SILICA

Silica is a common constituent of wood ash and most woods have only traces ( O. 01 pct.). However, many tropical species may have sili¥a contents of 1 . 0 percent and greater. Silica is determined by ashing the sample , treating the ash wit h hydrofluoric acid, and measuring the loss of silicon as the volatile silicon tetraflu­oride.

A timber is arbitrarily called "silica accumulating" when the silica can be det­ected microscopically and the lower limit of detection has been found to be approx­imately 0.05 percent.

At the l!>~er levels of silica content the timbers can be converted with 1 itt 1 e difficulty when carbide tipped tools are used. In the dry state, conversionrecomes more difficult with a very pronounced dulling effect on cutting equipment.

A relatively low silica content is not necessarily an indicator of the d if f i cult y that may be encountered inmachining. In certain timbers, the Moraceae in par­ticular, the silica occurs in the form of casts in the various cellular types and t h u s locally the concentration of silica may be very high.

The following list contains the siliceous timbers encountered in this project.

Species

Pouteria amygdalina Dialium guianense Pouteria mammosa Licania platypus Rehdera penninervia Brosimunl alicastrum Pseudolmedia spuria Vitex gaumerii Trophis chorizantha Vochysia hondurensis Guarea excelsa Castilla elastica All others

* Silica percent

1. 90 1. 32

.87

.89

.78

. 68

.36

.30

.29

.06

.04

. 02 less than 0.01

* Percentage based on ovendry weight of the wood.

73

Silica distribution

Uniform Uniform Uniform Uniform Casts Casts Casts Uniform Casts Uniform

. Uniform Casts

SHRINKAGE

Shrinkage across the ~-:"ain results when wood loses some of the absorbed mois­ture. Conversely, swelling occurs when dry or partially dry wood is soaked 0 r when it takes moisture from the air or other source. Shrinkage and swelling in the direction of the grain of normal wood is only a small fraction of 1 percent and is too small to be of practical importance in moust uses of wood. Exception to the lal ter occurs when compression wood (in conifers) or tension wood ( in hardwoods) is present in significant amounts . Excessive unbalance in these instances will re­sult in warp or where normal wood bounds either compression wood or tension WDd, tension failures are likely to occur during the drying process. •

The shrinkage of any piece of wood depends on numerous factors, some of which have not been thoroughly studied. In all species the radial shrinkage is less than

the tangential. Hence quarter-sawed boards shrink less in width but more in thick ness than flat-sawed boards. The smaller the ratio of tangential to tangentia l Ehri;;­kage for a species, the greater is the advantage to be gained through min i m i z i n g shrinkage in width by using quarter-sawed wood. Also, the less the d iff ere n c e between the radial and tangential shrinkage, the less ordinarily is the tendency of the wood to check in drying and to cup when its moisture content changes.

The srinkage tables here provide information relative to the ease of drying in the initial stages (green to equilibrium with a relative humidity of 80 per c e n t ) ; the dimensional change that may be expected in changing from a relative humidity of 80 percent to equilibrium with a relative humidity of 65 percent ; and the change that may be expected in changing from 65 percent to 30 percent which are the condi­tions commonly encountered in the more northern climates where central heating is employed.

Dried wood takes on or gives off moisture with each change in weather, or heat­ing conditions . The fact that time is required for these moisture changes causes a lag between atmospheric changes and their full effect on the moisture condition of the wood. The lag is greater in some species than in others, greater in heartwood than in sapwood, and is .much less in small than in large pieces . It is increased by protective coatings such as paint, enamel, or varnish, the shrinkage figures given do not take into account the readiness with which the species take on and give off · moisture, and therefore should be considered as the relative shrinkage bet wee n woods after long exposure to fairly uniform atmospheric conditions or with the sa­me changes in moisture content . .

In addition to the summary table, three additional tables are provided which are tabulated in decreasing order of tangential shrinkage for each of the moisture con­dition changes.

The total shrinkage from the green condition to a particular relative hum i d i t Y may be determined by simply adding the values in the respective columns . For exam­pIe, the radial shrinkage of cedro from the green condition to equilibrium with30per­cent relative humidity would be 1. 0+0.8+1. 4 or a total average of 3.2 percent.

74

MECHANICAL PROPERTIES

Modulus of Rupture in Static Bending

This value is the computed stress in the top and bottoJ fibers of a beam at th e maximum load and is a measure of the ability of a beam to support a slowly ap­plied load for a short time. The formula by which it is computed is based on assump' tions that are valid only to the proportional limit, hence modulus of rupture is not a true stress. It is however a widely accepted term and values for various species are quite comparable.

Since the modulus of rupture is based on the maximum load, which is directly determinabtle, it is less influenced by personal and other factors than proportional limit values.

