SHADE CLOTH FOR SOLAR KILN?

jimF · May 28, 2024, 09:35:24 PM

Let me step on some toes now and explain my drying philosophy. We have many types of drying systems from: airdrying to solar, dehumidification, vacumm, in the attic drying, to radio frequency drying to commercial elevated temperature or steam heat supply and finally to high temperature drying ( which is usually also steam heat supply and is normally used for construction lumber). They all have benefits and disadvantages. My suggestion on which systems to use is simply, use the one with the most control of the drying forces that is within you budget. I have air dried, attic dried after air drying, planned on building a solar kiln but have always been in a position of renting, used elevated temperature kilns, consulted for dehumidification systems and have been impressed with a innovative low cost vacumm system. What ever system you used all the physical processes that occur are the same , with maybe the exception being vacumm drying. My emphasis is know the physical processes occurring in your system and the benefits and limitations of your system. I think most people using or thinking about solar drying wants a system that is faster than airdrying, has budget limitations and like the idea of a gentle drying to avoid drying stresses and also maybe are environmentally minded.
As you might have gathered by now, I do not support the concept of the ideas of gentle drying and nightly conditioning in the solar drying system. It's not that I am against solar drying, just the misconceptions in solar drying and my explanation of why will likely be explained in the following posts. But not tonight.

doc henderson · May 29, 2024, 07:44:42 AM

Jim, I am having trouble following your descriptions. you have to reread your post and see if someone else can read it and follow along. I get the general concept but in trying to understand the details, I cannot follow the words. You need to specify what stress you are talking about. It reads like you are simply making a statement and then contradicting your statement. I do not doubt your knowledge but cannot follow the way it is written. If I was not trying to understand, I would not care and would not comment. i have seen diagrams of the prong test. It is described in the drying hardwood lumber manual I believe. You say it is in no manual and you have very specific recommendations. making a prong a percent of the thickness of the board, and making a prong that wide? Again, not following exactly what you mean. Is this your research? Did you just make it up? not following the slice test.

jimF · May 29, 2024, 04:56:00 PM

Doc, I just wrote this before opening thread to observe you posted. I'll post what I wrote and read you post.

