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Robinia Pseudoacacia
The Black Locust, scientifically known as Robinia Pseudoacacia, is a tree of North American "Introduced species" origins that was introduced to Europe 400 years ago, where it continued to flourish.
Isolated trees grown in parks and gardens can live up to 200 years; in wooded environments, the signs of old age tend to manifest themselves much earlier, when the tree is still under a century old.
The rapid diffusion of this species was initially fostered by man, who exploited it not only for its prized wood, but also for its nectar and ornamental qualities. In fact, the Black Locust boasts numerous advantages, including good resistance to adverse conditions, a rapid growth rate, and abundant and fragrant blossoming.
In addition to being highly elastic, Black Locust wood is also extremely durable and resistant to sudden climatic changes (dampness/dryness). The wood has a widely varied colouration, and tends to darken with vaporization; it is well suited to processing, finishing and polishing operations.
Thanks to these features, it is even capable of effectively replacing exotic species for a variety of uses, and can therefore help reduce the deforestation rate in tropical areas (South America, Asia and Africa). The seasoning process for Black Locust wood is extremely slow; this species also has a high calorific value and low water content in its fresh state.
The particular properties of Black Locust wood, above all its hardness and the fact that it absorbs very little water (see the summary table below), have the great advantage of increasing its resistance, thus prolonging its average lifespan. These properties, however, also render it rather difficult to stain ad/or paint the various components, since the colour adheres to the top layer of the treated area and is unable to penetrate deep down into the pores of the material itself. For these reasons, staining/painting treatments may need to be repeated in order to maintain their beauty and brilliance intact.
PARTS OF THE TREE: PROCESSING AND USES
For the solid wood species of greatest interest to the European market, the UNI EN 350-2 standard, titled “Durability of wood and wood-based products - Natural durability of solid wood ” establishes the natural durability indications in relation to fungi, insects and marine organisms. It also provides information regarding the impregnability of the wood, as well as the width and density of the sapwood. According to this standard, Black Locust wood can be safely used in its natural state for the creation of outdoor structures, above all in consideration of the fact that it is the only European wood of durability class 1-2.
The following table indicates the durability of Black Locust wood in comparison to other wood species:
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WOOD SPECIES
|
DURABILITY CLASS
(natural, against fungi)
according to the UNI EN 350-2 standard
|
DAUERHAFTIGKEIT
(natural, against fungi)
according to the UNI EN 350-2 standard
|
|
Teak
|
1
|
very durable
|
|
Black Locust
(robinia pseudoacacia)
|
1-2
|
very durable - durable
|
|
Chestnut (Castanea sativa)
Oak (Quercus robur)
Red Cedar - Redwood
|
2
|
durable
|
|
European Larch (Larix decidua mill.)
Douglas Fir
Scots Pine (Pinus Sylvestris L)
|
3-4
|
moderatly durable - slightly durable
|
|
Norway Spruce (Picea Abies)
Silver fir (Abies alba mill.)
|
4
|
slightly durable
|
|
European beech (Fagus sylvatica L.)
European ash (Fraxinus Excelsior L.)
|
5
|
not durable
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MECHANICAL PROPERTIES
In addition to being extremely resistant to load bearing and bending, Black Locust wood is also extraordinarily resistant to dynamic loads.
The following table lists various properties of Black Locust wood in comparison to those of other wood species.
Average properties of the most common types of wood materials at 15% humidity
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WOOD SPECIES
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d
Density
|
σc
Compressive strength in the direction parallel to the fibre
N/mm2
|
σ
Shear strength parallel to the fibre
N/mm2
|
σt
Tensile strength parallel to the fibre
N/mm2
|
σf
Flexural strength
N/mm2
|
Ef
Flexural modulus of elasticity
N/mm2
|
|
Norway Spruce
Picea Abies
|
470
|
40
|
2,5
|
80
|
24
|
11.000
|
|
Chestnut
Castanea Sativa mill.
|
600
|
48
|
4,0
|
100
|
88
|
12.500
|
|
European Ash
Fraxinus excelsior L.
|
690
|
52
|
12,8
|
165
|
120
|
13.400
|
|
European Larch
Larix deducia mil.
|
590
|
56
|
8,7
|
85
|
96
|
13.000
|
|
Scots Pine
Pinus Sylvestris L.
|
520
|
55
|
6
|
104
|
100
|
12.000
|
|
Oak
Quercus robur
|
690
|
61
|
11
|
90
|
88
|
11.700
|
|
Black Locust
Pseudoaccacia
|
770
|
73
|
16
|
148
|
150
|
14.800
|
|
Teak
|
640
|
72
|
8,3
|
119
|
148
|
13.000
|
Black Locust wood is being increasingly used for the production of recreational structures and furnishings, even rendering each structure unique thanks to its natural and irregular curves.
Play structures made from Black Locust wood are often installed over a loose layer of shockproof material, such as bark mulch, which can prove to be quite corrosive over time.
For this reason, Holzhof prefers to fasten its structures' wooden support elements to brackets, without embedding them in the ground.
Under typical environmental conditions, in fact, this practice can increase the wood's durability 3-fold with respect to that which is indicated by the UNI EN 350-2 standard.
LOCUST WOOD
The correct use of Black Locust wood requires the bark to be removed prior to any initial processing operations. It is also important to remove all the sapwood since:
• it can be more easily attacked by fungi and insects
• it decays at a faster rate
• it tends to separate from the underlying heartwood
• For these reasons, the only part of the wood that Holzhof uses to create its products is the heartwood. The heartwood, in fact:
• cannot be attacked by fungi or insects
• is extremely strong and durable
• can be used without being impregnated or undergoing other protective processes
