Whether to Thaw Frozen Wood Before Kiln-Drying

Here's a long and intense theoretical argument about whether frozen wood can be safely dried without thawing it first. June 13, 2014

Is there any good rule in regards to thawing frozen wood before kiln drying it? My first impression is that I should heat up wood quite slow to avoid evaporation of superficial water leaving a block of ice inside wood.

Forum Responses
(Commercial Kiln Drying Forum)
From Gene Wengert, forum technical advisor:
With white woods you must be certain to use the correct RH no matter what the temperature. There is no information in any drying texts about thawing the wood. It is not done and has not been done.

From the original questioner:
Interesting - if the temperature is near-zero, is the psychometric or cellulose or other special cellulose good to measure RH? You gave me a very clear answer for white woods. What kind of troubles with not white woods?

From Gene Wengert, forum technical advisor:
We have been drying frozen wood in Wisconsin in dry kilns for about 100 years without any special thawing - the same for MN and MI and other northern states. White woods need to be dried rapidly to prevent color loss. Controlling RH for non-white woods is not so critical.

From contributor B:
Using the standard kiln schedules for oaks will be safe. The DB is low and if the air flow is correct there will be no danger of sap stain. As the lumber thaws the air will move the moisture. You may have some water on the floor, but if the snow was shoveled off of the pack before loading the standing water would be minimal. If the correct schedule is used, all instrumentation is correct, the correct air flow is achieved and the snow is off of the packs you have no worry of any degrade.

From contributor D:
Instrumentation won't be 100% correct. When fans blow through a frozen stack toward a wetbulb the wet bulb will look very low.

From contributor P:
The best and only way to dry frozen wood is to thaw it out. Heat it up very slow monitoring the RH% and trying to keep the RH% above 30%. Eventually the RH will start to climb and this is when you poor the heat to in the kiln.

From Gene Wengert, forum technical advisor:
I do not know of any company that thaws before kiln drying, other than using a standard kiln schedule setting. I have been around this business for 40 years. I have not read a book in English that has a thawing schedule. The idea mentioned above must be for softwoods, as we would not dry hardwoods at 30% RH. It is not clear what you mean by saying that you keep the RH above 30% RH and then when it rises, pour the heat, etc. If you have wet lumber, why wouldn't the RH go to 100%RH if the vents are closed and there is a little heat (or even a lot of heat)? The water from the wood will quickly achieve 100% RH, even with very little drying. Certainly this sort of technique would work, but why not go right to the desired temperature? That is what 99% of the people do with softwoods.

From the original questioner:
I have a theory - in Russia thawing wood before drying is very popular. This is usually done keeping wood at 20C (68F) for a time proportional to wood thickness (with windows closed). This should provide thawed wood, then its dried.

There are two (similar) reasons. One is that if you start drying frozen wood with ice inside you risk overdrying the surface while keeping ice inside. With fast drying schedules I suppose there is water going from the wood core toward the outside. If it is frozen water can't move. The other reason is that inside wood water is tough to follow regarding the colder zone (because of lower pressure). This makes it harder to extract. Regarding Contributor P's thought I understand the philosophy - heat up wood but avoid water evaporation. I think that's good, but when did you stop the thawing phase?

From contributor B:
Separating the white woods from oaks (check prone species), what's the degrade potential on oaks when drying frozen lumber? I don't believe there's any degrade potential except sap stain (highly unlikely) if the lumber is fresh from the saw. The white woods will be more prone to stain/discoloring(not bright) if the drying is to slow for a long period, combined with poor air flow and high RH's. When you think about it, the regular schedule will have a slight thawing effect since the desired settings will take longer due to the lumber being frozen. I have loaded kilns with hard maple halfway overnight and filled them the next day to ensure that I have achieved rapid set points. I have been commercially drying hardwoods in WI and the south (GA,NC) for many years and I have never used a thawing schedule.

I have never experienced any degrade without using a thawing schedule (I do track degrade in all of our lumber), but if you use a thawing schedule, the elements are there for degrade by stain or discoloring in white woods. To prevent stain/discoloration the moisture must be removed rapidly. When drying for profit every hour of kiln time is money and wasting time by thawing is sacrilegious. When deciding what schedule to use consider all of the pros and cons along with your goals, then decide the best schedule. The 1/2 billion board feet that I have dried for profit over the years is proof that Gene is 100% correct.

