Guidelines for Successful Thermal Modification Pt. 1
Kiln operations must be effective and efficient for Thermal Modification technology to be successful. Effective thermal modification produces quality wood that meets the appearance and performance expectations. Outgoing wood is darkened and becomes better in biological durability, dimensional stability, and stain performance. Efficient operations ensure these wood enhancements are realized at a reasonable cost. Thermally modified wood qualities sought should be balanced with cost, workability, strength, ductility, and consistency.
Based on our experience, here are some guidelines to improve your ThermoWood TM Kiln operations to maintain wood quality and preserve efficiency:
1. Pay careful attention when stickering wood – When wood is not stickered properly, thermally modified wood develops localized stresses at load points which causes wood to not be straight. Each sticker should be located in the same place as the layer below it. When wood is thermally modified it goes through a few phases of moisture change and the net result is wood that shrinks. Well-organized stickers also ensure an even and smooth airflow across the batch of wood for an even drying and treatment of all wooden board in the batch.
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A simple stickering station like the one pictured above can help speed up stickering while improving wood quality.
2. Ensure wood is loaded with enough weight – After the wood is stickered but prior to being loading into the ThermoWood TM Kiln, the wood has to be loaded with weight on top. This weight on top ensures the wood on top is held in place so as stresses are relieved from the wood the wood remains held in place.
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A heavy weight on top keeps wood in place as internal stresses are relieved from wood during modification.
3. Rough lumber is better for processing than planed – When cutter blades strike the machined lumber, it compresses the outer surface of the wood. This discourages moisture transfer in wood and thermal modification becomes more difficult and expensive. It is more efficient to thermally modify rough lumber because the rough surfaces allow more moisture to be released. Despite the loss in kiln space efficiency from treating rough rather than planed lumber, rough lumber helps reduce required processing time and helps improve wood quality. Processing rough lumber also ensures that final dimensions of the material is within builder specifications even after wood is later planed to remove raised grain and extractives that are brought to the surface of the wood from thermal modification.
4. ThermoWood TM Kilns can be utilized to kiln-dry wood, but you shouldn’t – Because that is not what they are optimally designed for. Before thermal modification begins, green lumber needs to have its moisture levels brought down. If you do this on a ThermoWood TM Kiln rather than a conventional air-drying kiln, you lose valuable time on your thermal modification chamber. Using kiln-dried wood significantly reduces the required time for wood to reach the required 1% moisture content before thermal modification. For hardwoods the optimal incoming MC is 4-6% and for softwoods 12-16%. Lower MC wood allows thermal modification to occur much faster. A 1-2% MC differential can sometimes lengthen the process as much as 12 hours, ultimately increasing processing cost. It is recommended that only kiln-dried wood is processed to reduce cost and increase production capacity.
5. Thermally modify same thickness lumber – As with all wood drying processes the wood in the thermal modification kiln will be processed based on a uniform species and thickness to determine modification temperature and time. Varying thicknesses of lumber in the same load will cause lumber to be dried and modified unevenly, resulting in inconsistency in color and properties. This also increases the likelihood of internal checking in the wood which greatly adversely affects yield. By having all incoming lumber of equal thickness, this variable is virtually eliminated and a consistent production of thermally modified wood batch can be attained.
6. Prepare the next load before the kiln is emptied – Measure the height of the load to ensure it is not under or over-sized. Pre-drill the moisture and heat sensor holes, and if an extra weight is available, pre-load the wood batch with a weight. This enables the load to be ready for loading into the kiln as soon as the kiln is emptied. This will help maximize kiln uptime by minimizing idle time from wood being prepared for thermal modification.
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Unloaded modified wood (left), wood loaded onto the cart for modification (middle), and wood waiting for next modification cycle (right).
7. Location of moisture sensors – It’s important to drill the moisture sensors in different locations, typically away from knots and systematically placed around the load to get data from each area of the load. This helps better represent the moisture of the batch as a whole. The location of the moisture sensors should also be in the center of the wood to allow for readings of wood moisture content in the middle.