I was a little ambiguous in the description of performing the two tests. First cut off 6 to 8 inches from the end of the sample board to avoid end grain drying. Then cutting off about an inch along the grain for each sample. This makes the width of the sample the same direction as the length of the board and the length of the sample was the width of the board. Roll the sample 90 degrees onto the band saw so that the end grain is facing up. Then cut the slices or prongs starting at the end of the sample. I hope this makes it clearer.
The easy part of describing the two methods to test for stresses and insulting and stepping on toes have been covered. Now lets get to the technical explanations. To be specific and avoid confusions this will require picayune technical descriptions involving chemistry and physics. So be warned to wear your diving googles as we dive into the explanations. First definitions and chemical description of the material involved.
Strain is a change in dimension whether it is caused by change in temperature of a piece of metal, water turning into ice or a hydrophilic organic material like wood gaining or lossing water.
Stress is a force exerted over an area resulting from an external source like gravity or a pair of pliers or an internal nonuniform strain like two different metals ( a bimetallic strip) heating up or nonuniform gain or loss of water in wood.
Elastic strains are when the material returns back to the origin size and shape when the cause for the strain is removed, like letting go of a rubberband. Shrinkage and swelling strains are elastic because the wood returns to the original size when the moisture returns to the original level.
Polar molecules have locations that are electrically charged (+ or -) because the electrons are not uniformly distributed in the molecules relative to the protons in the nuclei of the atoms. Two examples are the materials we are discussing here: water and the components of wood, primarily cellulose. When two polar molecules are in vicinity of each other they tend to aline so that opposite charges are close to each other, opposites attract.
While the exact positioning of water in wood is in contention, for our purposes : the water is between two stands of cellulose connected like a chain, the positive side of water close to a negative site on one cellulose molecule and the negative side of water to a positive site on the other cellulose molecule. With higher moisture content the more water molecules in-between the cellulose molecules forming a longer chain. As water is gained the longer chains cause swelling to occur and the reverse as water is lost forming shorter chains shrinkage occurs. As you can imagine the simple popping in or out of a water molecule is an elastic reaction.
Now consider many cellulose molecules. Some are loossing water faster than others, shrinkage more than others. With different strains, a force is generated, pulling one cellulose molecule so that the water chain is being stretched so that another charged site become closer that the originally involve charged site. It is under tension, a stress. The water chain switches sites and some of the shrinkage strain becomes another type of strain called mechanosortive strain. This switching of charged sites occurs in other locations. This is initially occurring in the surface and this material, in this surface becomes longer. This mechanosorptive strain occurred when there was a moisture content change as there was a force exerted great enough to cause a water chain change the charged sites involved. This not an elastic reaction because if the moisture content changes back to the initial level, the initially involved charged site is not close since there is not a force pushing the cellulose back to the original position. If during the change in moisture content occurs when an equal or greater force is applied, like gaining moisture during conditioning, then the cellulose molecule might revert back to the original position and the material returns to the original length.
Now take a step back and instead of looking at molecules look at the layers in a boards. This mechansorptive strain is occurring in the surface, increasing in length but still under tension because all the shrinkage strain has not been converted to mechanosorptive strain. It does not provide the need stress level for charged sites to change. With the surface in tension and the core in compression the prong test would show reverse casehardened lumber.
This remains the case as drying continues until the center starts to loss moisture and shrinks. The tension stress in the surface reduces and with core continuing to loss moisture the stresses are eliminated. Since the surface was stretched mechanosoptively, when the stresses are eliminated the core still has a higher moisture content than the surface. As the core still continues to dry, the core shrinks smaller than the surface causing the surface to become under compression and the core in tension. And now if the prong test was to be performed it will display casehardening. At this point we passed what is called stress reversal and many drying schedules increasing the drying rate. The thought is that with stress reversal, the surface is not under any risk of developing new surface checks.
We are now at an average moisture content much higher than the desired moisture content of about 8%, so the drying is continued. The casehardening level increases until we get to the moisture content level where equalization is performed. The RH is now increased, the thought being to slow the moisture loss in the surface while the moisture in the core continues to move out to the surface. In an effort to reduce the moisture gradient through the thickness of the board and to bring the slower drying boards in the kiln to the level of the faster drying boards. What actually occurs is the high RH causes the surface to gain moisture from the air and from the core. Bells should be ringing. We have a relatively large increase in moisture content in the surface causing a high compression stress during a moisture change! The mechanosorptive strains are reversed, the size of the material is reduced, pre-conditioning occurs casehardening decreases.
The gain in surface moisture only occurs initially during equalization. Drying than continues and casehardening again increases. Until the moisture content is slightly below the desired level. Conditioning is started and drastic increase in RH is produced. The surface gains moisture, causing a very high reversed stress and more mechanosorptive strain is removed. And now there is no, or almost no stresses present – the desired out come.
You have been attending a graduate course in drying! Typically in undergraduate courses the students are told surface material losses moisture and shrinks causing stresses to build up, then the core starts to shrink and the stresses are versed and develops into casehardening. Conditioning relieves the stresses. Now follow the published schedules

jimF · May 29, 2024, 05:07:46 PM

The prong test is mentioned in manuals, but they don't go into prong thickness nor what an acceptable prong response would be.
When I say the prong thickness is 50% of the board thickness I mean to make one cut through the center of the sample. For a 25% board thickness for each prong on a 1 inch board, each prong is a ¼ of an inch thick.
The slice test I did not make up. Many other folks have used it before me. To create one start slicing the sample as you would a 10% prong but go all the way through and repeat this until you get 10 slices.

Doc, does this and the begining of my last post help?
As for a better description of stresses try the post just before this one, tell me if you are still having trouble. Don't worry about complaining, feedback is good.

doc henderson · May 29, 2024, 06:02:50 PM

Jim, I know how to slice a board up, but when I am done, the slices are all the same length. on the prong test do you have a pic? for the prong test are you interchanging the thickness and the width? On the slice test, do you let them lay and measure tip to tip to let them curl? In the prev. I think I figured it out, but you are cutting across the grain again not along the grain it the way I think and speak. in the prev. you say a 1-inch board, is that width or thickness. I assume thickness, and if the prongs are 1/4 inch thick, I assume you really mean wide. this is the issue, not the actual concept. In the prev. post (15:56) I can follow you, but I still think the solar cycling will relieve much of the case hardening on a daily basis. Thanks for the effort.

