Na u al Resou ces, 2016, 7, 505-514
h p://www.sci p.o g/jou nal/n
ISSN Online: 2158-7086
ISSN P in : 2158-706X
DOI: 10.4236/n .2016.79043 Sep embe 9, 2016
Checking o Radially Sawn Sco s Pine and No way
Sp uce Wood
Ma i Venäläinen1, Yonggang Hu2, E kki Ve kasalo3
1Na u al Resou ces Ins i u e Finland (Luke), Punkaha ju, Finland
2School o Fo es Sciences, Wood Ma e ial Science, Uni e si y o Eas e n Finland, Joensuu, Finland
3Na u al Resou ces Ins i u e Finland (Luke), Joensuu, Finland
Abs ac
In ex e io use, wood is subjec ed o wea he ing ha causes checking and o he de
e-
io a ion in he appea ance and echnical p ope ies. We s udied quan i a i ely he
su ace checking o adially and angen ially sawn specimens o Sco s pine (
Pinus
syl es is
L.) and No way sp uce (
Picea abies
Ka s .) wood in a cyclic clima e cha
m-
be es . The esul s s ongly sugges ed ha he sawing di ec ion de e mines he
checking pe o mance o bo h Sco s pine and No way sp uce wood. The adial su
-
ace o Sco s pine specimens had 62
% less checks han he angen ial one, and he
cumula i e a ea o checks was 74% smalle . Fo No way sp uce, he espec i e i
g-
u es we e: 83% less in he check numbe and 91
% less in he check a ea. Di e en
om pine, in sp uce specimens he checks o adial su ace we e signi ican ly smal
l-
e . Thus,
sp uce imbe gained clea ly mo e abou adial sawing pa e n. The e ec o
annual ing wid h was simi
la o pine and sp uce: he educ ion in annual g ow h
wo sened he checking. The inc ease in densi y wo sened he checking o sp uce bu
did no change he pe o mance o pine. In pine wood,
he inc ease o hea wood
p opo ion educed he luc ua ion o mois u e con en and he o ma ion o checks.
Keywo ds
Sco s Pine, No way Sp uce, Checking, Sawing Pa e n, Mois u e Con en ,
Wea he ing
1. In oduc ion
Checking o wood su ace is an undesi ed phenomenon ha sho ens he aes he ic and
echnical se ice li e o wooden cons uc ions [1]. Checking may lead o an excessi e
need o main enance, and can be he cause o choosing o he ma e ials ins ead o
How o ci e his pape :
Venäläinen, M.
,
Hu
, Y. and Ve kasalo, E. (2016)
Checking o
Radially Sawn Sco s Pine and No way Sp uce
Wood
.
Na u al Resou ces
,
7
, 505-514.
h p://dx.doi.o g/10.4236/n .2016.7904
3
Recei ed:
Augus 9, 2016
Accep ed:
Sep embe 6, 2016
Published:
Sep embe 9, 2016
Copy igh © 201
6 by au ho s and
Scien i ic
Resea ch Publishing Inc.
This wo k is licensed unde he C ea i e
Commons A ibu ion In e na ional
License (CC BY
4.0).
h p://c ea i ecommons.o g/licenses/by/4.0/
Open Access
M. Venäläinen e al.
506
wood in he end-uses subjec ed o checking. Checking akes place du ing he d ying o
imbe , as a esul o in e nal s esses ha a ise om aniso opic sh inkage o i egula -
i ies in wood, when he mois u e con en (MC) dec eases unde he ibe sa u a ion
poin (FSP) (e.g. [2] [3]). In he ex e io use, checking is one isible elemen o wea-
he ing, he g adual deg ada ion o wood due o sola adia ion, luc ua ing mois u e
and empe a u e, and mic oo ganisms (e.g. [1] and e e ences he ein, [4]). Pain s and
o he coa ings a e used o p o ec wood om wea he ing p ocesses [1]. In No dic
coun ies, bo h No way sp uce (
Picea abies
Ka s .) and Sco s pine (
Pinus syl es is
L.)
a e used widely in ou doo applica ions, such as cladding, decking, window ames,
sashes and shu e s and ou side u ni u e. In wooden acades, No way sp uce is used in
mos cases due o i s low pe meabili y and Sco s pine occasionally because o i s du able
hea wood (e.g. [5] [6]).
