HARDWOODS GROWING ON SOUTHERN PINE SITES1

E. T. Choong, F. O. TesoroProfessor of Forestry and Research AssociateSchool of Forestry and Wildlife Management

Louisiana State Uiliversity, Baton Rouge, LA 70803

andF. G. Manwiller

Principal Wood ScientistSouthern Forest Experiment Station, USDA, Pineville, LA 71360

(Received 22 July 1974)

ABSTRACl"

Gas permeability of hardwoods growing on southern pine sites is significantly affected bymoisture content in the longitudinal direction. The ratio of permeability in the transverse tolongitudinal directions is from 12,000:1 for post oak to over 1,000,000:1 for other oaks,but it is not affected by moisture. Although variation in longitudinal permeability variesgreatly between and among species, for most species there was no height effect. A significantdifference was detected between sapwood and corewood only in the longitudinal direction.Gas permeability tended to be somewhat less than liquid permeability.

Additional keyW0rd8: Gas permeability, liquid permeability, longitudinal permeability,transverse permeability, specific gravity.

INTRODUcnON

The permeability of wood has been thesubject of numerous studies because of itsimportance to wood processing ( Erickson1970; Siau 1970; Choong et al. 1972; Tesoro1973), and also because the methods ofmeasuring permeability differ considerably(Resch and Ecklund 1004; Siau 1971).Despite large amounts of work on perme-ability, only a few limited studies have beenmade on the variation within trees(Comstock 1005; Fogg 1969; Isaacs et aI.1971; Choong and Fogg 1972), or oncomparative permeability among variousspecies (Smith and Lee 1958; Tesoro et al.1966; Choong et aI. 1972). The low-gradehardwoods occur in great abundance in thesouthern pine forests, but they are presentlyof no great economic importance. Little isknown of their properties; consequently, aspart of a series of studies to increase theutilization of these woods, this paper reportson their permeabilities.

EXPERIMENTAL PROCEDURE

Sample preparationTwenty-two species of hardwoods were

selected for study. They comprise in excessof 95% of the hardwood volume growingon pine sites. These species, arranged bydecreasing volume on pine sites, and theirspecific gravities are listed in Table 1.

For each species, ten 6-inch (dbh) treeswere sampled from throughout that portionof the species range occurring in the eleven-state area extending from Virginia toeastern Texas. Samples for the permeabilitystudy were obtained from two heights ineach tree: 6 ft and 14 ft above ground.Longitudinal, radial, and tangential perme-ability samples, each about %-inch indiameter and %-inch in length, were takenfrom each height at locations near the bark( sapwood) and near the pith (corewood).Dowel-shaped samples were cut from theair-dried wood using a Greenlee plug cutter.The ends of the dowels were eitherperpen-dicular to the grain ( cross-section) , or to theradial or tangential planes, so that the move-ment of fluid in each of these dowels would

SPRING 1974. V. 6(1)

1 This study was conducted £rom funds providedby the USDA Southern Forest Experiment Stationand the Mcintire-Stennis Cooperative Research Act.

WOOD AND FmER 91

Made in the United States of AmericaReprinted from WOOD AND Fmza

Vol 6, No. I, Spring 1974pp. 91-101

PERMEABILITY OF TWENTY-TWO SMALL DIAMETER

92 E. T. CHOONG, F. O. ~RO AND F. G. MANWn.I.ER

TABLE 1.cific gramties

The 22 species studkd and their If)e-

Species Specific ~bc .

