Reclaimed Lumber Scalable-size Wood Kiln

Last summer a realtor friend gave my name to a family that had relocated and were looking for a custom sized kitchen table. I'm a full-time teacher, full-time dad, and a part-time woodworker who does some work for people. The family called me and we sat down to talk about their needs. We came up with a design for the table and choose Spalted Maple for the top - one of my favorite woods. For those who don’t know, Spalted wood is basically wood that has been felled and left to rot. It gets some amazing character and color from fungal growth and bugs breaking down the wood over several years. Normally, any bugs that are in the board die when the wood is milled and dried. I sourced wood from a mill about an hour from me and bought a beautiful 11 foot board that the mill resawed for me to get two 1.5 inch book matched pieces for the table top. I built the table and delivered it to the happy family. About 6 months later the family contacted me to tell me that they saw bugs crawling out of the table. And, before you ask, no, I didn’t see any evidence of it when I had it in my shop!

The family asked that I fix the table. I researched this issue and determined that kiln drying the wood was the best way to treat it, but obviously it’s difficult to find a kiln or buy one the size I needed, so I decided to build one. After much research I determined that a minimum internal temperature of 130 degrees at a minimum of 1 hour was necessary to kiln the bugs. After thinking it over during a long car drive I decided that a box shaped kiln, slightly longer and wider than the table top, made of foam board and with heat guns as the heat source, should work.

As a note, I consider this to be a scalable size kiln because you could use any size of foam board to make any shape that you need. I did this for a specific size table and wood, but change the dimensions of the components and you have what you need to be successful!

Step 1: Materials - Purchase

For this I decided on 2 inch thick foam board after standing in front of the foam at the big orange box store for about 25 minutes. I used HF for the heat guns after some research. They were rated well and a decent price - plus the 25% off coupons that seem to come every week helped as well. I have to say, I was honestly surprised at how good they are! I also decided to use a 4” to 3” metal reducer from the HVAC section of the orange store to avoid melting the foam board when I used the heat guns

Materials:

3 - 4x8 sheets of 2” foam board (because I need to get 10’ long)

Kreg pocket hole jig and coarse pocket screws

2 - 4” to 3” metal reducers

2 HF heat guns

12 - 1” thick 4” wide and 8’ long pine boards

8 - 10’ long 1x4 boards (I should have used 1x6’s)

Ratcheting tie down straps (I used ones I already had)

1 meat thermometer for temperature

Tin foil pan for water (I forgot this when I started and it’s not in the pictures)

Hole saw

2 - 2 inch "L" brackets

Step 2: Kiln Design - Cutting Support Pieces for Inside the Kiln

I used a brace design with the wood pieces inside. I made it that the pieces that were to be in the kiln would rest on those to allow air to circulate around the wood. Because my table is 27” wide I made the braces 28.5” wide and 15.5” tall to allow the foam board to surround it. I cut 10 lengths of 28.5” and 10 lengths of 14”. I used the Kreg pocket jig to make easy connections - for those who haven’t used it, it allows you to drill holes into the sides of the wood to create strong joints in the wood. I cut the pocket holes on the side supports and drilled them into the top and bottom pieces - then I added support crossbeams for the table at 5” up from the bottom (so I had space to add other support crossbeams later if I wanted to). The support was cut at 27” and pocket screws were used to connect to the side supports - I also added a small piece of wood underneath it just in the middle to provide some extra strength to the crossbeam. Make 3 of these with the support piece in the middle of the width of the uprights - these will be the main portions of the kiln. Make 2 with boards flush to the edges of the boards and at the top and the bottom of the support - these will be the end pieces.

Step 3: Kiln Design - Foam Board Exterior

For the outside of the kiln I used a box cutter to slice part way into the 2” foam board at 18” in from one side on 2 of the boards. I cut on the blue side and left the metallic portion on the interior and those joints were uncut. I placed the board over the wood (which was on sawhorses) and pushed down along to cut line to break the remainder of the cut cleanly and make the pieces fold over. This gave me one section that was 18 inches wide and 8 feet long and another section of the same board that was 30 inches wide and 8 feet long (18 inches wide and 30 inches wide = 48 inches wide, which was the wide of the foam board). Because I needed a 10 foot long kiln I used my third piece of foam board to add 30” to the 8’ length and cut those the same way - metal inside and blue outside, 18 inches and 30 inches.

