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Kiln question

Paul M. Kaplowitz

Paul M. Kaplowitz

Joined
Oct 6, 2008
Messages
395
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Location
North Charleston, SC
I'm thinking about building a kiln like the one Cindy Drozda did. (out of an old freezer ). My question is - since I live in South Carolina and the temperature is in the 90's all summer with a very high humidity, will this type of kiln work. It would be drawing in very humid air and the outside temperature is about the suggested kiln temperature. Would my rough turned bowls dry any faster than having them on a rack in my garage which reaches outside conditions.
 
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OK, lets call a spade a spade. Without monitor and control you don't have a kiln, you have a warm microclimate. If you had a kiln you could actually dry your stuff below the MC it will assume after it comes out. This is not really good.

Warmth provides a bit better carrying capacity for the air which is warmed. Makes warm air with the same absolute humidity relatively lower. Kiln operators use this phenomenon, but they warm up far beyond your guestimate bulb heater, stir the air with fans to get even distribution, monitor relative humidity and exhaust the humid air in measured amount to gradually lower MC.

If you haven't already,get over to the USDA FPL site and read about what they do in real kilns. Or, as I do, accept a bit longer dry time in exchange for a lot less work. Get a hygrometer, find the typical RH in a few available locations and store your stuff where it will come down slowly. Might take an extra month - out to ~4 at nearly an inch - over what you could do with a real kiln, or even the warm box, but you can just turn a few more blanks at the start, so that you'll always have something ready when the family gives you time to turn.
 
Paul:

You ask some good questions, and my response here is based on theoretical conclusions on how kilns work, not from personal experience in such a climate, so take this information as such.

Any time you increase the temperature of air without adding additional water vapor to it you will drop the relative humidity (...the air at that temperature is "drier"). Such a difference between the relatively high water content of the wet wood (specifically, the water evaporating from the surface of the wood), and the lower humidity air surrounding the wood, will improve evaporation and hence will increase the rate of wood drying - the function of a kiln. Convective air flow (or even forced air flow with fans) around the wood in the kiln will shorten the drying time - although as you note, feeding the kiln with very moist air (high humidity) will reduce the efficiency of drying (due to a smaller differential in air moisture levels). Since you do not want too high a rate of drying, this may be a good thing! Early in the drying cycle, high humidity around the wood is desirable, to be followed by a SLOW drop in humidity and gradual release of water from the wood over a period of time. Regulating the release of moisture-laden air is an important aspect of operating a kiln correctly.

Further, I doubt that your local temperatures remain at 90 degrees 'round the clock ("24/7"), and that dropping temperatures at night along with fluctuating humidity levels are the norm. Since the function of a kiln is also to control the rate of drying while maintaining controlled temperature and humidity conditions over an extended time, you will gain more control over the drying process in a kiln, than wood sitting on a rack in your garage "air-drying".

In short (?), I think that the wood will likely dry faster in a kiln than sitting in your garage without one. You will need to experiment with various temperature and time schedules to learn how your kiln works with different wood species, or blanks of different sizes, at different times of the year.

My "second generation" kiln is almost finished, and I'll be doing some wood drying experiments with it over the coming months.

Rob Wallace
 
Paul, I live on the coast of VA...hot and humid in the summer. Many years ago I built a bowl kiln from an old commercial freezer, a light bulb, a computer fan and a thermostat. It worked just fine. It was not too long before I have so many green turned bowls that were air dried that I didn't feel the need to bother with the kiln any more.
 
Paul, sorry I missed this post. It is wet,wet, wet here in Hawaii where I live. These low tech lightbulb kilns do a great job. The airflow is slow and convective and 24/7. At 90% humidity your bowl blanks will take a long time and never get rid of bound cellular moisture. But in the kiln it takes care of that. A test is two blanks. Oh, three. One air dried. One kiln dried and on the shelf and one popped back in the kiln a couple days before finishing. The winner will be the one you either took out of the kiln and never made the shelf or the one you popped back in. The air dried will have moved a bunch during tool work alone and depending on the wood be downright oval. The sanding will then will really give you grief as it really dries the thing out. Oh, and slightly S shaped.
 
A different way of doing that has worked well for me.

My opinion is that an old dishwasher makes a better dryer than a refrigerator. The ones I have used may not be perfect, but I have dried a lot of wood with one of them over tha past of 20 years.

A dishwasher makes a better box for a dryer than either a refrigerator or an upright freezer (my opinion) because it has venting already designed into it, there are no holes to cut, there is no need for a fan, there is no messy insulation to contend with, it fits under a workbench or counter, and it is easier to carry home.

To recycle an old dishwasher, remove the motor/pump, center nozzle and silverware basket. Keep the racks. Place a drop light with a 100-watt light bulb in the bottom of the box, running the cord through the hole where the pump was. If the motor/pump is left in place, remove the plug and pass the cord through a 1/4" hole drilled in the side of the box, and then attach a new plug. Also, drill a 1/8" hole through the door for a kitchen thermometer, the kind with a dial and a probe. The new dryer is complete.