From contributor B:
After read everything again I see that there's a concern about frozen cores. Moisture is moisture, frozen or not, like the volume in a glass of water with ice will not change when the ice thaws. In drying lumber you use a weight method to monitor the moisture. If the weight doesn't change neither does the moisture and if the desired set points are achieved in the kiln you have no potential in overdrying the shell (outside wood), because the EMC (equilibrium moisture content - the result of the combination of the temperature and RH, which will not let the lumber dry any further than that point) will not dry the shell any more than the set point.

This will make certain that the core dries at the same rate as a normal schedule. Contributor D, the instrumentation will be correct using a wafer or sock. That's why you see a spike in the chart recorder when the fans reverse, the kilns are recording the actual and adjusting to the desired. This is due to the moisture build up on the exiting side of the lumber and this is why the fans should be reversed every two hours for uniform drying. What I mean by correct instrumentation is that what the actual environment in the kiln is should be what is being displayed. I have seen many operations that have not had all of their kilns calibrated to display what the actual is. This is important because a heat valve may be stuck open and the dry bulb could be off by many degrees or the charts are working and the operator doesn't know what to look for.

From Gene Wengert, forum technical advisor:
I did work for a company in Upper Michigan drying hard maple over 15 years ago. They had a lot of pinking. As soon as we got rid of the thawing schedule and went straight to the normal schedule (and achieved the desired condition almost right away), then staining was gone forever. As I stated, in the USA and Canadian drying books and practices, thawing is not used as it has no benefit. However, some kiln controls (technically based or made in Germany) do have that option. Yet, if the process is not defined, how can one use it? From the previous postings and discussion, it seems that thawing has not been researched but is rather a variable process with people developing and using their own procedure. One could easily conclude therefore that the procedure used for thawing is not critical or necessary if anything seems to work.

From the original questioner:
Gene, I don't remember any drying controller with thawing function. Can you give me a name?

From Gene Wengert, forum technical advisor:
It is hard to refute an answer when there is no problem. Lignomat has a thawing function (called heating up or something similar), as well as Hildebrand.

From the original questioner:

The problem or not depends on many factors: quality level, wood type, energy cost etc. Anyway, "If you start drying frozen wood with ice inside you risk to overdry the surface while keeping ice inside. Fast drying schedules suppose there is water going from the wood core toward the outside. If it is frozen, water can't move." Is it false? What's the reason?

From Gene Wengert, forum technical advisor:
Heat transfer is much faster than moisture transfer in wood. This is sometimes referred to as the Lewis Number. So, what this means is that you cannot dry the surface too quickly with the core still frozen. Note that the heat conductivity of wood is high when it is wet. So, heat will be going into the core very fast. This can be easily calculated using basic heat transfer formulas.

"Water will follow the colder zone..." What does this mean? This is meaningless unless you have a non-adiabatic source of coldness internally, which we do not. Someone needs to go back to basic heat and mass transfer in a porous capillary bed to learn what really is going on.

As further proof, in North America, we dry about 7 to 8 billion board feet of wood per year and have dried that amount for many years and thawing is not used. It is also important that one appreciates that water in wood that is called bound water and is rough 0% to 30% MC) never freezes, as it is chemically held water molecules and is therefore not a liquid, vapor or solid form like free water. Also, when free water is in the living tree, does it freeze? Rarely. This is because of the various chemicals in the water. So, when the tree is cut, it is rare to have the water in the lumber freeze, except on the surface. If this internal water did freeze, it would expand greatly and then cause all sorts of internal failures before drying even begins. Yet this does not happen. Again, some basic understanding of water in wood is needed by anyone who claims that there is a block of ice inside the wood. Of course, if the water does not freeze, then it does not need to melt and therefore will warm up rapidly. I suggest that anyone interested in this from a technical standpoint take a class in heat and mass transport phenomena, read Transport Phemomena by Byrd, et al. and also read Water In Wood by Skaar.

From contributor D:
I'm running a vacuum kiln so this isn't apples-to-apples but I've been drying holly. People who want the stuff want it snow white. I have packs outside covered with ice. To keep holly white it has to go from frozen to well-on-it's-way to being dry in a hurry. Otherwise, you get an ugly, blue green stain. So I go from frozen to 95'F just as fast as I can pump in the heat. It doesn't hurt the wood and it does dry white.

From the original questioner:
1. "You cannot dry the surface too quickly with the core still frozen." If EMC is low and thickness is large that can be. But I don't know how much it is dangerous for wood.

