Water Absorption and Stability Test on Thermally Modified Wood

I received some samples of thermally modified wood and proceeded to test them for water absorption as I had been told that properly dried thermally modified wood will not allow active water to remain in the wood like other non-thermally modified wood products which in theory should make the thermally modified wood more stable and durable.

I took my moisture meter and got a reading of the moisture content of each piece of wood prior to the water absorption test. The moisture content reading of each piece of wood was 3% and 3.1% respectively.

I sealed the ends of the samples with a wax coating that does not allow water in or out once sealed which means the absorption had to take place through the surface of the wood. I placed the two pieces of wood in a pail of water for 24 hrs and submerged them using a bottle of my favorite Rum.

After the 24 hour period was up I removed the samples from the pail of water and let them air dry for another 24 hrs before I took a moisture readings of each of the two samples. The moisture content reading of each piece of wood were 3.3% and 2.9%.

I then inspected both pieces of wood for any cupping. I put the two pieces of wood back to back to check for cupping looking for any light coming through which would indicate the pieces were not laying tight to one another. I did not see any light indicating no cupping had occurred.

I then measured for variations in the width and length of the pieces of wood. I put each piece of wood tight to one another side by side and then measured over the total width of the two pieces. I measured both the height and the width. Prior to the absorption test the total width was 11″and the height was 6″. I had cut the samples exactly 6″ for length so they would fit in my pail of water. I did not cut along the width of the pieces as each piece measured 5.5″ and I had placed them on top of one another in the pail of water. After the 24 hr drying period I measured using the same method and the width was 1/64 or .015625 greater over a prior 11″ measurement and the height was 1/32 or .03125 smaller over a prior 6″ measurement.

Based on my test findings I believe that the thermally modified wood has performed exceptionally well for stability and I was glad to see that the moisture readings were within the same range prior to and after the water absorption test which tells me that the molecular structure of the wood cells have been changed to not allow the wood to keep free moisture within the piece’s of wood. The restriction of moisture that can remain within the wood allows the wood to be stable and more durable than other non thermally modified woods.

I performed my test on thermally modified ash wood which is a popular species of wood used in the thermally modified wood business.  The samples were courtesy of CFP Cladding & Decking.



Moisture reading on thermally modified wood after 24 hr. water absorption test.
Water absorption test of thermally modified wood siding. Samples were courtesy of
Moisture reading on thermally modified wood prior to 24 hr. water absorption test.


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