SHADE CLOTH FOR SOLAR KILN?

[DON FRANK]

This is the first spring/summer i have used my solar kiln. Our daytime temps have not reached 90 yet, mostly 70's-80's and I have had days where my kiln temps are 120-140. Much hotter than I want. My kiln is made with aluminum skinned foam panels for walk in freezers so the R-value is higher than most of the solar kiln builds. 

Did some searching on green house sun shade material and I can get a shade tarp that will work for about $60. The sell them in 30-60% shade. I'm leaning towards the 30%. I will have to rig up a system to raise and lower it easily depending on the day. 

If anyone has had some experience with this avenue your comments would be appreciated.

[Old Greenhorn]

I have had good luck just using that road fabric they lay down, I think it's called 'geo-tech' or something like that. It lets the rain through, so you don't have puddling issues and it breathes well. If you have a source, small pieces can be had cheap or even cheaper.
 I use it for shading mushroom log stacks and getting the rain through is important, keeping direct sun off is also important.
 Good luck, let us know what you come up with. What are your overnight temps in there?

[DON FRANK]

The temps drop back down at night within 15 degrees of outside temperature but come back up with some sun. It is 60 degrees out right now at 8 am and the temp is 81. Since this thing is on wheels I guess i could orient it south east instead of due south and see how much difference it makes in after noon temps. 

[scsmith42]

I use the Geotek fabric for baffling inside my kiln, and then tarp off a portion of the collector if I'm drying thick slabs from green.  Shade cloth should work fine for blocking off the collector if you're trying to slow the drying rate down.

But for most 4/4 species and a lot of 8/4, you shouldn't need to worry too much about the temps because the kilns allow the wood to condition every evening.    The only time I block off my collector if if I'm drying thick oak.

[doc henderson]

Don, what are the dimensions of the kiln floor and glazing.  How much material say in board feet do you dry at a time.  I would get a silver tarp at HF and try it and cover 30% of the glaze instead of more money for a % shade cloth.  Hard to tell the size from the pic.  cannot see joints in the plastic.  

[doc henderson]

Is that a twin wall plastic?  looks maybe 8 x 12 or so.  re orientation is also good.  are you concerned about your insulated panel melting?  do you know the rating for high temp?

[DON FRANK]

My clear plastic is double wall greenhouse panels. It is 8x12. The floor size is about 6x13. 
The concern on the higher temperatures is whether I'll case Harden thicker slabs. Everything I will be putting in will be air dried for a year or more, especially oak. The interior is black and the tarp coming down to the top of the wood pile is black. 

   My other question is how long to run the fans that circulate the air. Should they be on a timer and kick off at 10pm and then kick back on mid morning. I didn't know if the cooling down period overnight should be without airflow thru the stack.

I appreciate all of you that have commented.

[doc henderson]

so, as it cools down at night, the relative humidity comes way back up, and equalized the core to shell gradient and eliminates the stress.  At 180 however, you may melt fan blades and or insulation.  I would monitor for those reasons.  the solar kiln is almost fool proof according to @GeneWengert-WoodDoc who participated in its development.  nice kiln.  1000 board feet of lumber needs about 100 sq. feet of glaze except for oak needs less, or a larger charge of lumber.  You can cover it with almost anything that blocks the sun.  the fans can run continuous, or during the day on a timer, or from solar panel getting more air flow as the sun is brighter, or on a thermostat.

[scsmith42]

My clear plastic is double wall greenhouse panels. It is 8x12. The floor size is about 6x13.
The concern on the higher temperatures is whether I'll case Harden thicker slabs. Everything I will be putting in will be air dried for a year or more, especially oak. The interior is black and the tarp coming down to the top of the wood pile is black.

  My other question is how long to run the fans that circulate the air. Should they be on a timer and kick off at 10pm and then kick back on mid morning. I didn't know if the cooling down period overnight should be without airflow thru the stack.

I appreciate all of you that have commented.

After a year of air drying, the oak should not be a case hardening candidate unless it is very thick.

You can use a slide hammer probe on a meter to check the core mc% to know for sure.

[jimF]

What are your folks definition of casehardening? From the posts it seems to vary and if we don't have the same definition we are talking across each other.

[doc henderson]

My understanding of the concept is that the at the outer wood gets dry and therefore stronger/rigid much faster than the inside core of the board.  Casehardening is a stress induced in the wood from this larger gradient.  The outer surface "case" is done and therefore hardened while the core is still trying to dry.  the significance is when machining the kiln dried wood, it has lots of movement.  As an example, it may pinch the blade on a table saw or the kerf will spread apart.  Early in a drying cycle in a traditional kiln, the RH is actually very high to control the water loss and follow a schedule.  This controls the gradient between inside and outside and reducing the case hardening.  In a solar kiln, you get heat and a drop in RH during the day, but everything equalized each night.  The temp drops, the RH goes up, and the MC of the inner and outer parts of the lumber approaches equality.  This releases the stress.  There is a prong test that can be done to test for casehardening.  There is lots written about this @GeneWengert-WoodDoc .

