Vacuum Kiln for Drying Wood by Edmonton Woodturners Guild

 

Vacuum Kiln History 

 

Mr. Jeff Hankinson was kind enough to show us his presentation on drying wood with a vacuum kiln. This technology may seem a little bit like science fiction to some people, but it is at least a century old because vacuums have been studied in physics for a very long time, the particulars of vacuum kilns were sorted out in the 1980s and 1990s, less than half a century ago.

 

Wood-Mizer produced one and marketed it through the 1990s. However, they no longer produce it because it would be very difficult to pack in a large pile of wood and attempt to create a vacuum seal so that it could dry that way. Mr. Hankinson discovered, after looking through several techniques for drying wood that air drying takes approximately one year for every inch of thickness.

 

In the course of my research, I came across this book written by Joshua Salison, an ornamental turner in California. I bought the third edition of the book, but the fourth edition is also available on Amazon. I’ll now briefly outline the process before demonstrating how simple it is to construct the kiln.

 

 

 

 

How a Vacuum Kiln Operates

 

This is how it works, you can put wood in the vacuum and warm it up and you make sure the vacuum stays on for about three days. I automated something to keep the pump from running 24/7 and instead keeps it running for 20 minutes every hour. It works, it’s amazing. Out of those two pieces of wood that I showed you that I put in this thing I got three gallons of water.

 

Let me begin by explaining how quickly it operates. I have a sample wood for the demonstration last December 2016. I cut three pieces of 16 inches by 18 inches one I kept out, and two I put in the kiln on January 1st, 2016 at way over 55 moisture it was wet and frozen so I took it out of the kiln, three days later it is already at zero percent. 

 

I turned this heartwood platter out of that same stuff one of those pieces on January 6th, 2016. It should have cracked because this is the worst kind of wood to dry. But because we’ve used a vacuum kiln, the result is perfect and amazing! 

 

Why the Wood Cracks

 

Typically it’ll take me three days to dry wet wood. If you get wood that’s sealed with wax it’s wet, guaranteed. All of us know that wood is like a bundle of straws and the research I’ve read suggests that something like 95 of the drying comes out at the end of the straws, it doesn’t come out the side, it comes out the end and that’s why I think it cracks. 

 

It’s so much faster and so much gentler for the wood to do it in a vacuum kiln because it gets very minimal cracking and the wood doesn’t have to be much above body temperature. Heat is the enemy of wood and that’s where you get lots of cracking. If you leave wood out in the sun you’re going to see some cracking. 

 

 

 

 

What a Vacuum Kiln is Made Of

 

Mine is an 18-inch diameter steel pipe about ⅜ wall thickness on the far end. I used steel plates and welded them on one another, so the only place to look for leaks is at the end of every hole. That’s all you need. It works and it’s cheap. I even have a spare of this for 5 years. 

 

For the vacuum, I use a welsh vacuum pump that is used in labs, they’re piston pumps and they are bulletproof.  I got mine secondhand off of eBay. Josh Salison talks about it exactly what kind to use in his book. I have used a small one that I got for 80 bucks off of amazon and that’s a little oil diaphragm pump and it works. 

 

It’ll pull the right vacuum but there is so much water coming out of these things that the air coming out of the measure continues to pull. A vacuum kiln is just laden with moisture, because of the very very high humidity so you’re bound to get water in the vacuum pump. 

 

Tip: Do not use a $500 gas pump for your vacuum kiln, because it will destroy it. In my opinion, just get other cheaper ones.