Balsa

Balsa wood is the lightest and softest commercial hardwood timber. White to oatmeal in colour with high silky lustre. Density 160 (120-220)kgm3, commercially preferred density range 120-160kgm3. Strength and stiffness approximately 50% that of Baltic pine (Pinus sylvestris).

Other Names

Balsa wood

Botanical Name

Ochroma pyramidale

Common Form
Sawn
Species Type

Balsa is the lightest and softest timber used commercially. It exhibits an unusually high degree of buoyancy and provides very efficient insulation against heat and sound. The wood can be adapted to a great number of special end-uses where these properties are essential. The timber is white to oatmeal in colour with sometimes a pinkish tinge near the hart. Still most commercial material is from the sapwood.  It has a coarse and even texture resulting from the large and infrequent sap conducting vessels. Straight grain. It is a short lived fast growing tree native to tropical South and Central America but it is widely cultivated in the tropics. Trees grow to 21m high and diameter of approximately 0.5m within seven years, reach maturity in 12-15 years after which they deteriorate with increases in density leading to the formation of hard and heavy wood and heartwood which tends to become waterlogged. Although when planted outside their natural range, the wood produced tends to be heavier and outside the commercially accepted density range for balsa the species has been planted successfully in Papua New Guinea, Malaysia, Fiji, and Indonesia. 

Characteristics of balsa wood make it a unique hardwood albeit, when weight for weight basis is excluded, it is the weakest of all commercial species. The logs must be converted promptly after felling to prevent extensive splitting. Kiln drying is preferred to air-drying due to low moisture movement and very high green moisture content (200-400%). The timber works well with tools but cutting edges must be sharp to avoid crumbling and to ensure a good finish in planning.  Material can be nailed, but its holding power is poor. Glues well, stains and polishes satisfactorily but it is very absorbent. Timber is prone to attack by Anobium, termites and longhorn beetle.  The sapwood is permeable, heartwood resistant to impregnation. Timber cut form old trees is often brittle and decayed. Traditional uses include: surf boards, laboratory mounting boards, fish net floats, aircraft construction and model making.

Shrinkage

Very Low Low Medium High Very High
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Tangential : Provides a simplified rating for tangential shrinkage. This is the measure of the percentage reduction in dimension from the unseasoned to 12% moisture content condition.

4.90%

Radial : Radial shrinkage is perpendicular to the growth rings. It is shrinkage in the direction towards the centre of the tree. Measurement is % value

1.80%

Unit Movement Tangential: This is the percentage of dimensional change for each 1% moisture content change between about 3% moisture content and the fibre saturation point for the particular species.

0.27%

Unit Movement Radial: This is the percentage of dimensional change for each 1% moisture content change between about 3% moisture content and the fibre saturation point for the particular species.

0.10%

Strength Group Strength groups are given for unseasoned (S1-S7) and seasoned (SD1-SD8) timber in accordance with AS 2878. S1 and SD1 yield the highest strength and stiffness whereas S7 and SD8 yield the lowest.

Very High

High

Reasonably High

Medium High

Medium

Reasonably Low

Low

Very Low

Unseasoned:

S1

S2

S3

S4

S5

S6

S7

S8

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Seasoned:

SD1

SD2

SD3

SD4

SD5

SD6

SD7

SD8

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Stress Grade A stress grade is defined in AS 1720 as the classification of timber for structural purposes by means of either visual or machine grading. The stress grade indicates the basic working stresses and stiffnesses to be used for structural design purposes. Measured in MPa.

Structural
No. 1
Structural
No. 2
Structural
No. 3
Structural
No. 4
Structural
No. 5

Unseasoned:

F7

F5

F4

Seasoned:

F11

F8

F7

F5

F4

Density per Standard Seasoned density is based on moisture content of 12%. Unseasoned density is an approximation as it depends on the moisture content at the time of measurement. Measured kg/m3.

Seasoned:

160kg/m3

Unseasoned:

700kg/m3

Joint Group The joint group is a classification of the strength of a species in joint design. The values are from 1 (very high strength) to 6 (very low strength).

Very High

High

Reasonably High

Medium

Low

Very Low

Unseasoned:

J1

J2

J3

J4

J5

J6

Seasoned:

JD1

JD2

JD3

JD4

JD5

JD6

Colour The colour of seasoned heartwood can vary between species and often within a species. The information provided should be used as a general guide only. In most cases, the colour of sapwood is either a lighter shade of the heartwood or a white/cream colour.

