 43 43 fq 43 0 43 ,q 43 0 is 1 cd 0 p P q 0 q rd 1 43 rH q 0 O 0 0 0 a) 1 pH '4 1--1 & Pi to r%: (O P 1O l CO i -q t iw 10 1 Ph qQ< 1 1 P-o 43 01 p 1 Ph 1--1 *0 1 0 P= 1O Pi 1 w rH 0 1 40 0 1q q 0 { 1 *H 0 cd H-3 rH 43 Ph 1 1 Ph cd ,q & H P 1 1 G *H +03 . G 0 0 1 1 I ,q q ^ rH 0 Q 1 1 q H 0 nd 1 1 0 PQ l 'd 1 0 nd l 0 Pq h 1 1 0 O i rH cpd !-"r 1 1 LT\kO r*'\ r*~\ orf--^| vuoo|t ir<'r~\\cK\j\ 4o3 4o-3 tro^v c\] o ir\to to to to cd cd H rH 0 -P o d p o ,n 0 -h O rQ +3 ^q o W H FI 9 0 q in q inq *qh *phi A Ah Ph cd P 0 Pi H Ph ri rH 423 G| o| cd o| q P Ph o Ph rQ Ph w Pi c/1 cd c/5 d rH Cd q in tiQ 0 .h 0 q q rH d 1--| Cd rH O O p -h q a .h Pi 1--l P< mD Ph 0O4 -H4 POi IScd P*HI cd I--1 H 0 1--l h 0 H cd q -h 0 ,q cd m q0 q p. 43 o 0 OPP 30 q0 d P O <D t rH rq OJ qcd a4) 343q Hq o|. a) 4o3 Ph d 0 o o P< q *h 0 g ,q o p< 0 0 0 00Wcd }cdW0)d0qc0d 0cd 0cd rHppqqooo JS 0 0 *rH *H t H *rH *r1 5J LH fH & fH P-H P, fH 0 q G) H q Ph rH Ph 0 43 H * 0 43 8 H 0 rd q q H t- 0 p Ph q nd 0 Ph O tO43 H .q 43 H O P G> cd 43 up P CqO O CO 1 i t 1 \0 i 1<I G 1 G00 00q 1 P O0q qGp 1 cd i--1 rH rH H H q Pi 0 a a Ph Ph q c/> O 1 P 0 Pi d 1 ,-Q rd O ,ql O cd 0 Ph Ph * H 0 cd 0 r M rH 0 q Oh i 1 f 1 1 f 1 0 nd O O nd P 00 q q p- 0 0 0 43 P0h 0 43 cd 0 'nrj PI 0 H 0 Ph rH *5 nd OOO0 a rH H O 43 P H f*H 0 i>4. 43 <H IJSbM q p 0 43 0 cd PI q H 0 43 0 OO Ph P 00 *U) nd nd q O0 H P4 q p 403 0 cd W 3 r*H-1 G) 0 cd 1434 M CO 1 ,q O 1 In1 CO 01 Ph P Ph 1 <i{ I--1 <4 O O O 1 6 1 0 rH Pi 00 V) e 43 0 rMiO-H 43 43 cd rP t5 rd 0 O ^ i 0 rd q 1 i i P rH 0 43 Pi d fc) ip Pi 0 i pi Pi O 43 a q 1 00 0 q00 i p 0 -H q H rQ k rH 0 403 0 nd 1 000 i -p a nd 1q O 1 H 0 O cd is cd 1 cd 43 is Pr O 1 Ph cd rH rP G 0 43 j 1 ,q &o q 0 1 G0 H 0 q q .q 43 H d cd 0 i 1 j .q P 43 cj H 0 nd q & H rd 43 10 1 pi rH 0 00 rH 1 0 'd ,q 43 PQ rH 1 43 0 nd qH O0 O rH rd Fa 0 -H q ft ft 0 Pi nd 43 Pi ?aD i 1 i i i 0m dq 0 nd CO JO q 3 ,4q3 lI 1 OJ 1 0 <D~ 0p 01 rl q 43 ft -H p g fi cd CD a W +5 1H d ft 0 0 P H p *H ,P M -H & 0 o ft o ft (D rj i>a Co rl 1/3 H rH p O CO O P pCD -CHD r,QH *PHi S O O^ 0 CD rH O ft CD o 0 ; P W) <H Table 6 . -- C la s s ific a tio n o f N a tive 7/ood.s f o r A b ilit y to Hoi a Aged C oatings o f House P a in t (c o n t. rd 0* Ot-: O 0 r`Hdd Q 0 0 .. o <d P ft ,P -p *d oVJU VJ UJ rH -p p p -p o c\3 *H !>i o o P< 3 ft UO Eh C0D 0 aa CO O -P 00 0 pp 0 P p -H p 0 0 H ft *H OH Tpj fftt --" ft (fDt p [JOC0j 0 0 ft --I i--I p ,-P rH W) -H 0 W) 0 P ft P P $>* O OO r--I rp rQ p 0 J p -d P 0 ' rH p 0 ,p0ao rpPop -0ddd 0 *dd p rH 0 ft 0 concealed, greatly retards crumbling or flaking of paint from the bands of summerwood, thereby minimizing the discrepancies in durability between different woods. Graphite priming paint made by adding 1 pound of flake graphite to 1 gallon of long-oil. spar varnish seems to have.much the same effect as aluminum primer but there is as yet much less background of experience with graphite primers than with