Container Kiln Concept and Technology

Container Kiln Concept and Technology

The concept of the Global Container Kiln is to provide an affordable chamber and process to green lumber, timbers, poles and flooring for value-added profits.

Many lumber processors sell their wood green, while the real money in wood products is the kiln drying and secondary processing of the lumber.

Whether if it’s producing kiln dried siding, or log cabin home parts, a dry kiln is needed to stabilize the wood before it can be machined and sold.

The addition of a kiln to heat treat and dry lumber reduces inventory time, kills insects, and gives the customer a premium product that is stable and easily worked.

Huge kilns are expensive and require additional personnel to operate.  Our concept takes the large kiln and reduces it in size to fit inside a shipping container or insulated van.

You do the installation.  A hot water boiler (natural gas, propane, vegetable oil, or wood fired) provides the heat.  Single or three phase motors power the fans to circulate the air, and automated vents dissipate the moisture from inside the kiln to outside.  You can also use your existing building or put up a pre-fab metal building as long as it’s insulated to R-14.

Lumber is dried in a series of stages and the rate of drying is monitored and controlled automatically with a simple controller.

To operate the kiln, you refer to a kiln drying schedule.  The heat is increased incrementally according to that schedule.  The schedule will refer to the moisture content in the wood, which is tested periodically by using a hand-held moisture meter.  When the wood reaches a certain moisture content (according to the species being dried), the next set-point (temperature) is entered manually into the controller.  This is done anywhere from once to six times depending on the wood species.

You can dry down to any moisture content – even condition the wood if you’re going to be machining the wood for furniture or flooring.

The kiln is designed for ease-of-installation and operation, and allows your small business to benefit from the value-added profits that kiln drying has to offer.

Common Kiln Questions

The Most Commonly Asked Questions About The Global Container Kiln

1. What is a dry kiln and how does a conventional kiln work?

Conventional dry kilns have been in existence for over 50 years now. The basic concept is to dry wood products using airflow, humidity control and moisture extraction. Fans evenly distribute the heat which extracts moisture and the vents allow the humid air to be removed from the kiln. A simple controller constantly monitors and adjusts the heat and venting to maintain proper humidity in the kiln for accurate drying. Over a given period the temperature is raised and humidity reduced until a desired moisture content is reached.

2. Why is a dry kiln important to my operation?

A kiln can give you a door to value-added profits. Custom drying and filling customer orders in a timely manner enhance customer satisfaction and reduce inventory cost (especially costs associate with delays in drying from larger mills). Selling KD (kiln dried) lumber gives you access to $200-$1,000 per thousand board feet value added.  Selling KD lumber gives you better market access versus selling green or air-dried lumber. Customers perceive the KD stamp as one of quality and a stable product.

3. Why not use custom dry kilns from other larger mills?

Trucking your green lumber to a larger mill costs time and money. First, a larger mill may not schedule your lumber to be dried until space if available. Second, a larger mill may have such large dryers that they will mix your specie of hardwood with others, thereby compromising the quality of drying (a kiln chamber can follow only one schedule). Third, many green mill operators are concerned that large custom kiln drying operations may not handle their bundles with as much care. Probably the most critical factor now is the severe overtaxing of current kiln space. In most parts of the country there is simply no room for custom drying and many larger mills are no longer offering its kilns for the use of green mills.

4. What must be done daily to run the kiln?

Since the operation is automatically controlled by the industrial Controller there are only three basic checks every day.

(1) Make sure the equipment is operational.

(2) Check the moisture content of the wood with the included Moisture Meter.

(3) When a desired moisture content is reached, change the set point on the controller [this is done 4-6 times during a given schedule].

5. How is the kiln different from other kilns?

Global received a great deal of help and advice when it designed this kiln. It is in all means, a conventional dry kiln. The basic concept was to provide an affordable kiln which was easy to operate and simple enough to be put together by the buyer. There is nothing new about this kiln except that the kiln chamber is a aluminum shipping container or insulated refrigerated van. Vince Schneider who has built hundreds of kilns assisted with the design and made the blueprints. Since all of the parts are off-the-shelf there are no custom components to service or replace, saving on initial and replacement costs.

The basic design also keeps down maintenance costs. Since the kiln is put together by the buyer, he has a good knowledge where everything is and how it works within the kiln.

With more than 40 kilns sold and in operation, we have a long history of experience with these kilns.