I have used these dryers for over 20 years in the humidity and heat of North Florida, and the dry cold winters of North Idaho. I can keep the temperature inside the box at about 95 to 100 degrees with a 100-watt light bulb, and lower the wattage to 75 or 60 if it exceeds 100F in the summer. I have even done that with a 150-watt bulb when there was no heat in the shop during the winter.

A bowl that is finish-turned to 3/8" thickness will stabilize to around 15 percent moisture content in about 3 days, depending on the species, and the original moisture content. Thicker wood or a full 'dryer' will require a longer time.

Cracking and distortion is less of a problem than with air-drying. If I am concerned and want a slower drying rate, then I either fill the bowl with wet shavings, wrap the piece in a grocery bag, place a pan of water in the bottom of the box, or do all three. The objective is to raise the humidity inside the box and slow the evaporation rate from the wood surface.

A slightly elevated temperature and low humidity in the box also makes it an excellent drying chamber for varnish and oil finishes.
 
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Paul

I might add a few comments or say what others have in different wording. The air passing over the wood is what removes the moisture. At say 90 degrees and 80 to 90 % relative humidity that air cannot remove a lot of moisture but can do so, it just takes more time. There are figures in tables but I do not have the tables any more. The water in the air is expressed in weight by grains. 7000 grains to a pound. If that air is heated the grains of water remains the same but the ability to hold more water increases (the relative humidity goes down,lowers, the air expands.) Another method for increasing the rate of water removal is to increase the flow of air over the wood. You can do both or either increase the air temperature or the air flow. Without instrumentation the trial and error method will suffice but will take some time. Start with the light bulb for heat and a small fan for circulation. If drying results in no cracking increase the heat by upping the light bulb wattage, then if that works increase the fan speed or volume. You will reach a point where the removal of the water from the wood exceeds the ability of the wood to supply water to the surface, a steep moisture gradient in the wood will occur and subsequent cracking.

My intention is not to confuse the issue and I hope I have not done so.
 
Light Bulbs As Heaters In Kilns

Some interesting points have been made about the design and functioning of home made kilns for drying rough turned blanks which use light bulbs as the source of heat. I agree that you can, through trial and error, determine the wattage of the bulb(s) you use to set the temperature you want inside your kiln, e.g. the higher the wattage the higher the temperature. There is one major draw back to this method. Most consumer grade light bulbs have an average life rating of about 1,500 hours. In kiln operation they are on 100% of the time which gives them an average life of 62 days (1,500 hours/24 hours/day = 62.5 days) or about 2 months. A so called "long life" bulb may double this to 4 months, still a relatively short period of time in the scheme of things. Several years ago I built a kiln using a light bulb to heat it and was constantly annoyed at having to change burnt out bulbs every couple of months, assuming I noticed that the bulb was burnt out. I found a simple solution to this problem by using 2 light bulbs of the same wattage bulb running them at reduced power from a standard light dimmer switch available at hardware and home improvement centers for about $5. This has two major advantages. The first is that I can fine tune the temperature inside the kiln by simply adjusting the dimmer. The second is that as one lowers the power to the bulbs, their life increases almost exponentially. Examples (from a "bulb calculator" I have), if the power to the bulb is 90% of its rating the life is increased 3.6 fold, 80% = 14.5 fold, 70% = 72 fold, 60% = 459 fold, etc. So if you run the bulbs at say 60% power the life expectancy of the bulb is approximately 1,500 hours x 459 = 688,500 hours or 79 years! So much for the math, in practice I have been running my kiln with two 60W bulbs with the dimmer set to just above half power for over ten years now without having to change the light bulbs. At the calculated rate the bulbs should burn out when I'm about 146 years old. My daughters told me not to bother putting my kiln into my will, they are not into turning and don't want it...kids.
 
If you'd like an economical and predictable heat source, try something like these. http://www.yardlover.com/heat-mats-for-early-seed-starting.

Of course, as they say, it's not the heat, it's the humidity. Trial and accidental success is possible given the forgiving nature of the drying process, but without something like this. http://www.iaqsource.com/category.php/dehumidistats/?category=1525 you don't have a kiln to push drying, you have a warm room. Since I've never been able to finish pieces as fast as I can rough them, I don't bother.
 
Josh Salesin's Kiln Info

Josh is an ornamental turner who requires dry stabile wood for his pieces.
If you go to the url below - and scroll down looking for "10-2006.pdf",
he describes his kiln - with photos to accompany the description. Might find the info useful

http://svwoodturners.org/SVWnewsLetters.html

He's in Santa Cruz County - southwest of San Francisco - right up against the Pacific Ocean, separated from the rest of the Bay Area by the Santa Cruz Mountains. Humidity might be a little lower than say Florida, the temperature similar.

This thing works for him - or at least did as of Oct 2006.
 
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