  White, yellow, pale straw to light brown Pink to pink brown Light to dark red Brown, chocolate, mottled or streaky
   
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Mechanical Properties

Modulus of Rupture - Unseasoned: This property is a measure of maximum stress which timber can momentarily sustain when loaded slowly and continuously as a beam. Measured in MPa.

3.1

Modulus of Rupture - Seasoned: This property is a measure of maximum stress which timber can momentarily sustain when loaded slowly and continuously as a beam. Measured in MPa.

5.6 8.8-25.5

Modulus of Elasticity - Unseasoned: This property is of importance in determining the deflection of a beam under load — the greater the stiffness, the less the deflection. Measured in GPa.

0.5

Modulus of Elasticity - Seasoned: This property is of importance in determining the deflection of a beam under load — the greater the stiffness, the less the deflection. Measured in GPa.

1.8-6.4

Maximum Crushing Strength - Unseasoned:

Also referred to as compression strength. This property measures the ability of the timber to withstand loads applied on the end grain. Measured in MPa.

Maximum Crushing Strength - Seasoned: Also referred to as compression strength. This property measures the ability of the timber to withstand loads applied on the end grain. Measured in MPa.

4.9- 17.6

Impact - Unseasoned: Provides the Izod value, which is the energy taken in joules (J) to fracture the timber.

Impact - Seasoned: Provides the Izod value, which is the energy taken in joules (J) to fracture the timber.

Toughness - Unseasoned: This is a measure of timber’s ability to resist shocks and blows, and is synonymous with impact strength. It is measured in Nm.

Low - up to 15 Nm

Toughness - Seasoned: This is a measure of timber’s ability to resist shocks and blows, and is synonymous with impact strength. It is measured in Nm.

Low - up to 15 Nm

Hardness - Unseasoned: Refers to the Janka hardness test and is a measure of timber’s resistance to indentation.

1.6 (end)

Hardness - Seasoned: Refers to the Janka hardness test and is a measure of timber’s resistance to indentation.

0.4, 2.4

Durability

Low Moderate Reasonably High High
(0 - 5 yrs) (5 - 15 yrs) (15 - 25 yrs) (more than 25 yrs)

In-Ground: Durability is defined as the inherent resistance of a timber species to decay, or to insect or marine borer attack. All references to durability refer to the heartwood only. A scale of low to high (durability class 4 = low; 1 = high) has been adapted for above and in-ground durability.

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(0 - 7 yrs) (7 - 15 yrs) (15 - 40 yrs) (More than 40 yrs)

Above ground: Durability is defined as the inherent resistance of a timber species to decay, or to insect or marine borer attack. All references to durability refer to the heartwood only. A scale of low to high (durability class 4 = low; 1 = high) has been adapted for above and in-ground durability.

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(0 - 20 yrs, usually < 5) (21 - 40 yrs) (41 - 64 yrs) (More than 60 yrs)

Marine Borer Resistance: Indicates whether a species is resistant (R) or not resistant (NR) to marine borer attack.

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Lyctid Borer Susceptibility: Indicates whether a species is susceptible (S) or not susceptible (NS) to lyctid borer attack.

Susceptible

Lyctid Borer Susceptibility - Other:

Termite Resistance: Indicates whether a species is resistant (R) or not resistant (NR) to termites.

Not Resistant

Fire Properties

Appearance

Heartwood white to grey-white sometimes pinkish near the heart in older trees, not clearly demarcated from the sapwood; grain straight, texture coarse and even, wood with silky lustre.  Growth rings indistinct with medium to large pores (vessels) often appear as coarse brown scratches on the wood side-grain.

Common Applications

Ship and boat building, sporting goods, musical instruments, blockboard, toys and novelties, turnery, surf boards, model making

Workability

The timber works well with tools but cutting edges must be sharp to avoid crumbling and to ensure a good finish in planning.  Material can be nailed, but its holding power is poor.  Glues well, stains and polishes satisfactorily but it is very absorbent.

Origin of timber
SE Asia
Balsa wood comes predominantly from Ecuador, the main exporting country supplying 80-90% of the volume traded on the world market. available from hardware, arts and crafts retailers
Source of timber
Plantation

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