aluminum primers. Because of their dark color at least two coats of white or.light-colored paint are usually required over aluminum or graphite primers. I, Ac kn owl ed g e m e n t Exposure of the Forest Products Laboratory's test panels at stations other than Madison, Wisconsin, was muue possible by the cooperation of the following: U. S. Bureau of Standards. Bureau of Chemistry and Soils, U.S. Department of Agriculture National Lead Company. New Jersey Zinc Company. Pittsburgh Plate Glass Company. W. P. Fuller Company. Denver and Rio Grande Western Railroad. Southern Pacific Ra.ilroad. University of Florida. University of Washington. North Dakota Agricultural College. R 1053 -33- ? References 1.- Browne, F. L. , Spreading rates of outside white house paints on different woods. Drugs. Oils and Paints 42, 230 and 268 (Dec. 1926 and Jan. 1937); Painters' Magazine 54, 10 (Jan. 1927). o $1 ______________ __ Some causes of blistering and. peeling of paint on house siding. American Paint and Varnish Manufacturers1 Association. Scientific Section, Giro, 317, 48_0 TI927TT 3.- _________________ Properties of wood that determine the service given by exterior paint coatings. Federation of Paint and. Varnish Production Clubs, Offi cial Digest No. 95, 106 (Apr, 1930); American Paint Journal 14, 22 (Apr. 7, 1930). 4.- _________________ Effect of priming-coat reduction and special primers upon paint service on different wood.s, Industrial and Engineering Chemis-trv. 22, 847 7193077" 5. - _________________ Procedure used by the Forest Products Labora tory for evaluating paint service on wood, American Society for Testing Materials. Proc 30 "7277 852 (193077" 6. -- _________________ Adhesion in the painting and in the gluing of wood. Industrial and Engineering Chemistry. 23, 290 (1931). 7.- Browne, F, L. and Hrubesky, C. E., Effect of resin in longleaf pine on the durability of house paint. Industrial and Engineering Chemistry, 23, 847 (1931). 8.- Browne, F. L. Testing house paints for durability. J ournal of Chemical Education, 10, 529 (1933). 9.- _________________ .10 ________________ Effect of aluminum priming paint on the dura bility of house paints on wood, Industrial and Engineering Chemistry, 26, 369 (1934). Special priming paints for wood. Industrial and. Engineering Chemistry, (in press). R 1053 -24- 11.- Browne, F. L. Painting characteristics of hardwoods. In dustrial and Engineering Ohemistrv. (in press 12.- _____________ The effect of change from linoxyn gel to xerogel on the behavior of paint. Journal of Physical Chemistry, (in press). 13.- For the St. Paul Test Fence Committee, Fourth Progress Report, Priming-coat reductions for painting new wood surfaces (in press). 14.- Schmits, F. 0, , and Palmer, F. C., Drying of exterior paints under various weather conditions and over different woods. Industrial and Engineering Chemistry. 22, 84 (1930) and comment by Browne, F. L., on page 400. R 1053 -25-