Due o he aniso opy, angen ial su aces sh ink mo e han adial ones du ing d y-
ing. Consequen ly, one sugges ed solu ion o minimize checking has been he se ing o
sawing di ec ion o imbe so ha he p opo ion o adial su aces is maximized com-
pa ed o angen ial su aces [7]. Sandbe g and Söde s öm [8] ca ied ou an expe i-
men in which he check o ma ion o adially and angen ially sawn Sco s pine and
No way sp uce was ollowed in ou doo condi ions o se e al yea s. Besides he wo ks
o Sandbe g [9] [10] and Sandbe g and Söde s öm [8] he e a e no any quan i a i e
s udies a ailable on po en ial bene i s o adial sawing as ega ds he checking o pine
o sp uce imbe .
Besides he e ec on mois u e dynamics and checking endency, he e a e also o he
pu poses o op imize he sawing pa e n be ween angen ial and adial, such as isual
ou look (g ain pa e n, kno s uc u e), su ace ha dness, and pe o mance in gluing
and in su ace and p essu e ea men s [11] [12]. On he o he hand, he sawing pa e n
has e ec on olume ic and alue yield, ime expendi u e and cos s in he sawing
p ocess ha a e o high impo ance o he saw mill [6] [7] [13] [14].
The aim o his esea ch was o s udy quan i a i ely he checking o angen ially and
adially sawn wood o Sco s pine and No way sp uce in a cyclic clima e chambe es .
Fu he mo e, he aim was o ind co ela ions be ween he checking pe o mance and
basic wood quali y cha ac e is ics. The main emphasis was pu on Sco s pine wood
while No way sp uce wood was used as a e e ence ma e ial.
2. Ma e ials and Me hods
2.1. Sample P epa a ion
Logs o ees om ma u e- o- inal-cu s ands in Finland and Sweden we e employed
o he s udy. Sco s pines we e om i e egions (Sou he n Sweden, No he n Finland,
Cen al Finland, Sou h-Cen al Sweden and Sou h-Eas e n Finland) and No way
sp uces om wo egions (Sou h-Eas e n and Wes e n Finland). The Sco s pine ma e-
ials ha e been desc ibed in de ail in [15] and No way sp uce ma e ials in [16]. The
sampling heigh o logs in a ee was 0 - 2 m. The logs we e sawn o boa ds and selec ed
boa ds we e planed and cu o wood specimens o 17 × 50 × 100 mm (dimensions as
M. Venäläinen e al.
507
esh), aiming he 15 cm2 sized su aces o be ei he angen ial o adial. The longi u-
dinal leng h was always 100 mm. The o al numbe o specimens was 228 (Table 1).
The specimens we e condi ioned a Me la Resea ch Cen e in Joensuu, Finland, a 65%
RH and 20˚C o abou 12% MC, a e which hey we e weighed and hei dimensions
we e measu ed by calipe o calcula e he exac olume. The da a we e used o de e -
mine he speci ic g a i y (SG) o condi ioned specimens using Equa ion (1) and Equa-
ion (2):
12
ADW
LTW
ρ
=××
(1)
whe e:
ADW
= ai -d y weigh , g (a 12% MC);
L
,
T
,
W
= dimensions, cm (a 12% MC);
2
12
HO
SG
ρ
ρ
=
(2)
whe e:
12
ρ
= ai -d y densi y o specimen;
2
HO
ρ
= densi y o wa e .
Then, he c oss-cu ends o he specimens we e sealed wi h lacque o p e en he
longi udinal wa e so p ion in he clima e chambe es .
2.2. Clima e Chambe Tes
The condi ioned specimens we e exposed o a clima e chambe es a S o a Enso Re-
sea ch Cen e in Ima a, Finland. The wea he ing cycle included wa e sp inkling (20
min), a lag (30 min), and d ying unde ul a iole -in a ed (UV-IR) adia ion oge he
wi h en ila ion (130 min). The su ace empe a u e o he specimens was expec ed o
each 60˚C du ing he d ying phase. The wea he ing cycle was epea ed ou imes. The
du a ion o he s eps and he su icien numbe o cycles we e concluded acco ding o
[17] and a pilo es .
