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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.
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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.
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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. "
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Aspi d o ~perm a megalocarpon Mue l l.-Ar g . Ma l erio
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Castilla elastica Cervantes Palo de hule
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Dialium guianense (Aubl.) S and~.,. Tamarindo. Palo de lacandon.
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Enterolobium cyc!ocarpum (Jacq) Griseb Conacaste 0 guanacaste
lIymenaea courbaril L. uapinol ----
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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
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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
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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
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Samahea leucocalix Britt. & Rose Cen'lcero
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Simaro uba glauea PaRae, aeeituno
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Spondias ~~ L. Jobo, joeote.
~~~~~ia panamensis Benth • Chaete, ehiehipate
Llora sangre , cl t ao x ,~~lue he
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Swietenia macrophylla King caoba, mahogany
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_Talisia floresii Standl. Poloe, toloc, ixezul
86
Tabebuia rosea (Bertol.)DC Haculiz, roble
Talisia 01 i vae fod."mis (HBK) Guaya, tapaicoJote
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Terminalia amazonia (Gmel.) Exell. Canxun, cancxan, guayabo
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Frijolill Danto
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Trophis chorizantha StRn~r~ Ramon colorado, palo morillo
Vitex gaumerii
Yaxnic
Greenm.
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Zan t hoxylum belizense Lundell Zuelania guidonia (Swartz) Britt
Lagarto & Millsp-Palacio,Quacap,Tamay
88