Kiln schedules based on drying gradient


What are your responsibilities as a kiln operator and how can you implement them?

Operating one or more kilns is a big responsibility and should never be taken lightly. A kiln operator controls a process which can seriously effect the economy of the company. The cost of inattention to the daily operation can easily amount to more loss in value than an operator's own yearly pay. This job and its responsibilities require a person with a high level of self discipline and a flexible work schedule (extra overtime in order to attend to the kilns).

It is wise for any kiln operation to have a trained assistant available to relieve the head kiln operator from time to time; in addition, the lumber company will benefit from having trained and knowledgeable personnel as backup should it loose the main kiln operator.

Normally the kiln operator's responsibility is expanded to the extended drying process consequently, the kiln operator is responsible for quality control throughout a lumber yard. He is the one that the company will depend on to solve problems not related to the sawmill and secondary manufacturing, such as moulding and planning. The kiln operator’s job is very important and his responsibility is not limited to the four walls of a kiln chamber and it’s controls. The kiln operator must/should follow the lumber from the day it enters the lumber yard/sawmill to the day it ships, in order to spot and correct problems that are normally considered drying related problems. Many times these problems occur before and sometimes after the kiln drying process itself.

Partial list of the things the kiln operator should look for

Before the sawmill:
How long have the logs been in the log piles? This is especially important on whiter species that are prone to stain, mildew, mold, etc. It can make a difference whether the logs are under water.
After the sawmill:
How long does the lumber stay dead packed after it has been sawn? This is especially important on whiter species that are prone to stain.

Is chemical treatment needed:

If the lumber is going to stay dead packed for a longer period will it be dipped in an anti- stain/fungus chemical and what is the concentration?

On incoming green lumber:
During transport was the lumber covered to prevent checking on the surface of the packs?

 When placing the lumber on sticks:
1: Was the lumber stacked using dry sticks in order to prevent sticker stain?
2: Were the sticks placed in a straight line on standard spacing?
3: Were special sticks used with grooves to prevent stain?
4: Learn more about proper sticking procedures and methods.

Air drying/storage before the kilns:
1: Were the packs kept away from the direct sun and/or rain, either by placing it in a shed or with a cover on top of the lumber piles?
2: Were the faster drying species placed in areas with natural high air flow, in a forced air fanshed or in a predryer system?

During kiln loading:
Were the blocks (kiln runners) placed in line with the sticks and was the general loading performed to maximize air flow? Misplaced blocks will create bent boards during the drying process.
Learn more about how a kiln should be loaded.

After kiln drying:
Is the lumber stored in a dry area where it cannot pickup moisture before shipping?

We hope this information meets with your expectation and approval. We always welcome your opinion and suggestions. Feel free to email us anytime. Thank you for your continued interest.


In this section we will introduce you to some of the concepts of kiln drying in a very concentrated form. The subject is far too great to try to be too specific with detailed explanations for this web-page. However, there is much literature available (both in books and on-line) on the subject and we encourage everyone to educate themselves in the art of drying lumber. It is an exciting subject. We are always interested in improving our ability to advise you and value your input, so we welcome opinions and suggestions. We will always be available with suggestions, guidelines, and explanations to our customers.

First, it is important to understand that there is a general relationship between how fast the lumber is dried and the quality you get as a result of doing so; however, on some species slow drying can be fatal to the final outcome through stain and mold.

Drying lumber is in it’s nature a controlled removal of water from the lumber at a speed which does not damage the lumber by checking, collapse, splitting, etc. while trying to maximize production through the equipment. This is a place where the lumber company will loose profit if it cannot dry lumber with minimum degrade and still ensure adequate production through the kilns.

When we are drying lumber we have some control parameters that we can utilize:

-- Chamber temperature (dry bulb)
-- Relative humidity (wet bulb)
-- Main fan operation (either interval operation or variable speed)

Drying defects and their probable causes.

Normally you can adjust your drying program/schedule by the defect/s you see on the kiln dried lumber. See a list of corresponding drying defects and their probable causes.

Safe rate of MC% drop per day versus drying gradient.

Safe rate of MC% per day.
Each specie and thickness has an approximate rate at which it can lose water (moisture) per day at a safe rate. The “safe rate” is different from specie to specie and from thickness to thickness. Most kiln drying manuals will focus on the “safe rate” for maximizing the drying quality and production.

Safe drying gradient.
Safe rates are a good measure, however; we do look at the safe rate of drying per day, but we prefer to focus on the maximum safe drying gradient. In kilns with adequate air velocity and venting it is possible to use drying gradient, whereas with older kilns the kiln operator must sometimes focus on the safe rate even if he has to exceed the safe drying gradient.
The climate in a kiln has a corresponding equilibrium moisture content (see graph). This means that by controlling the climate you can control the moisture content on the surface of the lumber. Since lumber shrinks during the drying process, the surface which dries first will try to shrink more than the core with the higher MC%. The difference in the inside and outside MC% (the EMC% of the air) is called the drying gradient.
When operating a kiln with more than adequate air flow and venting, you can control the climate in the kiln to create the exact EMC in the air. This also enables you to control the surface MC% of the lumber and thereby the drying gradient. So it is important to have a kiln with sufficient heat, vent and air flow capacity.