jimF · May 29, 2024, 09:47:18 PM

For measuring the slices: lay them flat as you measure.
Maybe the problem in part is that I'm using the longest dimension as the length and the narrower dimension as the width, whether it is the test sample or the board. So the width of the board correlates to the length of the sample.
When you cut the sample from the board you are cutting across the grain. When you are cutting prongs or slices the blade is cutting along the grain and through what would be the thickness of the board.
Yes a, 1 inch thick board or 4/4 board.
In your 7:44 post you mention " ... you have very specific recommendations. making a prong a percent of the thickness of the board, and making a prong that wide? " Are you saying it is not a good idea to make specific suggestions? Without using a specific process that you repeat from one run to another run you cannot make any informed decisions.
Once I assisted in an undergraduate practicum, part of which included drying lumber and the students were to cut prong tests. When the groups were dispersed, one group came back and asked how thick the prongs should be. The instructor said "how thick do you think they should be". As second group came back and asked what should the results look like. Again the instructor said "what do you think the results should be". When the groups came back together to report their findings both questions were never answered! It is one thing to try to get the student to think, but follow through and see that the questions are answered. I am convinced he gave no answer because he did not have an answer; this was a professor in a well known Wood science department! Again no manual answers these question either. How is the kiln operator to do their jobwithout some guidance?

doc henderson · May 30, 2024, 05:25:23 AM

Yes, I think we are having language issues. It is good to make suggestions "and" offer where the information came from. so, the slices are only 1 inch long in your slice test, and you can find a small difference that indicates casehardening? when you cut the prongs in your prong test you go from width to thickness. so do the prongs remain the full thickness of the one-inch board, and are 1/4 inch wide, or do you cut the prong in two dimensions, and it becomes a 1/4 x 1/4-inch square prong on the end of a one-inch board? these are the things I cannot discern from your description.

doc henderson · May 30, 2024, 05:31:13 AM

Are you saying it is not a good idea to make specific suggestions? Without using a specific process that you repeat from one run to another run you cannot make any informed decisions. you asked.
No, I am trying to understand what you are trying to say.

jimF · May 30, 2024, 06:31:29 AM

if you start with a 4/4 radially cut board that is 6 inches wide. The samples are 6 inch long 1 inch wide.
For a 10% prong thickness the prongs are 1/10 of inch thick, about 5 1/2 inch long and 1 inch high. The slice test will be 1/10 of inch thick, 6 inches long and 1 inch high. For both the prongs and slices will be made by cutting across the growth rings.

doc henderson · May 30, 2024, 10:37:29 AM

way past being at risk of beating a dead horse. I thought on the slice test you cut across the grain to make the sample, but with the grain to make the slices. On the prong test I assume it is cut along the grain as well and the prongs are 5.5 inches long, but that would have come from a sample that is 1 x 6 x 6 inches. the sample in the slice test, the 6 inches of sample length is the original width of the board. Correct? I will PM you.

jimF · May 30, 2024, 09:11:51 PM

Then cutting off about an inch along the grain for each sample. This makes the width of the sample the same direction as the length of the board(height after rolling it 90 degrees) and the length of the sample was the width of the board. Roll the sample 90 degrees onto the band saw so that the end grain is facing up. Then cut the slices or prongs starting at the end of the sample.
if you start with a 4/4 radially cut board that is 6 inches wide. The samples are 6 inch long (radial direction) 1 inch wide (tangential direction). And 1 inch high (longitudinal direction).
For a 10% prong thickness the prongs are 1/10 of inch thick (tangential direction), about 5 1/2 inch long (radial direction) and 1 inch high (longitudial direction). The slice test will be 1/10 of inch thick (tangential direction), 6 inches long ( radial direction) and 1 inch high (longitudinal direction). For both the prongs and slices will be made by cutting across the growth rings, with the blade parallel to the longitudinal direction.

doc henderson · May 31, 2024, 10:32:34 AM

hard to explain and hard to read, but I was able to follow, but, if the pic in my brain is following, then the blade perpendicular to the longitude of grain. longitudinal direction of the grain and therefor the original board, or of the length of the sample?
can you provide pictures in a series that show the tests. Is the length difference in the slice test measured with a tape or micrometer.

jimF · May 31, 2024, 03:57:52 PM

The first figure shows both transverse (first small one) and longitudinal (two long ones) prongs test. We have only been talking about transverse, which is the enlarged sample. the second small sample in this figure is a core/surface MC test.

The second figure displays the first two slices if only four slices are to be cut.

A side note if you are comfortable with a bandsaw. The first figure displays a prong test finished. Notice that the top half of alternate sides on both prongs were removed. This is to allow the prongs of a very casehardened prong test to move their full potential. To achieve this:With the waste portion still in the center of the sample, rotate the sample back onto the side which was the face of the board. Cut about half way down the center. Then spin it onto it's corner going longitudinally, cut to the center or to the previous cut. You may have to use a screwdriver to snap the piece hanging on. Repeat on the second prong.