The objective of the work presented in this doctoral thesis has been to correlate the commercial output of  ThermoWood® process with a pre-defined, quantifiable, non-destructive measurable parameter - colour, and to show the feasibility of measurements to be the essential part of the internal and external quality-control system. Colour is the most evident property changed of thermally-modified timber (TMT). Colour can be thought of as describing the intensity of the process and as such is a sign of repeatability when working with same treatment methodology and materials. Intensity is usually related to the temperature and time, but elements like water or moisture, steam and pressure present on their own or when combined, have their own influence on the final properties of wood. There are several processes using these above-mentioned components as a modification method for wood. In addition, processes using oil as a heat-transferring element are included in this same modification category. Some of the methods are closely interrelated, with only subtle differences noted. Most of these methods are patented, introduced and processed in Europe, but the market and potential increase for these methods is worldwide.  TMT is a new product, with a relatively short industrial scale history (approx. 25 years). During this time huge technical improvements have been made. Processes that are computer aided, remote controlled, and containing recordable systems have replaced the traditional manual processes.  Building materials and technical parts regarding air circulation, ventilation, heating and measuring technology with much better thermal endurance has been introduced. All these, combined with several variety of wood species becoming commercially available, have set the frames for a successful enabling of the technology. TMT dominates modified wood. Nevertheless, it is only fraction of treated wood available when looking at preservative treated wood produced globally. 

Respective comparisons between thermally-modification processes ThermoWood® and WTT® have been undertaken. Due to different treatment atmospheres and wood moisture content, significant chemical changes of water-soluble compounds and degradation products, colour, acidity and strength properties were reported. All the material used in these studies was based on typical dimensions and lengths and they were industrially produced. There were several options for ThermoWood® sources, because the products are classified, and processes are certified. WTT® treatment process was lately presented and the initial experience and knowledge about process was minor. As a consequence. some unexpected reactions and observations were experienced. ThermoWood® represents the major part of TMT commercially produced in Europe. It has two treatment classes Thermo-D and Thermo-S, where the D describes “durability” and S “stability”, respectively.  Thermo-D products are usually exposed to external applications and wood species such as Norway spruce and Scots pine are used. Most of the ThermoWood® producers have been audited by certification body since 2006. At present there are several producers from Finland, Iran, Latvia, Poland, Sweden, Turkey and latest Canada which are under continuous control system and certified. New treatment kilns have been supplied to several new countries and as a consequence more potential customers are expected to apply membership and certification. The importance of required treatment parameters in each class, repeatability and internal quality control is highlighted. Non-destructive colour measurements (CIELab) system from wood surface has been applied as one quality control method and extensive colour data has been collected during external audits and even much more in continuous internal quality control. Colour measurements have been entrenched as a daily routine in the production plants. Much of this data has been used in this study and a correlation between L* and b* has been observed in both treatment classes. There have not been a lot of studies published dealing with Norway spruce and Scots pine colour measurements, produced with a dry process under superheated steam, but all the studies found regarding these two wood species suggested that colour might be a reliable quality control tool and the studies done in this thesis strengthened this opinion.