The modulus of rupture values are used to compare the bending strengths of dif ferent species, and in conjunction with the results of tests on timbers containing defects, to determine safe working stresses for structural timbers.

Modulus of Elasticity in Static Bending

This is a measure of the stiffness or rigidity of a material. The deflection of a beam under load varies inversely as the modulus of elasticity; that is, the higher the modulus the less the deflection. Modulus of elasticity is useful in computing the de­flections of joists, beams, and stringers under loads that do not cause stress beyond the proportional limit. It is also used in computing the load that can be carried by a long column, because for such columns the. load depends on the stiffness, and not on the crushing strength of the wood parallel to the grain.

Hardness •

Hardness is the load required to embed a O. 44-inch ball to one-half its diameter in the wood. It represents a property important in wood subjected to wear and rna!. ring, such as flooring, furniture, and railroad ties. Since there is no significant difference between tangential and radial hardness the two are averaged together as "side hardness. "

75

'" __ ,_ .... , ... . '/....w. .... ','l. -- ":",' ", ..... _ :oO .... J ... .

,,-

• , . . ..... <t _ _ .• l" _

• • I ' • • ~ , ; ';. 'j ,1 , 'j , 'i , 'i , LJ

Alseis yucatanensis StandI. Palo son; caca6-che

3

Aspi d o ~perm a megalocarpon Mue l l.-Ar g . Ma l erio

76

2

Ampelocera hottlei StandI. Lu!n y lu!n hernbra

Astroniurn graveolens Ja cq. Cu l inzisa

I 'I' I

I

5 I' , • • I •. , " .~ '1 , ~ , 'i r

Brosimum alicastrum Swartz. Ramon blanco. ujushte.masico y eapom6.

Hursera simaruba (L) Sarg. Chacaj.chie-chica.chicah. palo chino, ehae a h.ehacah colorado, jiot e . 77

6

Bucida bueeras L. Cacho de taro, puete, poete

Caloph y llum b rasiliense rekoi S t a n d l. Mario, Santa ~ a ri a

var.

Cassia grandis L.f. Bucut, carao

10 LLJ,j 'I iii 'I " 1 ", 1 '! , 'j 'J i 'j fJJ

Castilla elastica Cervantes Palo de hule

....... _CIl .. .u ... .. lli .. l 6 •• ' ... 4..,l'J _:er."". __ ""~.:... , l.a\L .... { .'b~.""~. ~'"""'" __ ~ :::: :":':"!!:'NP'''' . .....:::.c";;;---. .. , ~Ef:~;~··:~ - -. ~ .. ""'_I .............. !i;",-~ .. ~:'·k. ;:",".~

'::..:....::',_ ...

LLi .. LLLJ .. LJ .Ll.t:LLLLLL1J.,J

Cordia alliodora (R.& P.)Oken , .i:·. ' .. ::. :.~:.j .. : . '&c;lj-on • . 1aur~;L .. :' ... , .... . " ... : ..... ::, .". : .....

78

Dialium guianense (Aubl.) S and~.,. Tamarindo. Palo de lacan­don.

li\:l ':';t,:",·,· "'",*t~·':·l':!~L'·,_l.~Jc"f'. Guarea excelsa H. B.K. Cedrillo. lobfn

Enterolobium cyc!ocarpum (Jacq) Griseb Conacaste 0 guanacaste

lIymenaea courbaril L. uapinol ----

79

Licania platypus (llemsl.) Fritsch. Sunza

Luehea seemannii Triana & Planch

Yayo, tap asq uit, cotonr6n

80

Lonchocarpus castilloi Standi. Machich, manchiche

Matayaa oppositifolia (A.Rich) Britton Sacuayum 0 Zacuayum

21

Myroxylon balsamum nereirae (Royle) Harms. Naha. balsamo

Ormosia toledoan~ S tandI. ~ o tiene nombre , mun en Guatemala pero s ~ r. onoce

COmo colorin en V, c ecruz 81

Nectandra SJ?.. canoj

Pimenta dioica (L) Herr.

Pimienta

• \ f :2:,1 L,.LI."\.,.J,..\".l .• J,,j .t..l.,,1 .•. I .• .'1.,:LJ<.1.1,j •. Q

Platymiscium dimorphandrum Donn. Smith lIormigo

Pouted a mammosa (L) Cronquist Zapote mamey

82

Pouteria a my g da lina (Standl.) Baehni Silion

Pseudobombax ellipticum (HBK) Dugand Mapola. Amapola

_ .. _......... ..,....' .. _.r_, , ........... ..bt t 1". 1";'";1 ""!+

~~4i{;:~'::-,...-.-.--.Al-._~-----"

::;.-;--:- .. - "U' .-=.d1:~- .IoO.;iJI;"'r_~

2j3j U ,L.LiLI . l .. LU .1,.' , L'1.1"" .L , .t.Ll~ Pse~d6lmedia spuri~ (Swartz) Griseb Manax