2.3. Annual Ring Numbe and Ring Angle
The c oss-cu ends o he specimens we e pho og aphed a e he clima e chambe es
(Figu e 1). The images we e used o de e mine he ue annual ing di ec ion o he
exposed su ace since he su aces we e seldom exac ly adial o angen ial. The angen
Table 1. The ma e ials used in he s udy. Means (sd in pa en hesis) a e p esen ed o he basic
wood cha ac e is ics. The de e mina ion o annual ing a iables is p esen ed in Figu e 1.
Sco s pine No way sp uce
Su ace Su ace
Tangen ial Radial Tangen ial Radial
Numbe o specimens 82 84 31 31
Speci ic g a i y (SG) 0.603 (0.073) 0.621 (0.074) 0.452 (0.060) 0.448 (0.046)
Annual ings/17 mm 15.3 (9.0) 12.7 (7.2) 7.6 (3.0) 7.3 (3.5)
Ring angle, ˚ 13 (11) 84 (14) 2 (2) 79 (11)
Hea wood p opo ion, % 50 (46) 72 (39) No de e mined No de e mined
M. Venäläinen e al.
508
line o annual ings was d awn i s and hen he e ical line o his angen line. The
angle be ween he e ical line o he specimen su ace and he e ical line o he an-
gen line was used as a quan i a i e a iable o he annual ing o ien a ion. This a ia-
ble, called ing angle in his a icle, old how close he exposed su ace was compa ed o
exac ly adial ( ing angle 90˚) o angen ial (0˚) su ace (Table 1). The numbe o an-
nual ings was coun ed o 17 mm sec ion o he e ical o he angen line.
2.4. Mois u e Con en Change and Hea wood Pe cen age
The specimens we e weighed du ing he 4 h wea he ing cycle be o e and a e he las
d ying phase o de e mine he change in MC. The specimens we e a e wa ds o en-
d ied a 103˚C ± 2˚C o 24 h and weighed o ge he d y mass ha was used o calcu-
la e he MC. Finally, he Sco s pine specimens we e c oss-cu and one lacque - ee end
was s ained wi h sul anilic acid o de e mine he bo de line be ween hea wood and
sapwood. The hea wood p opo ion o Sco s pine specimens was de e mined as a pe -
cen age o he s ained c oss-cu a ea.
2.5. Measu ing o Checks and S a is ical Analysis
High esolu ion digi al images o he d y exposed specimen su aces we e examined
using Image-P o Plus 4.5 so wa e (Figu e 2). The da ke colo o checks de e mined
he objec s o be analyzed by he so wa e. The numbe o objec s as well as he ue
a ea, leng h and wid h o each objec we e eco ded. Only he objec s g ea e han 1.5
mm in leng h we e included in o de o a oid o he small da k-colo ed spo s, such as
esin duc s, o be eco ded as checks.
One-way analysis o a iance was used o es he e ec o sawing di ec ion on he
numbe o checks, he o al a ea o checks, and he size o checks. The non-pa ame ic
Spea man ank o de co ela ion analysis was used o in es iga e he ela ionships be-
ween he a iables measu ed om he specimens.
Figu e 1. De e mina ion o he numbe o annual ings pe 17 mm o adius and he o ien a ion
o annual ings, called ing angle (
α
), on he exposed su ace o sample.
M. Venäläinen e al.
509
252 261 357 292
Figu e 2. Example o images used o s udy quan i a i ely he checking pe o mance o angen-
ially and adially sawn Sco s pine (252 an., 261 ad.) and No way sp uce (357 an., 292 ad.)
imbe . Clus e s o da k colo ed pixels in e p e ed o be checks we e u ned ed o enhance he
con as . The numbe o checks was coun ed and dimensions o each check we e analyzed by
Image-P o Plus 4.5 so wa e.
3. Resul s and Discussion
Acco ding o he esul s o ou clima e chambe es , he angen ial su aces o bo h
Sco s pine and No way sp uce specimens we e signi ican ly mo e p one o checking
han he adial ones (Tables 2-4). On a e age, he checking o pine was clea ly mo e
se e e han ha o sp uce, as ega ds he numbe o checks and he o al a ea o checks
(Table 2). In pine wood angen ial su aces, he numbe o checks was 2.6 old (p <
0.000) and he o al a ea o checks was 3.9 old (p < 0.000) compa ed o adial su aces
while in he a e age size o checks he e was no signi ican di e ence (p = 0.103). In
sp uce wood, he espec i e igu es we e 6.2 old (p < 0.000) and 11 old (p < 0.000) in
angen ial su aces compa ed wi h adial su aces. In con as o pine, in sp uce he di -
e ence in he a e age size o checks was signi ican be ween he sawing di ec ions (p =
0.002). The checks o angen ial su aces we e 4.5 old in size.
