Purpose
To study various species of trees, whether leaves are combustible with or without flame, would contribute :
i) to find one condition to choose trees for the standard plantation of fire prevention,
ii) especially to find fit species for plantations standing very near to houses, because the leaves bursting into flame may cause a danger of spreading fire instead of being a fire cordon,
iii) to to examine experimentally the reliability of the reports on flameless combustibility of leaves written by forerunners who met with and observed past conflagrations : for instance, Pinus palustris were combustible without flame, and Cinnamomum Camphora blazed up easily etc.
Experimental methods
Fresh leaves were carried, hanging from hooks on a rail into a cylindrical furnace maintained at constant temperature by a trasformer box. On leaves were made observations of the type of combustion, measurements were taken of the time required to blaze up. They were repeated several times in a series of 7 trials.
Results
The experiments were made in March and April 1951, in which were used most of popular evergreen trees and shrubs for landscaping in Japan. Results obtained are shown in Table 1, regarding
1. ratio of flameless combustibility at 500° and 550°C,
2. time required to blaze up,
3. the type of combustion at 500°C heating. This is classified as follows :
A
1 No explosion; furiously jetting vapor and gas.
A
2 No explosion; calmly jetting vapor and gas (sometimes with faint crepitations)
B
1 Sharp, strong cracklings.
B
2 Successive weak cracklings.
B
3 Powerless explosions.
Discussion of results
1. The flameless combustibility of leaves depends probably on the heating temperature.
It could be hardly seen in the furnace at temperatures higher than 550°C among the plants in Table 1.
2. No correlation could be found between the flameless combustibility and the time required to blaze up. Probably the latter may be connected with the evaporation surface area, accordingly to the thickness of leaves, while the flameless combustibility may depend on existence of any conditions, which prevent the kindling of the inflammable gases produced by thermal decomposition of leaves.
3. Flameless combustibility of leaves at 500°C could be seen more often among broadleaved shrubs than among conifers and broad-leaved trees.
The excellent species in this regard are Camellia Sasanqua, Chamaecyparis pisifera, Daphniphyllum macropodum, Euonymus japonica, Fatsia japonica, Ilex rotunda, Ilicium religiosum, Myrica rubra, Nerium indicum, Osmanthus ilicifolius, Sciadopitys verticillata, Thujopsis dolabrata Viburnum odoratissimum.
The worst species are Pinus densiflora, P. Thunbergii, Cedrus Deodara, Magnolia grandiflora, Osmanthus fragrans, Quercus glauca.
4. Generally speaking, leaves take more time to blaze up among conifers than among broad-leaved trees and shrubs. Species taking long time to blaze up are Chamaecyparis obtusa, C. pisifera, Pinus palustris, Taxus cuspidata; Daphniphyllum macropodum, Ligustrum japonicum, Osmanthus ilicifolius, Pinus densiflora, Podocarpus macrophylla, Sciadopitys verticillata, Ternstroemia japonica, Viburnum odoratissimum.
Species on the antipole are Cinnamomum Camphora, Myrica rubra, Osmanthus fragrans, Shiia cuspidata, S. Sieboldi.
5. In this experiment Viburnum odor. (a conventional fire prevention tree was excellent both in regard to the flameless cobustibility and to the time required to blaze up. Likewise to be recommended is Thujopsis dolabrata.
Myrica rubra, a popular tree for fire prevention of forests in Japan was excellent in regard to the flameless combustibility, but it was one of the species which took the shortest time to blaze up.
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