The soils that are mostly wet, sandy, gravel – and clay-like, deep and fertile usually found above high underground waters are the most suitable habitats for the growth of oak forest (quercus robur). The largest oak forests thrive best on lowland and slight upland soils of the diluvial geological era, the valleys of river basins being especially suitable sites for this type of oak.
Variations in the water level, floods, marshes formation – all are conditions that in a very special way promote the growth of this particular tree. Because of a continuous change of the direction of the river flow on a greater or lesser degree, the mainstreams weave through the valleys constantly forming live meanders. In its meandering course, the river undermines the banks covered with forests, the trees fall in to the river and get drifted away in the water. When the trunk gets trapped by its branches and roots to the river bed, in time it becomes covered with the layers of mud, sand and gravel. Deprived of oxygen the wood undergoes the process of fossilization and a long process of abonos formation (or Croatian “Crni dub”, English bog-oak, German Mooreiche, in the world of pipesmokers known as “morta”).
During hundreds and thousands of years, under the influence of the minerals and iron from the water, the decomposition of oak timber is considerably slower. In the formation of abonos a special role is played by the currents of the underground waters, providing solution of the ingredients which make compounds with larger quantities of tanin from the wood and in this way induce darkening of the wood color. The centuries-long process of timber change, often termed “maturation” results in colour alteration from golden-brown to completely black, and its hardness is increased to such a level that later it can only be carved with the use of specially grind and exceptionally firm tools.
The time necessary for the oak to transform from the end of its biological growth to abonos is various. According to the conditions under which “maturation” takes place, this process can last two, three, or seven thousands years. Due to the ecological reasons mentioned above no two trunks can be found of the same color.
Sites of abonos wood in the world are very rare. In the sites expected to accommodate it (in Croatia mainly in the valley of the river Sava and its branches) abonos is hard to find, the access to the river bank and its bed is usually difficult, and abonos recovery usually results in failure. Therefore, excessive preparations and engagement of a large number of professionals skilled in diving in the murky waters and in complete darkness, are necessary. Abonos is found in the entire dark and its pulling out of the water is in fact the first sight of it after many thousands of years. The age of abonos found in Croatian rivers ranges from several hundred years in the rivers on the southern part to the oldest retrieved so far in the river Krapina being 8290 years old.
Saving the quality of wood as the material for potential further processing is a very delicate matter.
The process of wood desiccation is complex, but despite greatest care most of raw wood is unsuitable for further processing. For this reason the price of quality abonos raw material is very high.
Aesthetic effect of abonos is based on the very experience of naturally formed color, noticeable wood structure, its “imperfect” appearance, and the fact that parts of the wood are combined which by their variation in the color, directions of growth rings, or certain “damage” convey a strong aesthetic and ethical message about the immensity of the past times. Deep impression of the passing of time is also grounded on the knowledge of longevity of wood and the miracle of its intactness indicates that wood is not affected by usual weather conditions or vermin which naturally spoil the looks and strength of wood. Semi dry abonos wood is exceptionally hard, sometimes of the golden or copper color, or with a tint of some other hue. Older wood can be completely black. This color is particularly specific so every connoisseur of materials is able to notice the profound permeation of the wood structure by darkness, noble black color of “live” wood so different from that of black fossil or coal.
This value of deep darkness is a special feature of abonos as construction material whether it be used for the making of semi manufactured goods, veneer, planks or bams, depending on purpose of the final product, or for final goods: floor coverings, furniture, doors, window frames, sculptures, and various decorative objects and items for everyday use.