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Product Performance Profile

Wood Properties and Utilisation of Kiri


(i) Macroscopic Structure

The bark is grey brown, smooth and without fissures and the wood is pale yellow (honey/blond) to pale red. The demarcation between sap and heartwood is not clear. Usually there are only one of two annual rings in the sapwood. The heartwood is wide and its texture is fine to rather course. The latter is more applicable to fast grown KIRI where growth rings are wide. Pores in rings gradually diminish in size outwards and the place where the rings start usually does not form a clear early wood strip. The rays are not clearly visible to the naked eye but can be apparent with a hand lens or under a microscope. The wood is straight grained, light, soft and odourless with a gloss after planning.

(ii) Microscopic Structure

(a) Vessels: In cross-section pores are round or elliptical. The difference in the vessel size between early and late wood is three to five times; with a maximum variation of about ten times. The vessels are mostly solitary, sometimes whorled (cluster arrangement) or occurring in short linear multiples. Perforation is simple. Bubble-shaped tyloses (growth of cells into the cavities of others) are present in the vessels; more in the vessels of early wood than in late wood. The bordered pits (vessels with distinct rims of the cells walls overarching the pits membranes) are small, elliptical and with a wide opening.

(b) Rays are mostly homogeneous, occasionally heterogeneous, few are solitary, more are mixed, irregular, spindle-shaped, the lowest less than 0.5mm high. Rays are generally 1-50 cell high and 1-8 cells wide.

(c) Wood parenchyma is well developed, paratracheal, generally annular and wing-like, mostly clear around the vessels of late wood, wide strip-shaped in early wood. Bands are common.

(d) Wood cells are even in size in annual rings. Bordered pits are common. Wood fibres are arranged in vertical series, cell walls are thin, less the 3mm thick, medium in length, 550 - 1,700mm long.

Information extracted from Paulownia (KIRI) Report by James Davis, M.Hort.Sc., C.P. Ag.


(i) Weight and Quality

Lightness is a major advantage of KIRI timber. Its air-dried density (at 11% moisture) is 260 - 290kg/m3. The density varies between species and site conditions but all of the species produce exceptionally lightwood.

The strength-to-weight ratio (quality co-efficient) for KIRI is relatively high; most species are near medium grade and are suitable for many uses requiring soft but relatively high strength wood.

(ii) Deformation and Warping

The shrinkage co-efficient (0.27-0.37%) of KIRI is smaller than the commonly used coniferous and broad-leaved woods (see Table 1). Products made of KIRI timber do not easily warp, crack or deform. This is a conspicuous advantage. In experiments with KIRI, Poplar, Oak and other wood boards of 25mm thickness were simultaneously put into a drying room at 68C. After 12 hours, the moisture content of the KIRI timber reduced to 24%, without any deformation and damage. Under the same treatment, Poplar wood cupped badly and cracked at the ends. Oak checked and honeycombed and woods of other trees were severely damaged.

Table 1 - Wood Shrinkage Co-efficient of the KIRI Species

Radial Tangential Volume
0.094 0.268 0.362

(iii) Moisture Content

Furniture made from dried KIRI timber is very good even under humid conditions. Thus products made of KIRI timber, are generally not expected to be affected by humidity or insects. The moisture content per unit volume is comparable with other timbers that are in common usage. The humidity penetration is also said to be lower than the other timbers well known in the trade.

(iv) Thermal Insulation Characteristics

  • Thermal Conductivity: The thermal conductivity of KIRI species is in the range, 0.063-0.086 Kcal/m/hr/C which is one of the lowest values for wood. Therefore KIRI has a very high heat insulation properties
  • Temperature Conductivity: The temperature conductivity of KIRI species is in the range, 0.000561-0.000631m2/hr, the lowest in 40 species of timber that have been tested.
  • Electrical Insulation: The dielectric constant of KIRI species is lower than in other timber species when moisture content is constant, and the alternating current specific resistance is usually higher than other tree species. This shows that KIRI has better insulating properties than other tree species.

(v) Rot Resistance

KIRI timber is highly rot-resistant and that any rot appearing on the timber is only superficial. Clean, white wood appears when the surface is planed. In tropical areas where wood rots easily, KIRI boards have been soaked in water for ten years then used to make a coffin which has lasted for 30 years without decay. Coffins made of KIRI timber extracted after more than 200 years have been found in good condition. KIRI was compared with other timbers that were completely rotten. Experience in Australia from KIRI poles left in the forest for five years tends to confirm the rot-resistance characteristic.

(vi) Ease of Natural Drying

USA reports that KIRI boards, 25mm thick, can be dried to 10% moisture content in 25 days at normal room temperature. This property of KIRI is convenient for wood processing and can save the expense of artificial drying.

(vii) Sound Conduction

The sound boards of Chinese traditional musical instruments are always made of KIRI timber. KIRI has good resonance and sound radiation qualities (ranging from 14.06-20.84m4/kg/sec across all species).

(viii) Ease of Processing

KIRI timber is easy to plane, saw, mould, turn and carve without danger of chipping. It is easy to drive nails into KIRI timber but its nail holding ability is rather low. On the other hand screws hold fast and the wood hinges well. Tenon and mortise joints are best for furniture and cabinet making. KIRI timber is excellent in the absorption of glues, paints, oils, stains, dyes, varnishes and polishes.

(ix) Wood Colour and Grain

KIRI timber has a beautiful gloss like spun silk, straight grain and no odour. As it does not contain polyphenol material, red stain spots appear on furniture made of KIRI. This can take months or even years to come out. In order to solve the problem the timber has been soaked in clear water for decolourisation before processing. Particular attention should be paid to the proper felling seasons, which are autumn and winter, avoiding the growing seasons.

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