5.et.. ~ .t,..,.,tf1.ac l. 0.. - 0.57Trw hlctwy CarrA ~. 0.68 - 0."BI.ct 1..-10 ..- ..,lMti'"Z Marsh. 0.45 - 0.57Post 08 ~ .ulZak W~nh. 0.71 - 0.18Wlter 08 Q. ~ l. 0.59 - 0.78s...u red oak Q. f~k MlcIIx. 0.82 - 0.88Mlite o.t Q. alba L. 0.71 - 8."Yell~ poplar r.~ tltltpif- l. 0.31 - 0.55Sl8etbay --lOa ~~ l. 0.38 - 0.55Dlerrybirk Nt Q. fal«Zk .ar. ~foliG Ell. 0.63 - 0..BI.ck oak Q. ...lutiIIG lIL 0.65 - 0..Mlite ash ~ l. 0.64 - 0.76&reell ash P. , ,l~ Marsh. 0.51 - 0.71~8IPle ~ - l. 0.49 - o.m_ri- .1. rIz- -"- L. 0.52 - 0.64Millgod .1. u. aZak Michx. 0.82 - 0.77~ckb8rry c.ltto -'d.oIklu L. 0.51 - 0.70Northe... fWd 0Ik Q. rubm L. 0.65 - 0.81Scarlet 08 Q. _t- _. 0.64 - 0..S'-rd oak Q. -'--"Ht Booctl. 0.66 - 0.83laINI ou Q. z-tfoliG Michx. o.m - 0.74Blackjack oak Q...n~ _chh. 0.70 - 0..

ISpeclf1c grl.1ty *te..1ned f.. 1;;;1tIMl1..1 peralb111tys~les. buM on _dry w1g11t IIId d18.s;Gls.

always be in one of the three primary struc-tural directions. Permeability is greatlyaffected by surface smoothness (Choong etal. 1975); therefore, the ends of each testsample were individually and carefully cutclean and smoothed with sharp scalpelblades. Samples obtained from near thepith were classified as "corewood" since insome species the heartwood could not bedistinguished from the sapwood on thebasis of color.

sleeve in order to facilitate easy removal ofthe test samples.

Prepurified nitrogen gas was passedthrough a filter and two nullamatic regula-tors. One regulator was maintained at 75psig to control the sleeve pressure that heldthe sample inside the steel barrel; the otherregulator was used to regulate the upstreamflow to the sample. Burets of different sizeswere used as "flowmeters" since they aremore accurate and more reliable for mea-suring gas flow than the commercial ball-type flowmeters that require frequentcalibration. One leg of a Y -shape plastictube was connected to each buret. Theother leg was connected by a line to theoutflow end of the sample chamber. To thevertical end of the I was connected arubber bulb, which was filled dropwisewith a dilute soap solution. By carefullycompressing the rubber bulb, a single fiJmcould be formed and the gas flow rate easilymeasured by recording the time (with astopwatch) for the film to traverse betweentwo marks representing a precalibratedvolume.

All samples were measured at a meanpressure (P) Of 1.2 atm. Depending onthe permeability of the sample, the up-stream (PI) and downstream (P2) pres-sures were adjusted accordingly to give thesame mean pressure. The superficialpermeability K was calculated using Darcy'slaw for gas, as follows:

.a.P(AP)

(Darcy. or (82- cp/sec-aw) (1Permeability measurements

The gas penneability of each sample wasmeasured in a specially built apparatus(Fig. 1), which consisted of a sample holdermade of a stainless steel barrel fitted insidewith a neoprene rubber sleeve. The spacebetween the barrel and the sleeve was filledwith water and connected to a vacuum andpressure system. The wood sample, posi-tioned between two stainless steel plugswith a %-inch bore through the center ofeach, was sealed inside the rubber sleeve inthe sample holder by hydrostatic pressure.Vacuum was used to expand the rubber

where Q (cc/sec) is the flow rate at meanpressure P (atm) = (Pi + P2)/2, Q. (cc/sec)is the flow rate at atmospheric pressure P. =1 atm and ambient tempera~ of 72 :t: 3 F,b,P (atm) is the pressure drop (Pi-PI), II- isthe viscosity of nitrogen (0.0178 cp), A( cm2) is the surface area of the samplethrough which the flow took place, and L( cm) is the length of the sample.

All test samples were first dried in avacuum oven. Mter gas permeability mea-

93PERMEABn.rrY OF TWENTY-TWO HARDWOODS

.. I"

~-

~

6-

I~I ""I. I." .'

0

~B~0

...c

~

Vatuum

~! -0...