Quercus oleoide s S che~ht & Cham. Cholol

P t elT 0 car pus ' nay e s ~ ~ ~(e m 8 r . Palo de san g re, cheja

_ _ ,_~""""I'.~btU --~~poJ.o.~-- --_. _. ll"'."' .... _ ...... ........ . .n __ ..... , .• _ . ... Q ••• r""l .. l •• tD _ .J)<l. "_

~~::~~~:.-.----:::.~ --~-... ~,-.­::-:=~~~",;1,ttJ:t.~t.;t~ '~ .... I" • . N~h ..... 1.JI-J'"

::;,32 ;C! LLlLLJ-,.L i ,1, i, 1 ,J ,i ,~~~~i:f."ill:wi!;':~ Rehdera penninervia StandI & Molkenke Raspo sombrero.

33

Samahea leucocalix Britt. & Rose Cen'lcero

·-.... ~~ ... ,1 ... ~

.;~,:: .::~:'i:J.==i~~:'~~~~::~·· ~::.~;,~~~ .. ~ ,-~:.:~ :::.w""~~_

Sebasti aniR lon gicuspis S tRndl. Chech~n, chech em hlan c o,Icicheh

:~: "i~"~'~"-'" .- "'~ ... '"~~, I = j . . . . ~"1iIIII/IIIIiII '26 !··'.: ·,'1·. __ ,j :LL~L.J.J...Jw.h.h.Ll,)d;'l.h~!!!,_ "

S chi z olobium p a rahyburn (Vell)Si ck ingi a sal va dor e n s i s Sta n dl . Bl ake Salternuchc P lum a jillo, co p te.

Simaro uba glauea PaRae, aeeituno

DC.

39

".­.,

! ' ,' " \

.•• ..., lIl _ ..

''''''. ~ "" ,. _---........... -.,-,,~ . · .. ..... _ "u ...... IIlr._~ .1 • .-. .. '. ,".',. _ , 1 . . ' ..... ,

..!i.wartzia eub e n:: L~ (Britt & l.,li 11 s). S tan d 1.

Spondias ~~ L. Jobo, joeote.

~~~~~ia panamensis Benth • Chaete, ehiehipate

Llora sangre , cl t ao x ,~~lue he

4 1

Swietenia macrophylla King caoba, mahogany

~;:;';~~l_ s=::T'}- 1 _ .. tlJ: _ _ ._. ,. __ b.)t _ .~ ...

~~!::2;:,:~~ _.::~ .. ~ __ _ - --- ---- ~- -- - - ----.-....",.~-,-.-~.------

_.... _ . ! i,,_·.,_._ .... ,.; •• , • .. ;"_i.,,_~ _ ... '.- -!'.:::"l';i:~:·' :...:::-~"'~'~:~· .. ~:;;,;.:;:: ~~~i)

_Talisia floresii Standl. Poloe, toloc, ixezul

86

Tabebuia rosea (Bertol.)DC Haculiz, roble

Talisia 01 i vae fod."mis (HBK) Guaya, tapaicoJote

Radlk.

-4q

Terminalia amazonia (Gmel.) Exell. Canxun, cancxan, guayabo

, .... !~.,.!.,;~,;!.,.~~",."!!::: !:=--~~,,!~ '..-.!""_"""'._,~.o,l"~,).III.»4Jo~'"

S:~U-:-:':':::::~.Jf"~~ .... _' • . ', ". _ _ .1't "'" li'_, ..• .-Iiat ~Uo _.-. ~~' "

~:'~~~~~=?~Z~~ ':,"::~ll-J_I . ....... ~ .... ~ ... JI ._. m!....

lIat ~ irea <iHip

lundellii (Standl.)

Frijolill Danto

87

I:'~~~,~:~,";;;g~~~~:,;,:.~;, _.;....~.~'Q,..",.,._~ ;;i

~ .. 7""'~=o.:'- ... ,;;.., ~._ t!:;.,-i; .• ;~i:!Jtji .• ,,,,":~ •. ·._ ., ~.::;.t~"'~_ -'-"-- ~." ..

Trophis chorizantha StRn~r~ Ramon colorado, palo morillo

Vitex gaumerii

Yaxnic

Greenm.

• Voch y si a h on d uren s i s Sp r a~u e Wi rn meri a ba rtl e tt ii Lu n de l l San Juan. - sa y uc

51:""1 I' • 11.1. '

Ch i ntoc

I W2:,j LLLLi.L.LJ...LL.l ,1 'J ,7, 'j 1,,:1 ,ill W"" •.•

Zan t hoxylum belizense Lundell Zuelania guidonia (Swartz) Britt

Lagarto & Millsp-Palacio,Quacap,Tamay

88