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Robinia wood – Europe’s tropical wood

At Ziegler, our playgrounds are carved from a particular kind of wood – the wood of the robinia (Robinia pseudoacacia), which is also known as the black locust tree. We only ever use this wood because it is resistant and durable.

Of all the types of wood available in Europe, robinia wood deserves special attention thanks to its resistance to abiotic and biotic influences. It can be used everywhere outdoors without the need for impregnation. It is the best alternative to tropical wood and can withstand the harshest of weather conditions without any mould forming.

Under local weather conditions, the wood of the poplar tree for example succumbs to decay after two years, beech after three to five, ash after seven, larch after ten and oak after twelve to fifteen years. In this respect, robinia wood is far superior to these woods. Even if the quality is low, it will stay standing in the soil for at least 20 years (according to GÖHRE, 1952). Outdoors, robinia will last 15 to 30 years if in contact with the ground, and 60 to 80 years if not. The wood’s structure and constituent substances ensure that robinia is permanently protected.

The robinia tree grows up to 25 metres high and can reach a trunk diameter of 60 to 70 centimetres. It was mainly environmental criteria which triggered the tree’s considerable spread from the mid-19th century onwards. Because of its strong root system, the tree’s original uses included securing debris and slag heaps as well as railway and road embankments. Not until after the Second World War did the robinia also gain in importance as a forest tree for wood production. Robinia wood is grown and used in particular in South Eastern European countries like Hungary, Romania, the Czech Republic and Slovakia. Today, with an afforestation area of some 320 00 hectares and standing stock of 40-42 million m³ (1993), Hungarian supplies are the largest in Europe. Overall, with almost two million hectares worldwide, the robinia is the third most common deciduous tree from plantations after eucalyptus and poplar trees. Robinia acreage has quadrupled in recent decades. In Germany, three-quarters of recorded robinia acreage can be found in Brandenburg and Saxony-Anhalt. Robinia trees grow fast. Thanks to its solid character, the robinia is difficult to split, tough and elastic and has good dimensional stability. Its values for compressive, buckling, tensile and flexural strength as well as elasticity are better than those of all other native tree species. Robinia wood has also proven its immense strength under permanent and constantly repeated stress. The wood is extremely impermeable to liquids and gases. Robinia wood can dry quickly despite this impermeability and its high bulk density.

Robinia wood is odourless and has a high natural resistance to fungi and insects. Another important factor is that its hardness and resistance to abrasion mean it is also highly resistant to wear.

Due to these manifold advantages over other European woods, Ziegler only ever uses robinia wood for outdoor, but also indoor play equipment. To begin with the sapwood is removed, meaning we only work with the hardest heartwood. Robinia trees have one of the narrowest sapwood zones (1 centimetre wide), meaning the proportion of heartwood is 87% for a trunk diameter of 30 centimetres, and 96% for a trunk diameter of 90 centimetres. If the wood is intended for use as a piece of playground equipment, there is no need for a special protective coating. If left untreated, the surface will develop a silvery-grey patina after about a year. Translucent glazes can be used to enhance the wood’s vivid grain and at the same time add a certain colourful touch.

Wood is a material which adapts to the conditions of its environment and reacts to changes. This ability becomes particularly apparent during extended dry periods, because these can result in cracks in the wood. As mentioned earlier, such cracks are comparatively minor and will become slightly less visible once a dry spell has passed. However, cracks are not simply the unwanted “downside” you have to take into account; in fact, cracks in wood are a logical result and consequence of using a natural resource. And most importantly: Cracks in vertical support posts are not at all dangerous, which is important when it comes to manufacturing playground equipment. This was confirmed by the DIN Consumer Council in its conclusion from 1998. As such, robinia wood has numerous advantages over other European woods. At the end of the day, robinia wood is a reliable natural material which is, in Europe at least, unparalleled. Last but not least, robinia wood really does look the part. Playground equipment made from robinia wood will add a little piece of “natural enjoyment” to any urban area.   
(sources: www.robinienholz.de, www.botanikus.de, www.bfafh.de; from an article in the magazine “Freelounge”)