..3

VIR

Flc. 1. Flow diagram of gas perDleability apparatus. (A) wood sample, (B) pelDleability sampleholder, (C) robber sleeve, (D) steel plunger, (R1) 0-100 psig range pressure regulator, (R2)0-30 psig range pressure regulator, (F1) gas filter, (F2) spun glass filter, (H) humidifier, (M1) mercurymanometer for upstream pressure, (M2) mercury manometer for pressure drop, (M3) water manometerfor pressure drop, (V.) metering value, and (U) bubble meters for measuring flow rates.

tudinal samples. Only the longitudinalsamples were studied because it is virtuallyimpossible, under known techniques, tocompletely saturate woods of low perme-ability. Also, the rate of flow in the trans-verse direction is extremely slow and diffi-cult to measure.

The technique of determining waterpermeability is similar to the one alreadydescribed by Choong and Kimbler (1971).Samples were first saturated in a stainlesssteel impregnation chamber by filling themwith carbon dioxide gas in order to dissolveas much air in the wood as possible, next avacuum was pulled and deaerated waterwas introduced; then the samples weresubjected to mechanical shock (with a steelhammer) to induce cavitation. Flow mea-surements were made with a specially builtapparatus that includes a precise constant-rate, positive displacement pump equippedwith several gears to provide a choice of

surements were made at 0% moisturecontent, the samples were conditioned in anenvironment of nominal 92% RH ( about20% EMC) inside an Aminco Climate Labchamber. Gas permeability was then mea-sured at about 20% moisture content todetermine the effect of wood moisture onpermeability. Transverse permeability ofwood is low and its measurement very time-consuming. The difference, if any, betweenradial and tangential directions was ex-pected to be negligible; therefore, onlylimited numbers of transverse samples weremeasured at this higher moisture condition.To prevent severe drying effects duringmeasurement, the dry nitrogen was firsthumidified by bubbling it through achamber of water.

In an effort to verify the reliability of gaspermeability measurement as an indicationof fluid flow in wood, flow rates with liquidwater were determined for selected longi-

=z

94 Eo T. CHOONG, F. O. ~RO AND F. G. MANWn.LER

TABLE 2. Average gaa J)e1meabilUy oolues fO1' 22 hardwood 81J6cie8 in the longitudinal direction at oromoisture content, and ratios of penneability at ~ to ~ MC

Overall ..h'-an tr

(Darcy)

Sap.ood.:a - ~anNa (Darcy)

CGrewood..A MeanNi (Dircy)

~

Ferm.bi 1 ityRatio

(0%/l0% Nt)

Species

.etg..Tr.- HickoryBlack Tupelo

Post Oak

Water OakS~them Red OakWhite Oak

Yell~ Poplar

SwetbayCherryba~ OakBlack Oak

Illite AshGreen AshRed Maple

Aarican El.Wln~d El.

HackberryNorthem Red ~Scarlet OakSh.-rd Oaklaurel Oak

Blackjack Oak

.:»Z537:54'»383932293938394241352834393840

13.8797.8918.1050.161

32.37564.7791.359

28.95014.992

69.16266 . 8)4

1.9733.759

10.2915.815

3.97919.34061.92055.~372.527~.3402.492

.U17...34.3528.,40.,273737.,3437413t28

15.3271.~78.4050.067

16.5549.652

0.045

1.87311.43812.91628.4040.7551.7207.3871.274

1.07414.13956.733

35.~25.178

21.~0.5»

1.~1.681.121.431.~1.921.751.311.411.811.531.711.091.191.~0."1.291.371.461.541.472.40

y Nl8Der of observations

W Coefficient of ftriati~ (S)

some 30 flow rates. The apparatus alsohas two filtering systems, a transducer cellattached to a meter read-out for pressuremeasurement ( 0-5 psig range) , and apermeability cell similar to the one illus-trated in Fig. 1.

~ULTS AND DISCUSSION

Among and within-tree variability

Species averages and means by woodtypes (sapwood and corewood) for longi-tudinal and transverse (radial and tangen-tial) permeability are presented in Tables2 and 3 for 0% moisture content. The rangesof values for each species are shown in Fig2 for longitudinal permeability and in Fig.3 for transverse permeability. These perme-ability values are extremely variable, rang-ing from 124.7 to 0.0001 Darcys; and, asshown in tables, the coefficients of variationare very large. As expected, highly signifi-

cant differences (P < 0.01) were found byanalysis of variance (Table 4) amongspecies, structural direction, and wood-type;but the effect of height was negJigible.

