Another Kiln Question

[Richard Baker]

Not wishing to horn in on the other thread, will start another.

There seems to be a bit of a dichotomy between the need to slow down drying in order to reduce cracking and checking, and the use of a kiln which presumably is intended to speed up drying.

What am I missing? Does the use of a kiln not result in much cracking and checking?

 

[Ken Grunke]

I see merit in having a drying kiln for rough-turned bowl blanks and perhaps lidded boxes, but I use a microwave for that. With only a 12" lathe swing, I'm not turning bowls larger than 10" or so but if I had a larger lathe I may want a kiln for bowls that won't fit in the microwave.
I hope the person in the other thread doesn't intend to dry solid chunks (flitches) of wood in a kiln.

 

[john lucas]

For all practical purposes I have a kiln. I don't use it to dry wood I use it to keep my glues and chemicals at a decent temperature in my cold shop. It is a steel office cabinet that I lined with 1/2" foam that's silver on the inside. With a 60 watt bulb a light dimmer and a thermometer I can set it to maintain 60 degrees. It would be very easy to add a larger bulb and put vent holes in the top or possibly a fan to circulate the air. I built my own shelving to go in it and routed slots in the shelves to let the heat rise.

 

[Steve Worcester]

I hope the person in the other thread doesn't intend to dry solid chunks (flitches) of wood in a kiln.

I hope so as well, but my philosophy does not go towards production work. If I was doing production, I would rough for 6 months or so and after some of the first batch has dried suffuciently, finish turn and then do a mix of rough and finish turn to keep both supply lines filled. And maybe setup a kiln to quicken the process. I can see merit in it, but it still needs to be done correctly - slowly, not forced.

Personally, commercially kiln dried blocks tend to warp more because the outside is dry before the center and leads to alot of tension around the wood from being forced dry. the process needs to be more calculated that stickered and stacked lumber.
I personally feel air dryed wood turns better than commercially kiln dried. Seem to cut easier without the burnishing of the gouge. But that is a debate that will rage on throughout history.

Like exotics I buy , they get wrapped and dated, then used a few years down the road when they have evened out in tension and equilibrium moisture content.

 

[Bill Boehme]

Not wishing to horn in on the other thread, will start another.

There seems to be a bit of a dichotomy between the need to slow down drying in order to reduce cracking and checking, and the use of a kiln which presumably is intended to speed up drying.

What am I missing? Does the use of a kiln not result in much cracking and checking?

You might want to check with Trent Bosch (ya shoulda' been at SWAT) to see if he would share his plans with you. When he taught a week long series of classes at our club about three years ago, he gave us details on his set-up. I do not think that anybody in our club was really interested because the thing is rather large and elaborate and would only make sense if you were a production turner.

From what I recall, his contraption was 8 feet long by 4 feet wide by 4 feet high and held a couple hundred roughed out bowls at a time. It used a number of light bulbs for heat, several fans to keep air circulating, a thermostat to stabilize the temperature, and a dehumidifier that controlled the humidity level and ran on a schedule that gradually reduced the humidity over a period of several months. My memory of the details is a bit sketchy since turning that many bowls was not on my radar screen -- and still isn't.

 

[MichaelMouse]

There seems to be a bit of a dichotomy between the need to slow down drying in order to reduce cracking and checking, and the use of a kiln which presumably is intended to speed up drying.

What am I missing? Does the use of a kiln not result in much cracking and checking?

Drying is about letting the moisture out into the less moist air. Kiln drying is about helping it out. The process has a huge "fudge factor" built in, which is why simply increasing the temperature of the controlled airspace to allow it to accumulate more moisture seems effective. This is normally balanced by the restriction in circulation. Some put them in frost-free refrigerators where circulation is great, but the air is saturated almost all the time.

How much airspace do you want to control, and why, is what you should ask. I throw the roughs out into the garage in the fall so they won't be subject to too-rapid drying. In the summer I keep them on the floor in the basement for a couple weeks. Enough fudge there to do all I want. They don't crack because they're shaped and shrug drying stress they develop due to the moisture gradient. If I put them directly into my 30% RH heated space, they'd probably crack. Example of what happens when you leave an evening rough on the saw without realizing it's under a heat duct down below.

So if you want to discover how fast you can dry without degrade, prepare to pay attention throughout. If you just want things to survive, 'nother matter. Kiln schedules for all manner of woods have been developed over the years and are published in the FPL site. You should be able to move even faster than the schedules, because they're for boards, where the moisture has to take a devious path to escape rather than run through the vessels.

The shape you give your rough determines how much of the distortion inherent in the species affects you. A block will do the full 6/11% shrink, but it will only cost you 3-4% in diameter on a tapered roughed bowl.

 

[Bill Travelstead]

This site has a lot of stuff on building kilns and other drying methods:


http://www.woodturningonline.com/Turning/Turning_articles.html#drying

 

[Richard Baker]

(ya shoulda' been at SWAT)

Yeah Bill, I shoulda been there but couldn't quite justify (rationalize) the expense at the time. Maybe next year.

Sounds like a kiln of whatever size would be useful only if one had a reliable means of adjusting and controlling both the temperature and RH. The temp. would be fairly easy, but not so sure about humidity measurement and control.

Thanks guys.

 

[Malcolm Tibbetts]

In case anyone is interested...
As a segmenter, I don't use a kiln for drying bowl blanks, but keeping my stock of boards as dry as possible is very important to me. I live in location (Lake Tahoe), where the seasonal humidity changes are extreme. Summertime humidity can stay well below 20% for days or weeks; but the wintertime snows create a very different situation. Faced with this situation, years ago, I built what I call a drying shed (not really a kiln). It's a heavily insulated space, attached to an outside wall of my shop, about 9' long by 6' tall by 4' wide. It's got a lot of adjustable horizontal pipe supports for the purpose of holding lumber. Most of my stuff was kiln-dried at some point in time so most of my challenge is to keep the stuff dry. Although occasionally, I also store air-dried wood in this space. It has several large doors for easy access and they form a pretty tight seal when closed. At floor level at one end, I have a very tiny electric heater with a tiny fan. At the other end, at roof height, there is a about a 3" diameter opening. The incoming air is provided through a few small holes into the shop space, so that the cold outside wintertime air is not sucked in. The temperature stays around 90 degrees F. I hate the energy consumption of somewhere around 300 watts, but it does do a good job of keeping my stuff dry. Also, the entire outside is paint matte black, which helps with some solar heating. With all this effort, keeping wood at a kiln-dried status of 6% is not possible, but I can keep everything well below 10% (usually around 8%) and that seems to work for me. The combination of temperature and the constantly circulating air seems to do the trick. When I first built the thing, I also operated a small de-humidifier. It didn’t seem to make much difference. Another problem – another solution.

 

[MichaelMouse]

Sounds like a kiln of whatever size would be useful only if one had a reliable means of adjusting and controlling both the temperature and RH. The temp. would be fairly easy, but not so sure about humidity measurement and control.

The only thing that counts in defect-free drying is RH, and the same device that controls your dehumidifier is available. Heat just lowers the RH compared to the outside air, and counts for rapidity. That's done on a schedule.

You do realize that you'd have to batch the loads anyway, or the last wet one added would set back the first dryer ones?

 

[Richard Baker]

Seems like a lot of hassle for relatively minimal return. Also seems like Malcolm's application is the most useful, making sure your dry stuff stays dry.

Guess I must continue to rely on my whole-shop dehumidifier (aka air conditioner/heater wall unit), and lengthy storage time of stickered boards on shelves before using. Thanks again for your replies.