The MC change was calcula ed o he g oss d y mass o he specimens and i ap-
pea ed o be ai ly small. P esumably du ing he es cycle, he MC change in he su -
ace laye o he specimens was much bigge han he gi en g oss alues and MC o he
su ace laye exceeded FSP. In Sco s pine, he MC change o angen ial su aces was
bigge han ha o adial ones. The sawing di ec ion had no e ec on he MC change in
sp uce. The a e age MC o sp uce samples emained on a lowe le el (19% - 13% s
28% - 18%) du ing he wea he ing ea men , howe e he MC change was nea ly equal
(5.3% uni s s 5.3% - 7.7% uni s) in sp uce and pine wood (Table 2).

M. Venäläinen e al.
510
Table 2. Mois u e con en (MC) and checking pe o mance o adially and angen ially sawn
Sco s pine and No way sp uce wood in a clima e chambe es . The igu es a e means (sd in pa-
en hesis) o 17 × 50 × 100 mm sized specimens o MC and 15 cm2 sized exposed su aces o
checking a iables.
Sco s pine No way sp uce
Su ace Su ace
Tangen ial Radial Tangen ial Radial
MC a e sp inkling, % 28 (13) 23 (11) 19 (5) 18 (2)
MC a e d ying, % 21 (8) 18 (8) 14 (3) 13 (1)
MC change, %-uni s 7.7 (5.5) 5.3 (3.5) 5.3 (1.2) 5.3 (1.1)
Numbe o checks (>1.5 mm) 58 (45) 22 (25) 23 (27) 3.7 (7)
To al a ea o checks, mm2 43 (45) 11 (24) 32 (35) 2.8 (6)
Leng h sum o checks, mm 248 (201) 67 (81) 111 (118) 17 (32)
Wid h sum o checks, mm 9.1 (7.9) 2.7 (3.3) 5.2 (5.8) 0.53 (1.0)
Mean a ea o check, mm2 0.70 (0.56) 0.48 (1.06) 1.6 (1.95) 0.35 (0.70)
Table 3. Spea man ank o de co ela ion analysis o he explana o y a iables and he checking
pe o mance o Sco s pine wood ( he co ela ion coe icien and p- alue in pa en hesis) (n
= 168).
Checking a iable
Numbe To al a ea Leng h sum Wid h sum Mean a ea
MC change 0.211 0.212 0.227 0.207 0.156
(0.006) (0.006) (0.003) (0.007) (0.044)
Speci ic g a i y (SG) 0.118 0.099 0.097 0.112 0.088
(0.127) (0.202) (0.210) (0.146) (0.259)
Numbe o ings 0.157 0.199 0.198 0.138 0.230
(0.043) (0.010) (0.010) (0.074) (0.003)
Ring angle −0.448 −0.464 −0.474 −0.472 −0.387
(<0.001) (<0.001) (<0.001) (<0.001) (<0.001)
Hea wood % −0.186 −0.221 −0.226 −0.186 −0.164
(0.017) (0.005) (0.004) (0.017) (0.036)
Ou ma e ial was no op imal o compa ing he pe o mance o he wo ee species
ega ding he e ec s o ing o ien a ion, since he g ow h a e o he sampled pine and
sp uce ees was no equal. All pines we e ai ly slowly g owing, he a e age annual in-
c emen o adius being only 1.2 mm. By con as , e y slowly g owing sp uces we e
missing om he ma e ial se and he a e age annual inc emen o adius was 2.3 mm.
Thus he gene aliza ion o he di e ences be ween species needs o be done cau iously.
The esul s o Spea man ank o de co ela ion analysis a e p esen ed in ou g oups:
he co ela ion coe icien s among he explana o y a iables o pine (Table 5) and sp uce
M. Venäläinen e al.
511
Table 4. Spea man ank o de co ela ion analysis o he explana o y a iables and he checking
pe o mance o No way sp uce: he co ela ion coe icien and he p obabili y (in pa en hesis)
(n = 60).