Differences in permeability betweenlongitudinal and transverse samples variedgreatly from species to species; therefore, aseparate analysis of variance for each direc-tion was made with each species. In eachcase, the data were analyzed as a factorialwith completely randomized block designwith the trees as blocks.

Analyses for the longitudinal samplesshow that differences among trees are gen-erally highly significant. A few species,however, show significance only at the 5%level of probability (i.e. black tupelo, yellowpoplar, black oak, winged elm, and black-jack oak). The effect of height is generallynonsignificant, except for sweetgum, whiteash, and red maple. This general lack ofheight effect is understandable, since the

95PERMEABll.rrY OF TWENTY -TWO HARDWOODS

TABLE 3. Aoerage gtJI1J8"fnetJbiZit" ooZues for.22 hardwood apecieI In the tranlvene diredionl dt oromoiItu1'e content. and raIioI of "enneabaltll dt 0% to .20% MC

Owrall. KNJIAL ~t~1 IMa.ftTl~ ~Wt.~NI - .-.Specfl$ ~an Cvb a SaiafOcD I;or~ a Sapt«)od Cof'eWoOd Per8abilit;y

(Darcy) "'-8I.-an" ~an .-. RAtio(Darcy) (Darcy) (Darcy (Darcy) (01/20111:)

SWet.- 0.~(157.7)D ~ 0.0125 0.0056 29 0.0104 0.0065 0.«1True Hickory 0.0013 (123.2) ~ 0.0012 0.1xm 42 0.0010 0.0019 0.44Black Tupelo 0.0003 (32.1) 22 0.«* 0.0003 24 0.0003 0.0002 0.39Post Oak 0.0015 (234..9) 31 0.0028 O.~ 35 0.0011 0.0015 0.43Water Dak 0.0004 (219.5) 41 O.~ O.~ 46 0.0003 0.0001 0.76Southem ~d Oak 0.0011 (75..8) 31 0.0020 0.0008 35 0.0010 0.0005 0.26White Oak 0.0004 (122.5) 31 0.0010 0.0004 34 0.0002 0.0002 0.37Yell~ Poplar 0.0093 (251.3) 48 0.0143 0.0100 38 0.0031 0.0040 0.48Swetbay 0.0068 (123.2) 43 0.0059 0.0054 34 0.0100 0.0055 0.57Cherrybark Oak 0.0004 (1..5) 57 0.0005 0.0004 50 0.0003 0.0004 0.30Black Oak 0.0023 (212.9) 34 0.0044 0.0016 35 0.0021 0.0016 0.84Illite Ash 0.0004 (125.6) 39 0.0004 0.0004 29 0.0005 0.0002 0.39GNen Ash 0.0023 (142.3) 39 0.0034 0.0017 41 0.0019 0.0024 0.44led Maple 0.0010 (134.8) 45 0.0013 0.0009 39 0.1x. 0.0007 0.27-,-rican Ell1 0.0070 (125.3) 40 0.0067 0.0066 28 0.0075 --- 0.67Winged El. 0.0102 (82.8) 41 0.008Z 0.0090 ~ 0.0124 -- 0.57

Hackberry 0.0025 (162.0) 38 0.0044 0.0024 38 0.0013 0.0014 0.50 .Northem ~d Oak 0.0041 (103.8) 44 0.0035 0.0051 32 0.0013 0.0045 0.40Scarlet Oak 0.0070 (106.7) 61 0.0106 0.0074 62 O.~ 0.0044 0.54Sh_rd Oak 0.0018 (111.1) 40 0.0022 0.0021 31 0.0014 0.0012 0.19Laurel Oak 0.0023 (223.9) 46 0.0045 0."0022 34 O.~ 0.0005 0.55Blackjack Oak 0.0055 (100.2) 22 0._3 0.0033 19 0.0020 0.CMJ22 0.53

!/Wi8Ier of ~se"at.'ons

WCoeffic1ent of variation (1:)

breast-height diameter was 5.5 to 6.5 inches,and at both heights their growth rates werequite similar.

