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The process

Wood science

The process, phase by phase, and what it means for the wood you buy.

Bringing Wood Science to Wood PeopleScroll

The method

Wood modified with heat, to a published standard.

The wood is modified with heat in a kiln, over a set time and to a set temperature. The industrial method was developed in Finland in the 1990s.

The International ThermoWood Association sets the classes, temperatures and quality control for its members. ThermoCraft™ partners LDCwood in Belgium and HJT-Holz and SWM-Wood in Finland are members.

2000
The International ThermoWood Association is founded in Finland.
18
Permanent members in eight countries, October 2026.

Inside the kiln: three phases, start to finish

  1. Stickered boards on a kiln cart before modification

    Before the kiln

    Into the kiln

    Sawn boards are stacked in layers with spacers between them, so the heat reaches every face, and the loaded carts are rolled into the kiln.

  2. Kilns at LDCwood

    Phase 1

    Drying

    The kiln heats quickly to 100 °C, then rises step by step toward the modification temperature. The wood dries as it goes.

  3. An open kiln chamber with a loaded cart inside

    Phase 2

    Modification

    The kiln holds a steady temperature and the wood is modified. For Nordic softwood in the Thermo-D class, that is 212 °C. Heat and steam change the wood for good, with no chemicals used, so it comes out more durable with a rich, even tone.

  4. The same kind of cart after modification, the boards darkened through

    Phase 3

    Cooling and conditioning

    The kiln is cooled with a water spray. Once it is cool enough, moisture is brought back into the wood so the boards are ready for machining.

  5. Ends of a stack of milled Ayous boards

    After the kiln

    Milled to our grade specs

    The modified boards are milled into profiles: Fineline T&G, Bevel, Ribbed, S4S and Decking. Custom profiles on request. Talk to our sales team. ThermoCraft boards and profiles are milled to our grade specs.

Kiln temperature
212°C
100 °C212 °C
  1. 1Drying
  2. 2Modification
  3. 3Cooling and conditioning

Schematic for Nordic softwood, class Thermo-D, after the producers' association handbook. Times and temperatures vary by species, class and board size.

Before and after

Stickered boards on a kiln cart before modification
The same kind of cart after modification, the boards darkened through
After the kiln Before the kiln

Classes and temperatures

One process, set by species and class.

The association defines two standard classes, Thermo-S and Thermo-D, and sets the modification temperature for each by wood type. Each kiln schedule is then tuned to the species and the board size.

Modification temperatures by wood type and class
WoodThermo-SThermo-D
Nordic softwoods: Nordic Pine, Nordic Spruce190 °C212 °C
Ayous190 °C208 °C
Red OakNot in Thermo-S212 °C
Radiata PineNot in Thermo-S230 °C

All temperatures plus or minus 3 °C. Source: the International ThermoWood Association's handbook, 2025.

Control screen showing a kiln temperature record, with wood samples on the wall behind
A kiln temperature record on a producer's control screen, Belgium.

Quality control

Every plant audited, every year.

Production at the association's member plants runs under a quality control system that an external, accredited inspection body checks.

  • 01

    The plant keeps a written quality system covering its process, equipment, deviations and product checks, with named people responsible for production and for quality.

  • 02

    The plant is audited at least once a year.

  • 03

    Once a year an approved outside laboratory tests product from the line. The auditor picks the pieces.

  • 04

    The checks cover moisture, surface and interior cracks, colour and the process settings.

  • 05

    A plant that fails to meet the requirements can lose its certification.

History

From Finnish research to a standard used worldwide.

  1. 1900s

    Scientists begin to study heat treatment of wood in kilns.

  2. 1980s

    The first commercial plant to modify wood with heat opens in Germany, short of industrial scale.

  3. 1993

    VTT, the Technical Research Centre of Finland, and Finnish wood companies develop the industrial-scale process.

  4. 1998

    A Finnish kiln maker delivers its first kiln built to modify wood with heat.

  5. 2000

    The International ThermoWood Association is founded in Finland.

  6. 2001

    VTT and the association sign the licence agreement for the process.

  7. 2007

    European technical specification CEN/TS 15679 defines wood modified with heat as its own category of timber.

  8. Today

    Eighteen permanent members in eight countries produce to the standard, three ThermoCraft partners among them.

What this means for your business

A process you can explain to your customer.

Your customers will ask how ThermoCraft is made and who stands behind it. The answer is a published process, a named producer and a yearly outside audit. Our team is on hand to advise your buyers, builders and specifiers.

Buying for stock or for your shop

Buying for a job

Questions about the process?

Our team can advise, species by species.

Talk to our team