2. At about 271K (-2C) (pure water freezes at 0C) most free water inside wood is ice (some variations with species). Moreover an experiment in Romania monitoring wood core temperature shows that warming up from -10C to 25 C wood boards stay much time near 0C: looks pretty like a melting process. "Again, some basic understanding of water in wood is needed by anyone who claims that there is a block of ice inside the wood." Sorry, not a block of ice, many little ice blocks.

3. "If this internal water did freeze, it would expand greatly and then cause all sorts of internal failures before drying even begins. Inside the wood there is much air: volume of ice goes up without wood damages.

4. "Water will follow the colder zone..." What does this mean? This is meaningless unless you have a non-adiabatic source of coldness internally, which we do not. Someone needs to go back to basic heat and mass transfer in a porous capillary bed to learn what really is going on." Heating wood creates a temperature gradient from core to outside (ice or not). If you take a Mollier diagram you can easily see that at the same relative humidity lowering temperature you get a lower vapor pressure. That's why with HighFrequency you cut drying times: you heat up wood from inside and water is pushed out by pressure.

From Gene Wengert, forum technical advisor:
1. No you cannot because the heat will melt any ice.

2. The article you cite indicates that there is still liquid at - 15C. "Liquid water is apparently present in the stems of trees at even the most extreme winter temperatures based on our TDR measurements."

3. Indeed there is a lot of gas (probably not air) within wood. However, once ice is formed, it is a solid and then it expands in all directions. The "air" in the cells would not absorb the radial and tangential swelling of this sold; only the longitudinal direction would be affected. Consider a hollow tube with some water in it or even a glass of water that is 1/2 full. Freeze it at -1C, and then when frozen, cool it further to -10C and you will see that the glass breaks even though there is no lid on the glass. The ice can expand vertically without damage, but not transversely.

4. Indeed heating wood creates a temperature gradient, but this gradient in most cases is quite small (just a few degrees) with wet wood. Certainly above 100C or with a high mass flow process such as RF drying, you can get different results. But this discussion has been about drying at much lower temperatures (50C or so) and not 120C.

From Gene Wengert, forum technical advisor:
1. You might be able to save some time if you look in the literature about heating of veneer logs.

2. At -15C the article you quote reports that over 25% is still liquid (assuming that their interpretation of their data is accurate). In other words, it is an ice/water mixture that we call slush. But they do not say that as you go under -15C that it turns to solid ice. (Your initial posting used the term "block of ice.") Further, the water in the cell wall does not freeze as it is not a liquid, solid, or gas, but is chemically bound water. This means it does not behave like free water. This is based on a basic understand of adsorbed water and is not something I dreamed up.

3. I do indeed know that wood is not glass, but you have ignored the expansion in the transverse direction which will cause internal damage. I was merely using the glass as an analogy. I recall that Erickson at the Univ of MN froze wood at -40 and this then showed internal damage from expansion. The process is called pre-freezing. Perhaps some of the literature on pre-freezing has more information about internal structural damage.

4. If there is a large temperature gradient, with the high conductivity of wood, there will be a lot of heat flowing to the interior to melt any ice, so any gradient will quickly become very small and of little consequence. In conventional drying, the temperature gradient is not the controlling process of drying. Rather it is the mass flow or diffusion gradients that control the process. This is the point of calculating the Lewis number.

From Gene Wengert, forum technical advisor:

Absorbed water (bound water) in the cell wall is not able to freeze. It is not liquid or solid. It does not have a phase change when the wood goes under 0C. The water in the wall is chemically bound water (bound to the wood's hydroxyl sites) and does not have the hydrogen bonding between water molecules that makes a liquid or solid.

Please note that at -40C, the free water will freeze solid, but this is an extreme condition. On very cold days (-35 C or colder) in the woods, with a little wind, you can hear the trees snap as the ice breaks. The stems will then have a crack which is called frost damage.

From contributor P:
In comparison to dry air, humid air has the ability to carry much more energy due to the high specific heat of water. A good example to illustrate this point is the sauna. If you are in a sauna which has a temperature of 180F and a very low RH it would feel comfortable. However, if you threw some water on the rocks, the energy content in the air would rise quickly due to the moisture making it very uncomfortable, quickly. More energy in the form of heat is carried to the lumber in a humid environment. Heating a kiln up too fast at first can do a lot of damage to the outer cells on the surface of the lumber, causing twist and warpage. It should be noted that to benefit from the characteristics of high humidity, lumber must be heated up properly so as not to create any internal stress in the wood. Therefore the heatup must be accurate and dynamic. You could not achieve this with the conventional schedule type controller.