The difference between sapwood andcorewood is very pronounced in most of thespecies studied, with the sapwood showinghigher longitudinal permeability than thecorewood. Some species that exhibited nodistinguishable heartwood-sapwood colordifferences still displayed greater longi-tudinal permeability in the sapwood thanin the corewood. However, in four species,namely sweetgum, black tupelo, hackberry,and northern red oak, there was no dif-ference in permeability between woodtypes. In northern red oak, heartwood waswell developed, but corewood permeabilitywas not different from that of the sapwood.It was higher than corewood permeabilityof the other eight species of the red oakgroup. This may indicate that heartwoodvessels of northern red oak contain fewertyloses than do the other red oaks tested.

In the analyses of transverse permeability,only among-tree variations within a specieswere generally significant. Tree height,wood type, and structural direction in mostcases had no significant effect on perme-ability.

The largest difference in permeabilityoccurs between the longitudinal and trans-verse directions. Anisotropic permeabilityfor the various species is shown in Table 5.There is a wide range of permeability ratiosin a given structural direction as well asbetween structural directions. When theextreme range of values is taken, the ratiofor longitudinal to tangential permeabilityvaries from 12,<XX> to 1 for post oak to ashigh as over a million to 1 for other oaks(i.e. scarlet, shumard, and laurel). Theseratios are considerably higher than for soft-woods (Comstock 1970; Choong and Fogs1008, 1972). Longitudinal flow is obvious-ly much greater than transverse flow inhardwoods, mainly because of the very high

97PERMEABILn'Y OF TWENTY-TWO HARDWOODS

SIlEETG~

TRUE HICKORY

BLAcK TUPELO

PoST OAK

WATER OAK

SoUTHERN RED OAK

WHITE OAK

y~ POPLAR

SW£ETBAY

CHERRYIARK OAK

BLAcK OAK

WHiTt AsH

GAlEN ASH

RED MAPLE

MRICM ELM

WINGED W

HACKBERRY

bTIERM RED OAK

ScARLET OAK

SIMIIARD OAK

lAUREL OAK

Bl.ACUACK OAK

I ..1 ... . . ...1 ,..1. .. . ... ...1111111.1 I"""" ".."". ""'1"."""'"0.0001 0.001 o.G 0-1 . 1 0

TRANSVERSE PERMEABILITY. (Darcy)

FIc. 3. Ranges of penneability values in the transverse directions for various hardwood species at0% moisture content. Shaded bars are tangential movement; unshaded bars are radial movement.

indication that the ray cells serve as pas-sageways for transverse conduction of fluidin wood.

erally higher for radial than for tangentialperD1eability. The average ratio for all 22species is 2.3 to 1. Most of the oaks showsomewhat higher R/T ratios than the non-oak species. Higher radial perD1eability hasbeen reported in the literature (Buro andBuro 1959; Comstock 1970; Isaacs et al.1971; Choong and Fogg 1972) and is an

Moisture content effects

The gas permeability of wood is affectedby moisture content. When the flow is inthe longitudinal direction, permeability is

98 E. T. CHOONG, F. o. ~RO AND F. G. MANwn.LER

Analysis of oorlonce for gas permeabilitll at two moisture content levehTABLE 4.

os IIIISTURE C(111OO

d.t. ... squire f rlt1o201 IIIISTIIE ~1md.t. ~.. ~re .. ratIo -

Variable24.15**21

'971221

7.649.74316.8113.21

30.159.200.0

6.008.33

2f

198

1

2

2

.1

1

2

2

21

42

42

21

21

2814

3191

22.55**

0.19 "5430.94**

0.00 "585.85**

2.29 lIS

60.76**0.00 "50.72 NS

17.83**0.10 KS

14.69**1.11 "5

4.

1211.

O.

lit.