[scsmith42]

Excellent explanation by Doc above.  That fits with my definition as well.

It's all but impossible to case harden lumber once it's below 25% MC.  

Regarding the fans, Mine come on around 1 hour after sunrise and shut down around an hour before sunset.  So the run time varies throughout the year depending upon the number of daylight hours.

I also have an RH% sensor inside my 4 solar kilns that shuts the fans off when the RH% drops below 20%. 

[DON FRANK]

A little update. I rigged a way of stretching the shade cloth above the panels. Had a sunny day today with no clouds. The temp did not get above 113.  before doing this it would have been 135-140. 
   Thanks again for everyones input. I feel better about the hotter months ahead.

[Old Greenhorn]

Your comfort is our pleasure Don. Glad you found something that works for you.

[jimF]

As you can see this will be a long one.
If you look back at Gene;s writings he says that casehardening can only be measured when the core and surface is at the same moisture content. The prong test is used to test for casehardening. But if you look at all the drying manuals world wide, they don't say how thick the prongs should be, which strongly influences the resulting display. Nor do they say what the results should look like, they only imply that the prongs should be straight. But how much can they be bent and still be ok? This is because no one else has performed any testing to determine this. And, how the lumber is processed and the final use strongly determines acceptable results. Construction lumber is usually casehardened but is acceptable. If a board is evenly resurfaced on both sides a highly casehardened board will not be a problem. The more a board is not uniformly resurfaced on both sides the more casehardening causes problems


As for the definition or description: casehardening is when the core is in compression and the surface is in tension while both are at the same moisture content at the end of drying. As Doc mentioned, pinching of the table saw can occur, but these are longitudnal stresses where most talk is aabout transverse stresses, as well as cupping of a board. As a side note, moisture content determines the density, strength and stiffness a piece of wood displays. So when a board has a moisture gradient one location will be stronger than the other. This includes when a board is rewetted during poor storage. Doc, I'm not saying you inferred differently, just that someone may read it as if you did.


The high RH at night in a solar kiln causes a gain in moisture in the surface of the lumber may reduce the drying stress level or it may increase them depending on where you are at in the drying process. And this is only a temporary reduced stress level. With the loss of gained moisture the next day the stresses develop again and continues to become high with additional moisture loss. True drying stresses only develops or reduces when the shrinkage induced stress level is high and the moisture content changes. Also, depending on where you are at in the drying process existing surface checks can be made worse or develop into "honeycomb" which may actually be bottleneck checks. ( A side note to clarify thoughts: drying stresses are not stresses induced by shrinkage. Drying stresses are stresses resulting from shrinkage induced stresses converted to drying stresses through a specific molecular reaction.)
Similarly to this daily cycle in the solar kiln, in a commercial kiln the airflow is reversed every three to six hours; causing the entering side of the stack to become the exiting side. Since the exiting air has a higher RH the lumber gains moisture. This can be observed by continuously recording the width of the board, displaying swelling at these times.


While the term equalizing is often used for the daily cycles in a solar kiln: technically, equalization is an increase in RH towards the end of drying in an attempt to slow the loss of moisture from the surface with the expectation that the moisture in the core will continue to move to the surface. What this also does is to preconditions the lumber; making the "conditioning" stage more effective at reducing casehardening.


In a commercial kiln when the RH is kept high early in the process, it is to avoid surface check to develop, not to avoid casehardening. This one of the two periods in which a board is very prone to develop surface checks; the other is on the green chain just after it is cut.


Casehardening can start to develop at low moisture contents if the drying rate is increased. Such as in a solar kiln during a sudden warm spell or if solar or air dried lumber is "finished" in a more powerful kiln.


I'm not against solar drying. I'm only interested in people understanding the process they are using so that they can make wise choices and be satisfied with their results. The goal is to obtain lumber with a low uniform moisture content with low stress level so that the lumber does not distort over time or with further processing or machining. This can be accomplished with a numb er of different drying systems.

[scsmith42]

Jim, excellent write up above, thank you,

Would it be safe to say that causing case hardening due to poor RH% / temp management is more likely to occur during the initial or mid stages of drying, as compared to when the lumber is below 25% mc?

[jimF]

The thing to realize is that casehardening will occur. It is not a matter of initial, mid or final stage of drying. The questions are: how will the lumber be processed, what will the lumber be used as, can you control these factors, how will you reduce the casehardening during the end of drying? If you try to minimize the drying stresses, it will be more difficult to reduce these stresses. I know this goes contrary to what many attempt to achieve, the but data is there.

