Chemo-physical analysis and assessment of color alterations of selected hardwoods and development
The quality of wooden products and the efficient application of the raw material (wood), as well as monitored processes in the production decisively determine the economic success of a wood-processing enterprise. Lack of knowledge about discoloration processes during storage, drying and steaming of hardwood leads to considerable high economic losses and a depreciation of the material.
On account of new technologies new possibilities are given for the treatment of wood by drying or steaming. Compiled knowledge from practise must be analysed critically and be checked by basic research and investigations to ensure stable hygrothermal processes in the future.
The purpose of this project was to create a scientific basis for the specific hygrothermal treatment by means of steaming and drying of beech, oak and Common alder timber. The possible application of spectral-analytical measuring methods for controlling and supervising the change of colour during the wood drying process were examined. In addition, new knowledge about the influence of the history of the logs and the timber on the colouration should be developed.
Sample material was available of the three abovementioned species of wood from Europe. At the beginning, the colour of industrial used samples, sorted by colour were defined as controlling values for the colour results of the fresh air and exhaust air drying experiments. Steaming and convectional drying experiments were carried out in the industry as well as in the lab. For an objective comparison of the samples the colour values were taken using the CIE-L*a*b* colour system with a colour measuring instrument.
Depending on the initial moisture content, the temperature and the duration it is possible to influence the change of the colour significantly. Irregular discolorations have their cause in the presence of fungi, an irregular distribution of extractives in the board as well as a locally concentrated activity of enzymes. Depending on the temperature level of the heat treatment the activity of the enzymes can be used for the discoloration mechanism or also be prevented.
Streaky discolorations occurring after the drying process on wood which was frozen at the beginning of the thermal treatment can be avoided by a low temperature level in the early stage of defrosting the timber. Steady bright tones across the whole board cross section are also possible during the winter months.
After a thermal pre-treatment, oak with an initial moisture content above 30 % can also be dried at higher temperature levels without inclining to spotted brown discolouration.
Dr. Robert Putz
Kompetenzzentrum Holz GmbH
Project or cooperation partner
- Michael Grabner
BOKU Wien / Kompetenzzentrum Holz GmbH
- Johannes Pöckl
Kompetenzzentrum Holz GmbH
- Hermann Pretzl, Jürgen Pimmingstorfer, Georg Strobl, Monika Größwang
Alfa Massivholzplatten GmbH
- Günther Gruber
Mühlböck Holztrocknungsanlagen GmbH
- Manfred Reinkemeier
Abalon Hardwood GmbH
- Herbert Wimmer
Rupert Wimmer & Co