Species (S)Tne/Species (ErI'Or' A)Hei~t (H)Structural Oirectf~ (0)H X 0Wood Type (W)

(Sapwood n. Corewood)

H X WOXWHXDXWS X HS XDSXHXOSXWSXHXVError 8Toul

1

2

2

21

4l

4l

21

21

1545

1921

160.49

4,252.540.0

50.28

1,248.137.~

1,028.3777.9769.98

140.58

24,401.19225.21196.98

6,953.370.66

2,926.5587.92

117.41

1.20 NS

207.84**1.92 NS1.68 "5

59.23**0.01 "5

24.93**0.75 NS

* Si~ificant at 51 level of p~ilit,y** Si~ificant at lS level of probability

NS Not si~ific8lt

about 1.5 times higher at 0% than at 20%moisture content (Table 2); but in thetransverse direction (either radial or tangen-tial), the permeability at 0% moisture con-tent is about one-half lower (Table 3). Thecorrelation of permeability between the twomoisture conditions is very high (r = 0.92,N = 1460 in the longitudinal direction; r =0.81, N = 461 in the transverse direction).The corresponding regression equationsfor 0% MC vs. 20% MC permeabilitiesare K2o = 2.05 + O.58(Ko) for longitudinal,and K20 = 0.0027 + 1.47(Ko) for transversemovement.

Increased longitudinal permeability atlow moisture content was also reported byFogg (1969) on southern pines, who lentsupport to the theory of Tiemann (1910)regarding the formation of minute checksin the cell-wall structure as the possiblecause of increase in apparent permeabilityof wood during drying. This reasoningcould explain the results of this researchsince the wood samples in this study hadbeen previously air-dried, so that minutechecks could have occurred in them; whenthe samples were conditioned to higher

10.799.52478.88513.20

148.102.310.0

15.181.60

31*

41**

OONS

31**

moisture content. the openings could haveclosed by swelling of the cell wall so thatthe flow path was mainly through the celllumens of the coarse capillary structure.

In the transverse directions, flow mustpass through some openings of the cross-walls. Despite the apparent absence ofopenings in the pit membranes, Behr et al.( 1969) reported penetration of oily preser-vatives in the fibrous tissue of beech, redoak, and American elm, and thus showedevidence of passage of fluid through theinterconnecting pit hairs in hardwoods.The slightly higher permeability values at20% moisture content. as compared withthose at 0% moisture content. are not con-sistent with the explanation of minute checkformation. Such a phenomenon suggests thepossibility of increased pit membranepermeability when vacuum-dried wood isresaturated. The possibility of specimenirregularity contributing to leakage is dis-counted because tests at both moisture con-tents show no evidence of gas passingbetween the rubber sleeves and the edges ofthe samples. When their ends were sealedoff with layers of epoxy adhesive and Saran

99PERMEABILITY OF TWENTY-TWO BARDWOODS

TABLE 5. Ratios of longitudinal, radial, and tangential ( LI R/T) goB pemIeQbilIt" valua tit 0"moisture content

(J.-cf"1.598 : 1.10: 13.481 : 0.79 : 1

27.423 : 1.00 : 1

100 : 1.73 : 1~.603 : 2.00 : 143.339 : 1.56 : 13.558 : 3.50 : 1

2.753 : 2.09 : 11.SE : 0.67 : 1

143.045 : 1.33: 125.2M: 12.1 : 1t.712 : O.MI : 1

1.318:1.24:111.379 : 1.38: 1

491 : 0.8 : 1210 : 0.&9 : 1

12.871 : 2.77 : 117.902 : 1.39 : 18.956 : 1.~ : 1

37.124 : 1.6' : 147.687 : 5.83 : 1

844 : 4.25 : 1

147.807 : 1352.264: 1

167.513 : 112.144 : 1

8~.848 : 1958.710 : 1133.432 : 12~.572 : 129.203 : 1

691.618: 1708.224 : 1

119.890 : 164.708: 1

113.117 : 135.361 : 1

181.477 : 1758.454 : 1956.606 : 1

1.247.256 : 11.141.323 : 11.136.673: 1

118.299 : 1

SWetg18HickoryBlack TupeloPost OakWater OakSouthern Red OakWhite Oak

YellIN P~lar

S-tbayQlerrybark OakBlack Oak

Illite Ash&reenAshRed Maple_riCIn El.Winged El.

HackberryNorthern Red OakScarlet OakSh_rd OakLaurel OakBlackjack Oak

aRati0 of hi~st 1CX1gitud~1 pe,..i1it.y to l_stt;;;~~rse- (-t;;'~t1.1) ~.i11t.y~

coating. the flow of gas was completelyinhibited.