Checking a iable
Numbe To al a ea Leng h sum Wid h sum Mean a ea
MC change −0.432 −0.387 −0.420 −0.409 −0.351
(0.001) (0.002) (0.001) (0.001) (0.006)
Speci ic g a i y (SG) 0.402 0.380 0.404 0.396 0.283
(0.002) (0.003) (0.001) (0.002) (0.029)
Numbe o ings 0.469 0.419 0.472 0.423 0.318
(0.001) (0.001) (0.001) (0.001) (0.013)
Ring angle −0.476 −0.503 −0.490 −0.507 −0.439
(0.001) (<0.001) (<0.001) (<0.001) (0.001)
Table 5. Spea man ank o de co ela ion analysis o he Sco s pine wood a iables ha we e
used o explain he checking pe o mance ( he co ela ion coe icien , and he p- alue in pa en-
hesis) (n = 168).
Speci ic g a i y (SG) Numbe o ings Ring angle Hea wood %
Numbe o ings −0.024
(0.755)
Ring angle −0.001 −0.191
(0.992) (0.013)
Hea wood % 0.052 −0.060 0.198
(0.507) (0.448) (0.011)
MC change −0.203 0.055 −0.205 −0.851
(0.009) (0.475) (0.008) (<0.001)
(Table 6), and he coe icien s be ween he explana o y a iables and checking pe o -
mance o pine (Table 3) and o sp uce (Table 4). In he co ela ion analysis, he ing
angle was used as a quan i a i e measu e o he su ace o ien a ion. Specimens wi h
angen ial o ien a ion had alues close o 0˚ and he ones wi h adial o ien a ion alues
close o 90˚.
In pine wood, he e was a s ong nega i e co ela ion be ween he hea wood pe -
cen age and he MC change indica ing hea wood o be slowe han sapwood o adso b
wa e (Table 5). Fu he mo e, he e was a weak bu signi ican nega i e co ela ion
be ween he SG and MC change indica ing hea ie wood o be slowe o each FSP. In-
e es ingly, SG and numbe o annual ings did no co ela e which indica es ha slow-
ly g owing pines did no p oduce hea ie wood. The sligh nega i e co ela ion be ween
ing angle and MC change sugges ed ha adially sawn su aces we e less abso ben .
The co ela ion coe icien s o ing angle, as well as he means in Table 1, showed
M. Venäläinen e al.
512
Table 6. Spea man ank o de co ela ion analysis o he wood p ope ies ha we e used as ex-
plana o y a iables o No way sp uce: he co ela ion coe icien and he p obabili y (in pa en-
hesis) (n = 60).
Speci ic g a i y (SG) Numbe o ings Ring angle
Numbe o ings 0.687
(<0.0001)
Ring angle −0.078 −0.167
(0.555) (0.202)
MC change −0.617 −0.590 0.086
(<0.001) (<0.001) (0.512)
ha he wo sawing di ec ions we e no simila as ega ds he numbe o ings and
hea wood pe cen age. Radially sawn samples had sligh ly less annual ings bu mo e
hea wood compa ed o angen ially sawn ones. This was p ima ily due o he ac ha
he adial su aces we e ob ained close o he pi h and he angen ial su aces close o
he su ace o a log. The di e ence in he hea wood pe cen age, a he han he sawing
di ec ion as such, migh be he eason o he sligh nega i e co ela ion be ween ing
angle and MC change.
The ing angle had he s onges and mos signi ican co ela ion wi h he a iables
measu ing he quan i y and size o checks (Table 2). The nega i e coe icien alue in-
dica ed, consis en ly wi h he means in Table 3, ha adially sawn su aces we e less
p one o checking. As ega ds he ole o o he explana o y a iables, he bigge he MC
change he mo e he e was check o ma ion. On he o he hand, a bigge p opo ion o
less abso ben hea wood educed he checking. SG did no co ela e wi h checking bu ,
in e es ingly, e y na ow annual ings enla ged he size o checks.
In con as o pine, in sp uce wood he numbe o annual ings and SG had a s ong
posi i e co ela ion indica ing ha slow g ow h o sp uces leads o hea y wood (Table
6). Fu he mo e, slowly g owing hea y wood seemed o be signi ican ly slowe o ab-
so b wa e .