A dynamic controller (not a schedule) runs according to what the lumber is doing and not by time. A dynamic controller has may advantages some of which are faster drying times, energy savings, and better lumber grade. It is energy in the form of heat that dries lumber. With high humidity more energy can be supplied to the lumber in respect to time. Keeping in mind when humid air is raised in temperature the relative humidity goes down. To bring the relative humidity up you need more water vapor. The lumber adds water vapor to the air but only at the rate water is flowing from the center to the surface of the lumber. This rate is different for all wood species. Since water can only exit lumber from the surface, removing the surface moister from the lumber too quickly will case harden the lumber making it almost impossible to remove the water in the center. Transferring heat to lumber allows the water in the center of the lumber to flow to the surface and exit into the air. A dynamic controller will automatically adjust set points to keep the rate to a maximum limited to the species of lumber being dried. Back to the thawing out frozen lumber question a dynamic control has a thawout function. This thawout function is based on the humidity in the kiln and the outside temperature.

From Gene Wengert, forum technical advisor:
Your analogy to a sauna is not good (when trying to explain lumber drying) as what really happens is that at low humidity the body is able to cool itself, while at high humidity, cooling can no longer occur. The amount of cooling in a sauna is related to the wet-bulb or dew-point temperature. When the dew-point is higher than 98.6F (the body's temperature), then the body has trouble cooling itself. In fact, if the dew point is higher than the body's temperature, moisture from the air condenses on the body releasing the heat of vaporization (or heat of condensation). Heat transfer from air to wood (or air to a body) is not affected by humidity unless there is also moisture transfer. At low humidity, there is evaporation, so the wood cools.

It is true that humid air has more energy, but it does not have more heat. The specific heat of dry air is roughly 1.009 kJ / (kg C). The specific heat of moist air at 40 C is about 1.013. This is not much difference - less than 1%. Understand that most woods, except softwoods, are dried at about 40 C when very wet, which is when there would potentially be ice in the core - the subject of this posting. At 40C, the air at 100% RH would have about 46 grams of water per cubic meter. Dry air is at a density of 1.2 kg / cubic meter. So, although the specific heat of water is four times that of air, the amount of water in air is very small. So, I have trouble with your statement about humid air having much more energy (unless you go to very high temperatures). But even at high temperatures, having more energy does not mean more heat transfer unless there is also mass transfer.

You state that "More energy in the form of heat is carried to the wet lumber in a humid environment." How can this be unless the moisture in the air is condensing on the lumber, releasing the heat of vaporization (or the lumber's surface temperature is below the dew point of the air)?

You state "Heating a kiln up too fast at first can do a lot of damage to the outer cells on the surface of the lumber causing twist and warpage." Although excessive heat can cause damage, the rate of heating does not. That is, heating cold lumber at 0C to 40C quickly or slowly at the same humidity will not make any difference in wood quality. In fact, some kilns actually heat the lumber with saturated steam (over 100C) at first to relieve stresses and obtain flatter lumber. Further, some lumber is steamed before drying (the Elder process or steaming for color enhancement). You state "removing the surface moisture from the lumber too quickly will caseharden the lumber making it almost impossible to remove the water in the center." This slow drying of the core due to casehardening has not been shown to occur. First, casehardening is a condition of stress; actually the technical term for casehardening is tension set. It has nothing to do with moisture movement restrictions. In fact, tension set means that the wood cells are actually stretched out large than if they had been free to shrink. This stretching would seem to encourage moisture flow. Further, when drying Southern pine, for example, the temperature in the kiln will be in excess of 100C within an hour or less. In veneer drying, it is over 100 C within minutes in many cases. There may be casehardening, but the core has no problem drying. Appreciate that in conventional drying, the factor that limits the rate of drying is not heat transfer but moisture loss. In other words, the process is controlled by mass transfer and not heat transfer.

Your idea of using certain temperature and humidity settings to control the drying rate is indeed a good idea and is used quite often in conventional drying. This concept was initially patented by Dallas Dedrick in the 1970s and was called CRT (Constant Rate Drying). He used sophisticated controllers to achieve CRT. Experience with the CRT drying system resulted in some changes and improvement in drying, so it is not used as pure CRT today in many facilities.

From contributor P:
Your right, the analogy to a sauna is no good to a true scientific mind. You said it is true that humid air has more energy, but it does not have more heat if the air has more energy but not more heat then what is the energy? The way I understand energy in a humid environment is that the Specific Enthalpy of moist air is equal to the Sensible Heat (Specific Enthalpy of Dry Air) + the Latent Heat (Specific Enthalpy of Water Vapor).