Gas permeability os. liquid permeability

A relationship between superficial gaspenneability and liquid penneability hadnot been truly established, althoughComstock ( 1968 ) reported that thespecific penneability (i.e. superficial gaspermeability at various mean pressuresextrapolated to infinite pressure) and thenonswelling liquid penneability are thesame for longitudinal flow in hemlock.Non-Darcy phenomena, however, can beexpected to occur with gas, since the struc-tural components of wood are changedwith variations in moisture content in thehygroscopic range. However, if superficialgas penneability is a meaningful indicationof fluid flow, it should be well correlatedwith liquid penneability. As shown in Fig.4, such a relationship occurs for longitudinalflow in the hardwoods, but the liquid

permeability shows slightly higher valuesthan gas permeability when the range isover 1 Darcy. Below 1 Darcy, the liquidpermeability is not significantly differentfrom gas permeability. Such a correlationbetween gas and liquid permeability, how-ever, is contrary to the generally acceptedexplanation that slip flow with gases andlow moisture content should make the gaspermeability higher than the liquid.

Recent work by Tesoro (1973) indicatesthat the gas permeability of both softwoodand hardwood longitudinal specimens wasindeed significantly higher than the waterpermeability. He measured the gas perme-ability at moisture contents near the fibersaturation point by drying the samples veryslowly from the green condition; even so,the water permeability in never-dried sam-ples was found to be consistently differentfrom that obtained after resaturation. Itappears, therefore, that drying bringschanges in the inner structure of wood insuch a manner that the permeability of

101PERMEABn.rrY OF TWENTY-TWO HARDWOODS

Moisture content of wood significantlyaffects longitudinal penneability. Gaspermeability decreased with an increase inmoisture content from 0 to 20%; also, theeffect of moisture seems to increase withpenneability. No such effect was observedwith the transverse samples.

Gas penneability was found to be slightlylower than liquid permeability when therange is over 1 Darcy, probably because ofthe effect of resaturation. The relationshipbetween liquid and gas permeability, how-ever, is highly correlated.

permeability of wood. (Submitted to WoodSci~ )

CoMSTOCK. G. L. 1965. Longitudinal perme-ability of green eastern hemloc*. For. Prod. J.15(10):441-449.

-. 1968. Relationship between permeabilityof green and dry eastern hemlock. For. Prod.J.18(8):20-23.

-. 1970. Directional permeability of soft-woods. Wood Fiber 1(4):283-289.

ERICKSON, H. D. 1970. Permeabiilty of southernpine wood-A review. Wood Sci. 2(3):149-158.

Focc, P. J. 1969. Longitudinal air permeabilityof four species of southern pine wood. WoodSci. 2( 1) :35-43.

ISAAcs, C. P., E. T. CllOONG, AND P. J. Focc.1971. Permeability variation within a cotton-wood tree. Wood Sci. 3( 4) :231-237.

RESCH, H., AND B. A. Ecn.UND. 1964. Penne-ability of wood. exemplified by measurementson redwood. For. Prod. J. 14(5):199-2M.

SIAU, J. F. 1970. Pressure impregnatbn of re-fractory woods. Wood Sci. 3( 1): 1-7.

-. 1971. Flow in wood. Syracuse Univ.Press, Syracuse, N.Y. 122 pp.

SMrrH, D., AND B. LEE. 1958. The longitudinalpermeability of some hardwoods and soft-woods. Spec. Rep. For. Prod Res. (London),No. 13. 13 pp.

TESORO, F. O. 1973. Factors affecting flow ofgas and liquid through softWoods and hard-woods. Ph.D. dissertation, Louisiana StateUniversity, Baton Rouge.

-, E. T. CllOONG, AND C. SOAR. 1966.Transverse air permeability as an indicationof treatability with creosote. For. Prod. J.16(3):57-59.

TIEMANN, H. D. 1910. The physical structure ofwood in relation to its penetrability by preser-vative fluids. Am. Railway Eng. and Maint.Way Assoc. Bull. 120:359-375.

REFERENCES

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