Example: With a drying gradient of 2, and core MC% of 24%, the EMC should be 12% (24/2).

The maximum drying gradient allowed is different depending on the specie and thickness. Slow drying and harder species; such as oaks, have a low drying gradient, whereas faster drying and softer species; such as pines, have a high drying gradient. On softer species the wood can flex and stretch more, which allows for greater shrinkage difference between the core and the shell, therefore the drying gradient can be higher than on softer species.

The difference between pulling moisture out and forcing moisture out of the lumber.

Normally when you have well air dried lumber in a kiln you are trying to force the moisture out through the drier outside shell, in contrast to drying fresh sawn lumber. Here we try to pull the moisture out without over drying (case-hardening) the surface.

This aspect is much more important than it sounds.
Let us explain: Just as you know from a wet wick or sponge , once they dry completely they are actually hard to get wet again. You must dip them in water and squeeze, thereby forcing the water/moisture back into the fabric. This is nearly the same thing that happens to lumber that is over dried. Once the shell of a board has been over dried, either through air drying or drying too fast in the beginning of the drying process it becomes much harder to get the moisture out through the dry shell. This will normally mean a higher temperature is required to force the moisture out.

The best possible drying quality is achieved
by starting the drying process from fresh sawn lumber and keeping the temperature as low as possible. The lower temperature also makes the wood slightly stronger and thereby the chance of degrading is further reduced. This low temperature approach to drying can have many additional advantages.

We hope this information meets with your expectation and approval. We always welcome your opinion and suggestions. Please feel free to email us anytime.  Thank you for your continued interest.


Since 1920, Universities in Europe and the United States have been making suggested drying schedules on different species and thicknesses. There are three general types of drying schedules available:

North American:
Is based on constant fan operation and Wet/Dry bulb temperatures. These can easily be converted into a Temp/RH% schedule for those kiln controllers which use an RH sensor instead of a wet bulb.
These schedules have numbers like T4-E6, T2-C1, etc.

These schedules are also based on a Wet/Dry bulb temperatures and can be used the same way as North American schedules.

Drying gradient:
The species are normally divided into 4 groups depending on the drying characteristic. Each group and thickness is then given a maximum drying gradient that one can use and still expect a high quality drying.
The temperature will depend on the specie you are drying. Generally the slower and harder the specie dries, the lower the initial temperature is.


During the last couple of decades the average kiln drying temperature has been gradually lowered in order to increase quality. So if you are looking on an older drying schedules to help you make a new drying program/schedule you should be aware that you probably need to reduce the temperature significantly.

There is much good literature available on drying lumber; but you can also find some which is less clear and sometimes has questionable accuracy. When you are studying the subject always use your common sense and try to compare it to your own experience.

We hope this information meets with your expectation and approval. We always welcome your opinions and suggestions. Feel free to email us at


Developing drying programs/schedules is a continuing process of trying to perfect the drying quality and the efficiency of the kilns. On some occasions the same specie will require a little different drying schedule if it comes from a different region. One example is the difference between oak grown in warmer climates and oak grown in colder climates.

Finding the right temperature schedule.

In most cases finding the correct temperature for drying is a choice between:

Lower temperature/Higher RH%:

-- Slower drying
-- Less degrade
-- Better color in some species.

Higher temperature/Lower RH%:

-- Faster drying
-- A little more degrade
-- Little darker color in some species.

It should be noted:

-- In some species it is desirable to reach a high temperature at the end of the drying cycle to kill insects, set (crystalize) the resin; or in oily specie, to even out the color a little.
-- On a few species it may improve the quality to operate at a higher temperature for a short period of time to kill any stain or mold in the lumber. This is often used on Yellow Poplar in North America. The same practice is used on Beech in Europe; normally in a steam chamber. <

As a general rule the following temperature for most hardwood species can be used:

Over 25%MC (predrying phase):

Slow and careful drying for best color

25 C or 80 F

Medium and normal drying

28 C or 85 F

Faster drying (not color sensitive species)

35 C or 95 F

Slow and careful drying for best color

45-50 C or
115-125 F

Medium and normal drying

50-65 C or
125-150 F

Faster drying (not color sensitive species)

65-80 C or
150-175 F

On several species, especially tropical woods, the temperature schedules are higher than our normal drying pattern. In this case we suggest you study existing publications and use the temperature and RH% settings.


Older drying programs/schedules normally use higher temperatures and lower relative humidity due to insufficient air flow and venting in older kilns.
Later you can increase the drying pressure if it seems to be drying slower than the recommended safe rate.