The co ela ion coe icien s be ween he ing angle and checking a iables indica ed
ha also in sp uce he su ace o ien a ion is an impo an ac o o de e mine he
checking pe o mance (Table 4). In addi ion, he o he explana o y a iables had mo e
signi ican ole han in he case o pine. In sp uce wood, mo e clea ly han in pine,
slowe g ow h led signi ican ly o mo e se e e checking. Di e en om pine, in sp uce
wood highe SG led signi ican ly o mo e se e e checking. Su p isingly, also dec easing
MC change seemed o be associa ed wi h mo e se e e checking. The nega i e co ela-
ion coe icien is con using since such causali y is i a ional. Ou explana ion is ha
inc easing SG which wo sens checking has so s ong impac on he MC change ha his
s ong co ela ion co e s he e ec o MC change on checking.
Ou esul s abou he signi ican e ec o he su ace o ien a ion on he checking
pe o mance o pine and sp uce wood we e in acco dance wi h he esul s o Sandbe g
and Söde s öm [8] who concluded ha a e a long- e m ou doo exposu e “ he an-
M. Venäläinen e al.
513
nual ing o ien a ion was he mos impo an ac o a ec ing c ack de elopmen on
wea he ing”. Fu he mo e, he esul s suppo ed he ecommenda ions o use adial
sawing pa e n o educe checking gi en by Sandbe g [7]. Howe e , Sandbe g s esses
in [10] ha , ega dless he sawing pa e n, he wood close o pi h is always p one o
checks and dis o ions and should hus be a oided. Acco ding o ou quan i a i e
measu e o he su ace o ien a ion,
i.e
. ing angle, mos o ou obse a ions we e close
o 0˚ ( angen ial o ien a ion) o 90˚ ( adial o ien a ion), especially as ega ds he dis-
ibu ion o sp uce specimens, while he obse a ions in he middle o he ex eme al-
ues we e ew. Thus ou da a does no enable any eg ession analysis o ind ou wha is
he c i ical annual ing o ien a ion o adial su ace o gain he low le el checking pe -
o mance.
The aniso opy o solid wood is a well-known phenomenon,
i.e
. he dimensional
changes a e no simila o angen ial, adial, and longi udinal di ec ions when he MC
change is causing swelling o sh inking. The in e nal ensions due o as d ying sh in-
kage and aniso opy a e conside ed o be he ul ima e eason o he checking o wood.
Al hough he easons o he aniso opic beha io o wood a e no ully unde s ood (see
e.g. [8] and [9]) he unwan ed consequences o aniso opy can be educed by managing
he sawing di ec ion.
In ex e io uses wood is exposed o wea he ing and he e o e wood de ec s like
checks h ea en o sho en he se ice li e o e.g. wooden açade, decking, windows o
ou doo u ni u e, and anyway inc ease he need o main enance. Based on ou esul s
we conclude ha when Sco s pine is used as a aw ma e ial o cladding and ex e-
io -use p oduc s, u ilizing adial sawing pa e n and using hea wood seem o be e i-
cien ways o educe su ace checking. In con as , he use o e y slowly g owing o
highe -densi y wood does no imp o e he checking pe o mance o pine. No way
sp uce imbe ha is gene ally less p one o checking gains e en mo e han pine om
he adial sawing pa e n since i educes he numbe o checks close o ze o and educ-
es he size o he ew checks. The use o slowly g own and dense sp uce wood should be
a oided because such wood seems o be mo e p one o checking.
Acknowledgemen s
The au ho s a e g a e ul o Ms. Paula Machon o ca ying ou he pilo s udy and o
de eloping he quan i a i e check analysis p ocedu e based on Image-P o Plus so -
wa e, Ms. Eija Ma ikainen o ca ying ou he pho og aphing and he image analysis o
he ma e ials, M . Janne Manninen and M . Janne Pynnönen o he possibili y o use
S o a Enso esea ch acili y a Ima a, and M . Mika Kankkunen o ca ying ou he
clima e chambe es he e.
Re e ences
[1] Feis , W.C. (1990) Ou doo Wood Wea he ing and P o ec ion. In: Rowell, R.M. and Ba -
bou , R.J., Eds.,
A chaeological Wood
:
P ope ies
,
Chemis y
,
and P ese a ion
, Ad ances
in Chemis y Se ies 225, Ame ican Chemical Socie y, Washing on DC, 263-298.
h p://dx.doi.o g/10.1021/ba-1990-0225.ch011