Methods for processing carbonized wood and solutions for producing raised glue lines at the joints of large-board tabletops

Hello, Mr. Shen:

I'm a reader of Asian Wood Panels & Furniture and work at a woodworking factory in Indonesia producing solid wood furniture. Thank you for explaining the differences between traditional kiln drying and high-temperature kiln drying, as well as the pros and cons of high-temperature drying.

I've recently been discussing with industry insiders the feasibility of carbonizing wood using high-temperature kilns, as well as its advantages and disadvantages. I'd appreciate your assistance in preparing carbonized wood. A while ago, while producing two-inch-thick solid wood tabletops, I discovered that the glue lines where the planks are joined had some raised areas. Could you analyze the cause and offer suggestions for improvement? Thank you!

I wish you good business!

Mr. Lin

Thank you for your letter. The following is a brief summary of your questions based on the information you provided:

01. About carbonized wood processing

Carbonized wood treatment can be done by first drying undried wood at high temperature. Coniferous species are most suitable, such as spruce and Pinus sylvestris. Broad-leaved species that are not prone to honeycomb cracking, such as poplar and rubber wood, must be selected for high-temperature drying.

1. High temperature drying of wood: The drying temperature of wood is between 110-150 degrees Celsius. The drying kiln can quickly heat up to the required temperature when necessary, and then dry it quickly to ensure that there are no honeycomb cracks. The average moisture content of the wood is 8%;

2. Wood carbonization treatment: gradually increase the drying kiln temperature to 185-215 degrees Celsius, and maintain the highest temperature in the kiln for about 2-3 hours after the moisture content of the wood drops to 0% to reach the predetermined carbonization depth;

3. Cooling and moisture content treatment: Start the water spray system in the kiln to spray water to reduce the temperature in the kiln to between 80-90 degrees Celsius. Adjust the moisture content of the carbonized material in the kiln to 4%-7%.

02. The main advantages and disadvantages of carbonized wood

As shown in the table below (click on the original image to enlarge it).

The wood pile in Figure 2 is light yellow before being carbonized at high temperature.

Figure 3: The light yellow pile wood turns brown after being carbonized at high temperature.

03. Producing 2-inch thick solid wood slab tabletops (slab table)

According to your analysis, the reasons for the local bulge in the splicing glue line during shipment and the suggestions for improvement are as follows:

Figure 4: The 2-inch thick solid wood slab table top has a high moisture content in some areas. When the table top is stored in an air-conditioned room or a dry warehouse, it will continue to dry out and release moisture, causing the table top to shrink or collapse in some areas, resulting in a bulge in the glue line.

We hope the information above can help address some of the issues with high-temperature carbonized wood treatment and resolve the problem of glue lines protruding from the tabletop. We hope this brief overview of high-temperature carbonized wood treatment is satisfactory.

Mr. Shen Yuxin

Born in Kuching, Sarawak, Malaysia in 1948, he graduated from the Forestry Department of National Chung Hsing University in Taiwan in 1974. After graduation, he worked for the Taiwan Forestry Bureau. From 1975 to 1995, Mr. Shen worked in the wood processing and wood-based panel industries in Malaysia, Singapore, and Kalimantan, serving as an engineer, production supervisor, production manager, and general production manager. In 1996, Mr. Shen joined the Asian Woodworking Group of a world-renowned chemical company as Regional Technical Service Director.

This article is excerpted from the May/June issue of Asian Panels and Furniture