Introduction
Modification of timber properties by high temperature treatment has been explored for over fifty years. During the early 1990s a variety of heat treatment processes have been developed in northern Europe to produce timber that is claimed to be more stable and more resistant to rot, a property that is particularly useful for upgrading normally non-durable softwoods without the use of chemicals.
A pack of Welsh beech was sent for trial treatment (as part of an EU funded ‘Shortlength’ project) by one method that uses temperatures of between 180c and 230c and we were impressed with the results.
Heat treatment for Wales
Coed Cymru’s role is to provide help and advice about managing and improving woodlands. This includes the production of timber (mainly hardwood) and adding value to that timber and it is our experience that whilst oak has a ready market, beech, sycamore and even ash can be difficult to sell. Heat treatment might be a way of raising the value of this wood.
Purpose built ovens are available commercially but they are large and extremely expensive. We felt that there is a need for a small oven that would be more appropriate to the scale of hardwood timber production and much of the manufacturing in Wales. There was also a need to test the process on Welsh hardwoods and to obtain data on the most suitable treatment protocols. We are also collaborating with Marcos Gonzales-Pena at the School of Agriculture and Forest Sciences at the University of Wales in Bangor. He is carrying out a detailed study of heat treatment of birch and other timbers and is providing significant technical input.
The oven
An oven capable of treating up to 1m3 up to 250c was built by Caltherm, a specialist company based in Newcastle under me ((01782 563865). It is important to exclude oxygen so ports for steam and nitrogen were included and the interior made reasonably airtight. Air is circulated by a powerful fan. A robust trolley loading and clamping system to secure the timber was provided. The cost of this oven was around £13,000, although it could be made larger without significantly adding to the cost. More detailed information regarding the design of the oven is available on request. The Countryside Council for Wales funded the purchase of the oven.
Results of heat treating timber.
A range of samples of timber species have been treated for different lengths of time. Examples of results are as follows:
The timber darkens according to the temperature and duration of heating. It has been mistaken for tropical timber, and we understand that there is an increasing interest in darker timbers. The colour is throughout the timber, representing a big advantage over stained finishes, creating a whole new range of timber products.
The darkening depends on the duration and temperature of heating. The samples of beech, below, were heated at 200c for up to 8 hours.
The longer the treatment, the darker the colour, so the required colour can be chosen.
Heat treatment renders the timber more stable as shown below. Samples of beech treated at 200c were re-dried and allowed to swell in water saturated air. The movement was reduced by up to 50%, slightly better than a sample that had been treated in Finland. This is particularly useful for beech which has high movement and expands the potential range of products that beech can be used in.
We have found that lower temperatures for a longer time gives a more reliable and uniform product. A slow run up to temperature that includes holding at 120c for initial drying produces a good product.
Kiln dried timber is used although it is possible that manipulation of the process could be used to partially dry some species to reduce kiln drying times.
Heat treatment tends to change physical properties of timber. We have some data on this but much good work is being done by Marcos Gonzales-Pena at the University of Wales in Bangor. For many purposes (furniture, flooring) where the timber is not being stressed to its limit, these changes will not be significant.
The machining properties appear to be improved.
Durability of the samples of treated timber have not yet been tested, though this is planned for the future. It should be noted that these tests should be carried out by a competent laboratory who can provide proper certification.
Conclusions
We feel that heat treatment of hardwoods can create a new range of useful products from hardwoods grown locally in Wales. Certain softwoods may be amenable to heat treatment, although the reduced strength and increased brittleness may be a problem
For further information please contact Andy Stewart at Coed Cymru, 01686 650777
(OR see Staff Page for email addresses and officers' telephone numbers).
Coed Cymru, The Old Sawmill,
Tregynon, Newtown,
Powys SY16 3PL
Tel 01686 650 777
|