For the end pieces of the kiln I cut out 2 pieces of foam board that fit into the back of 2 of the supports. I traced the inside diameter of the end piece on the foam board and cut on the shape to fit inside the support. When placing the foam board inside the support make sure to keep the blue side against the cross pieces.

Step 4: Kiln Design - Heat and Outside Support

As I mentioned earlier, I used 2 HF heat guns to heat the kiln. I needed a way to heat the inside of the kiln without melting the foam board so I used a hole saw to cut a hole in the foam board of the end pieces. It was big enough for the 3 inch end of the reducer to push into with the 4 inch side on the outside of the kiln (the blue side). I used the "L" brackets and drilled a small hole in the reducer. I screwed the bracket to the wood crossbeam and one into the reducer.

The last picture is the corner supports that will cover the edges of the foam board and prevent the straps from crushing the foam. I used 10' x 8" boards that I ripped down the middle - when i use this again I will remake these with wider boards (5"). I simply joined these edge to edge and used the pocket screws to connect them.

Step 5: Putting It All Together for Use

1. Put down the straps that you will use to hold the pieces all together where ever you are planning on putting your support pieces. I put the end ones 10 feet apart and them about every 30 inches inside.

2. Place 2 corner brackets on the floor and rearrange your straps (Only once is necessary, but I did it 4-5 times until i was happy!)

3. Place one of your foam boards into the bracket - the 30" section is one the floor and the 18" section is on the side. I leaned the 18" section against my table saw to keep it upright

4. Place the center supports to the set up - add one support end in.

5. Place the other foam board across the open top and side - this one is the easy one to add as it just lays atop the supports

6. Add the last 2 corner braces across the top

7. The tie-downs will go across the area where the supports are and ratchet them down.

8. Place the boards inside the kiln across the center supports and add a tin foil pan of water (water is optional - it helps for the bugs)

9. Close up the end and ratchet the end clamp down

Step 6: Starting and Using the Kiln

With the kiln all set up it is time to add the heat guns on the ends and the thermometer to the top. I used a meat thermometer and stuck it through the foam board in middle of the top so it was equal distance from the heat sources. There are 2 heat guns - one for each end. I set mine up so that the heat guns are on opposite sides of the end, to make a more circular path of the air in the kiln. When I started this up the first time it actually surprised me - the temperature jumped to 125 degrees in under a minute (I told you I was surprised how good they were)! I kept the wood in the kiln for 5 hours, but turned off one of the heat guns after about 30 minutes. I alternatively turned one or the other off over the time and maintained an internal temperature of about 180 degrees over the time period. After 5 hours I turned off the heat guns and pulled off one of the end pieces to allow the heat to dissipate and the wood to equalize.

Step 7: Final Thoughts and Rationale

I did a lot of research before building this kiln - here is one of the pieces I read to determine if it would work: http://ucanr.edu/sites/WoodyBiomass/newsletters/FP... Good luck deciphering some of it!

As I said at the beginning of this instructable, I needed the internal temperature of the wood to reach 130 degrees for at least 1 hour. I had no way of knowing the internal temperature of the wood, but I figured that if I heated it to 180 degrees for 5 hours I would raise the internal temperature the correct amount (sort of like cooking!). I have not seen any more bugs in the table since it was in the kiln, so I'm taking this as a win! As I mentioned at the very beginning, I consider this to be a scalable size kiln because you could use any size of foam board to make any shape that you need. I did this for a specific size table and wood, but change the dimensions of the interior supports and the foam board and you have what you need to be successful!

On another note, I did not purchase a moisture meter until after I did this. I haven't needed to set it up again yet, so I don't know what the change in moisture content change would be, but I could feel a change in the wood after bringing it out of the chamber. next time I will definitely monitor that.

Could this be used to dry wet wood? To a point, but I'm not sure of the time effort required. This is, however, an effective way to kill bugs and fungus. With our love for reclaimed wood and barn wood this is also a good way to make sure that the furniture we make is bug and fungus free. If you have any questions or thoughts, please feel free to share them!