Developing drying programs/schedules is an ongoing process of trying to perfect the drying quality and the efficiency of the kilns. On some occasions the same specie will require a little different drying schedule if it comes from a different region. One example is the difference between oak grown in warmer climates and oak grown in colder climates.

Finding the right relative humidity schedule (wet bulb) based on the drying gradient.

The relative humidity (and temperature) is closely related to the equilibrium moisture content on the air. The equilibrium moisture content of the air is a measure of the moisture content the lumber will react to if placed in a certain climate. This is due to the fact that lumber is a hygroscopical material, which permits it to absorb and retain moisture depending on the outside climate (temp and relative humidity). The relative humidity setting is very important since the wrong setting can cause either stain (to high RH%) or surface checking (to low RH%). We suggest you calculate the RH% setting by using the drying gradient.

Table with different species divided into 4 groups used to determine drying gradient in table.
Guidelines for approximate drying gradient for different groups of species and different thicknesses

Example on determining the RH% setting using drying gradient:

28mm (4/4") white oak.


Temperature set point

45 C or 115 F

Current MC% in the core

(This is not by weight method.)

Oak belongs to specie group


Suggested drying gradient for
28mm (4/4) group 4 species

1.5 to 2.0
(1.5 for careful drying)
(2.0 for faster drying)

Drying gradient

1.7 Example.

Calculated EMC% of the air

12.4 EMC%
(21 MC% / 1.7)

Use EMC graphto determine

approx. 72% RH
(12.4 EMC% at 45 C)

The relative humidity setting should be.


If the MC% of the wood is above 28% you must use 28% in your calculation. When drying lumber, the shrinkage starts around 28%, this is the reason why MC% above 28% is considered 28% for purposes of calculating the RH settings. We are trying to prevent the excess shrinkage of the shell (in comparison to the core) to be stretched beyond the breaking point when surface checking starts.

Kiln-direct believes this is the best method to determine the relative humidity or depression schedule for species and thicknesses that you cannot find good and accurate existing data. Using this method does require a thorough understanding of kiln theory and very close attention during the first drying cycles to insure that drying defect and other degrade becomes excessive.

We hope this information meets with your expectation and approval. We always welcome your opinions and suggestions. You can email us at Thank you for your continued interest.



Developing drying programs/schedules is an ongoing process of trying to perfect the drying quality and the efficiency of the kilns. In this section we focus on some of the parameters which do not relate to temperature and relative humidity. Kiln-direct's kiln controllers have been designed to allow the kiln operator more methods of fine tuning the drying process. These parameters can be used to optimize the kilns to the best drying quality and lower operating cost. Many of the parameters in this section are probably not relative to the standard kiln controllers.

Learn more about the advantages of kiln-direct controllers.

This system has been designed for drying all types of hardwoods and softwoods. The setup parameters described in this section can be used to fine-tune the kiln controller for each type of load you will be drying. When using the Kiln Link Direct network you can build a database for each type of wood and thickness. Each drying schedule/program can have their individual setup parameters, which is automatically sent to the kiln controller when a load is started initially. This system makes the operation of kilns very simple, without compromising your ability to take advantage of all the fine-tuning you need to maximize the kilns.

Setup control parameter. This is the basic parameter, which will allow you to change your kiln controller from a sensitive hardwood kiln to a fast reacting pine kiln and anything in between.
Energy calculation setup. On all our controllers we have incorporated a method to estimate the energy consumption of each kiln charge. As energy cost rises this ability becomes increasingly important and will allow you to find better schedules and kiln setups, which reduces the energy consumption and thereby the drying cost.
Main fans forward/reverse. The forward and reverse operation of the main fans can be set differently for the predrying and the kiln phase as to provide you maximum flexibility.
Main fans interval operation. In the kiln phase the operation of the main fans can be forced into interval operation when there is no moisture evaporation from the lumber. This prevents excessive case-hardening and reduces the electrical consumption.
Minimum venting temperature. Also known as minumum exhaust temperature (MET). This parameter is only used in the kiln phase and very useful during initial startup to prevent venting before a desired kiln drying temperature has been reached. It is also useful on kilns with insufficient heat capacity.
Predrying to kiln phase period. This is a unique innovation by kiln-direct and Wooddryer System. This feature combined with the interval operated main fans is especially useful on slow drying and color sensitive species. There is no other system which can detect free water in load and act upon this information in order to preserve the drying quality.
Conditioning. Although our controllers can help you dry the lumber with less stress than a conventional "slave" type kiln controller it may be necessary to condition the lumber. When using the Kiln Link Direct network the conditioning can be started automatically or you can elect to engage it manually (most common). When the conditioning period has been started it can run indefinitely (until stopped manually) or for a preset time period after which the controller shut down the kiln automatically.

We hope this information meets with your expectation and approval. We always welcome your opinions and suggestions. You can email us at


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