Specific Enthalpy of Moist Air = Specific Enthalpy of Dry Air + Specific Enthalpy of Water Vapor

Specific Enthalpy of Water Vapor Hw = Cpw T + Hwe
Cpw = specific heat capacity of water vapor (kJ/kg.oC)
T = water vapor temperature (oC)
Hwe = evaporation heat of water at 0oC (kJ/kg)
Specific Enthalpy of Dry Air Ha=CpaT
Ha = CpaT
Cpa = specific heat capacity of air (kJ/kg.oC)
T = air temperature (oC)

You can see that the enthalpy of moist air would be much higher with the above formulas because the specific heat capacities are multiplied by the temperature. Yes my theory does work a lot better at high temperature drying.

You said appreciate that in conventional drying, the factor that limits the rate of drying is not heat transfer but moisture loss - does heat not increase the rate of moisture loss, or even yet the rate of melting solid water to a liquid?

The rate of heat up would cause case hardening when drying at high temperatures like 200(F) (softwood) probably not much at low temperatures 100(F) (hardwood). I do believe how ever that stripping the moist off the surface of lumber to fast will cause the cell fibers to bond very much limiting the water in the center to exit the surface. I am trying to keep on track of the how to dry frozen wood question but I believe that it is important to understand the energy that is used to both dry and thaw the wood.

From contributor P:
To elaborate on the case hardening of lumber a little:. Internal stresses are caused when the outside of a piece of green wood dries below the fiber saturation point and tries to shrink before the interior is ready for shrinkage. The surface MC is always trying to become (EMC) with the atmosphere. The center MC is always trying to become the surface (EMC). This is what I mean by moisture content gradient.

From contributor J:
It is true the vapor pressure of ice is lower than liquid water. Just because the vapor pressure of water is higher than ice does not means the water will enter the solid ice. Therefore the idea of inside water will follow colder zone is not true. The heat of the surface will be transported into the wood where it is colder by a temperature gradient much faster than cool water will evaporate from the surface of the wood . The energy used to heat the interior of the wood thereby robs the water that may tend to evaporate the heat necessary to evaporate.

While this supports Gene's statements I disagree with him on if there can be ice in a tree. The chemicals in sap can act as an antifreeze to just so low of temperature. Also, some of the bound water has more energy than ice therefore it will transfer to ice as the total thermal energy in the tree is decreased. There are hydroxyl sites on bound water to promote hydrogen bonding as in free water and ice. To adjust the volume of water in the tree, trees can allow transporvaporation to occur faster than uptake of water from the roots, thereby reducing the amount of water in the tree. It has been some time since I read Erickson's paper on pre-freezing wood by I don't remember reading about internal damage.

As for heating wood from the center out with high frequency is a misnomer. Most of the high frequency heating energy is absorbed by the outer layer of wet wood. This may be the surface or an inner layer depending on how far one is into the drying process. This type of heating does not "push the water out by pressure. This energy can bypass to some degree the drier outer wood and have the energy absorbed by the outer most layer of wet wood only.

As to casehardening making it almost impossible to remove the water in the center - casehardening has absolutely no effect on the rate water can be transported through wood. If the humidity is too high, water cannot evaporate no matter how easily it has the potential to transfer heat to the wood. If the wet-bulb temperature is the same as the wood surface temperature no heat will transfer. As Gene says no mass transfer - no heat transfer.

From Gene Wengert, forum technical advisor:
Tension set (casehardening) has nothing to do with how easily moisture can move in wood. Southern pine is heated to above 100 C within an hour. Veneer is heated to above 100 C within a minute or two. The rate of heating is not an issue with respect to moisture movement.

From contributor P:
Gene, you said Tension set (casehardening) has nothing to do with how easily moisture can move in wood. Southern pine is heated to above 100 C within an hour. Veneer is heated to above 100 C within a minute or two. The rate of heating is not an issue with respect to moisture movement." I have to disagree with that I will give you an example. A kiln drying southern pine is heated up to 240(F) in four hours and the drying time took a total of 44 hours. The same kiln with southern pine and the same dimension was heated up to 200(F) in eight hours and the total drying time was 28 hours not 44 hours. Can you explain that?

From Gene Wengert, forum technical advisor:
At 240 F, SYP two inch dimension will dry in about 18 to 20 hours. It dries that way all the time.