[doc henderson]

Jim, I am not sure what you mean by the next to the last line.

[jimF]

Doc, if you mean " If you try to minimize the drying stresses, it will be more difficult to reduce these stresses."?
The two things to remember is: 1) "The thing to realize is that casehardening will occur" and 2) "True drying stresses only develops or reduces when the shrinkage induced stress level is HIGH and the moisture content changes. ". So no matter how hard you try casehardening will develop. If the drying stresses are not high enough these drying stresses cannot be reduced. The only time I have recorded COMPLETE relief of drying stresses was when the drying was accelerated keeping the stresses high but below the fracture stress level. This was confirmed by performing both the four prong tests of different prong thicknesses and slice test.

[doc henderson]

Jim I am sorry, but I am not following you.

[jimF]

Doc, Where do you think I lost you? Statement 1 or 2?

[doc henderson]

Well, with all due respect I have read several times.  it seems contradictory, so I guess I need explanation with specific words used so the contrast is understood.  so, you say it (casehardening) will develop as a fact.  We are trying to mitigate that.  Fine it will but to varying degrees.  what factors or schedule can be used to make this less.  conditioning ect.  I have been informed that solar actually can releive that like conditioning each night.  Then you seem to say a range, " True drying stresses only develops or reduces when the shrinkage induced stress level is HIGH and the moisture content changes".
The first part seems to be two conditions, either develops or reduced, seems to be go up or go down, but under only one condition with shrinkage induced stress level high and with Moisture content change.  moisture content change could be going up or down.  If the drying stresses are not high enough these drying stresses cannot be reduced.
Are you saying to destroy the board, to relieve stress?  I doubt it.  I guess maybe when you speak of stress you are considering two types such as drying stress and casehardening as a separate stress, but in many sentences, that is not specified, and repeatedly referred to as stress, seem to contradict yourself. 

If you try to minimize the drying stresses, it will be more difficult to reduce these stresses.

You referred to the prog test somewhere as not reliable, but refer to it and a slit test later?

You seem to know what you are talking about.  I am a conceptual learner.  I am not able to follow to the point of an ah ha!  sorry for the criticism.  I am aware others may be having the same issue.  

[Machinebuilder]

Doc I think what he is saying is
if you minimze the stress, its difficult to further reduce the stress

IE its easier to take 10% off 90 than take 10% off 9

sorry if I confused the issue even more

[jimF]

Doc, don't be sorry about critiicism. I can take it and others likely are not following also.
Machinebuilder, your close, but it is easier to take of 89% from 90% than it is to take 1% off of 9%.
Give me some time to get some thoughts together on how to say what i want to say. I'm going to work on a project downstairs.

[jimF]

I will post a separate reply for each topic or point of question Doc listed. Hopefully this will help digesting and understanding my thoughts.


I don't think I implied that the prong test is unreliable, just that no manual says how thick the prongs should be nor how to interpret the results nor how to relate the results to how the lumber will be used. In a commercial drying operation they cannot predict how the lumber will be used, someone needs to come up with a method and interpretation for the most general situation. But again the manuals don't specify anything.


In my studies four prong tests and the slice test were performed for all sample boards. The thickness of the prongs were: 10%, 25% 30% and 50% of the board thickness. Ideally all four tests should display straight prongs. These four prong tests were used to correlate these results with the results of the slice test in an attempt to provide a suggested thickness and interpretation of the results for the prong test which kiln operators perform. The slice test is performed by using the same type of sample and the same orientation to the saw blade as the prong test. The difference between the two is where prong test is performed by cutting a "U" shaped profile and allowing the arm's of the "U" to move. For the slice test you cut 10 slices of equal thickness from the test sample. Instead of recording the amount of movement of the prongs or arms, you measure the length of each slice. For a board with no stresses all the slices will have the same length. For a sample from a casehardened board, the outter slices will spring to a longer length than the slices from the center or core. And similarly, a sample from a reverse casehardened or from a board at an early stage in drying will have the outter slices spring to a shorter length than the slices from the center. The one slice test is more descriptive and concise than cutting the four prong tests, but involves more work and measurements.
Suggestions for prong thickness and interpretation:
Prong tests will give the same results as the board being used if processed similarly the how the prong test is processed.

• Make sure the surface and core have the same moisture content
• if you know the lumber will be resawn into two pieces use the 50% thickness for the prongs
• if you know the lumber will be resawn into thirds use the 30% thickness for the prong
• for general use or unknown final product use both 10% and 25%
• Ideally for whatever thickness you end up using the prongs should stay straight. If they don't, you need to make a decision on how much you can get away with in the final processing and finish product.