6. What about DH kilns and why is there such a high kiln turnover sales rate?

DH or dehumidification kilns are initially less expensive to purchase, but have higher operational costs. The advantages of a lower acquisition cost is soon diminished from the high monthly electrical bills. Many DH buyers switch to conventional dry kilns as soon as they start paying the high utility bills and maintenance bills. As you look through the classified/want-ads under kilns you will notice a very high

proportion are DH Kilns.  In addition, most DH kilns cannot set pitch or heat treat since they are very low temperature kilns.  Many DH kilns have supplemental heat sources, which essentially make them a conventional kiln.

7. As far as operational costs, what can a kiln operator expect to pay for electricity and heat (hardwoods and softwoods)?

Typically for conventional hardwood kilns you can expect to pay about $15 per 1,000 BF on the electrical side (for the fans) and about $15 per 1,000 BF for the heat (depends on the price of natural gas or propane). You can save 15-30% of the fan electrical costs by turning off some fans near the end of the hardwood drying schedule since the wood does not require as much airflow. For SYP and other softwoods you can expect about $3-6 per day for the fans and about $3-8+ per day for heating costs depending on the current price of natural gas.

8. Why is the Global Kiln Kit a better value?

The Global Kiln is a better value because it is a time proven kiln which you, the buyer, can put together using simple hand tools. In addition it uses a fully portable shipping container as the kiln chamber which can be moved or sold without any modifications. The shipping container is self-supporting and does not require a foundation. It is also wind/watertight. The savings in labor, foundation and standardized off-the-shelf equipment result in large savings for both acquisition and maintenance costs.

9. Are there other kilns of this type or size?

There are no other portable dry kilns of this size. The Koetter Dry Kiln has kilns from 400-4,000 BF with a foundation and uses a method of drying called constant venting. Constant venting is a method which is always venting and costs more per BF to operate because of the heat loss. Since the temperature never gets above 140 degrees F, the

schedule is also a bit slower. While drying hardwoods this is not a big concern but with softwoods, such as Pine, the difference becomes greater. The smaller Koetter kilns use air-over motors which are more likely to receive heat/moisture damage than TEFC (Totally Enclosed Fan Cooled) motors which are used in the Global Kiln. In addition, the motor insulation in the Global kilns are rated above 200 deg. F.

10. Can I use my wood waste to provide heat for the kiln?

Yes. Using a Waterstove (like the Central Boiler) or solid fuel burner, you can use your scrap wood to heat water which is then routed to the kiln where the heat is extracted by the use of a hot water to air heat exchanger (radiator). Not only will this save you on fuel costs but may actually save you money from the cost of disposing of your wood wastes. The Waterstove is a hand-fired unit which requires loading 3 to 4 times per day. The Log Boiler offers an even better solution by accepting large format wood waste including logs, stumps, pallets, etc. The issue of a wood waste fired system versus a natural gas or propane heating system is one of convenience. The natural gas / propane system is fully automatic.

11. Do you offer an alternative to the Wet Bulb sensor?

Yes. We offer a solid state humidity probe which does not require a wet bulb well (container of water) to operate. The probe operates in conjunction with a Watlow or other digital controller which is sent in place of the standard controller. This is an expensive alternative, and best for larger kiln operations.

12.  Do you offer alternative heating systems?

Yes, we offer two other heating systems for the kiln:

(a) Cogeneration: Using a Log Boiler and a Infinity Turbine (waste heat to power), you can produce electricity and pure hot air which is introduced directly into the kiln.  Excess electricity can be used by your mill equipment.  For more information, please visit our Infinity Turbine site.

(b) Heat Pipe Kiln:  A large diameter pipe (10-15 inches) is used as the heat exchanger.  It is installed just below the ceiling where the airflow can circulate around it.  At one end of the steel heat pipe is a natural gas gun, or other heating direct fired device such as a solid fuel burner.

Process of Kiln Drying Firewood – Questions/Answers:


Wood is loaded (randomly) into wire baskets and then put into the kiln chamber. If you own a firewood processor, you can convey the wood right into the baskets and then forklift them into the container kiln, or load them onto a rail system for further rolling into the kiln chamber.

Kiln Drying

Chamber is heated and fans circulate hot air around firewood in baskets. Vents exhaust moist air out. The more heat and faster airflow, the better the firewood dries (and faster it dries).


The most value added for firewood processing is in the kiln drying and packaging. Packaged wood sells for 2-3 times more per unit volume, but only costs slightly more to package and deliver.

Best Strategy

The firewood dealers that make the most profit have their wood split by other processors. They have the wood split and delivered to their kiln and packaging operation, where the wood is dried, packaged and then distributed. The other benefits are less workman’s comp and insurance.


Global can assist you in marketing your firewood. We highly recommend a classified ad, yellow pages ad, Google ad, postcards, and word of mouth. A satisfied customer is your best form of advertising – keep them happy and they’ll rave about your product, generating additional sales. Provide a good quality, dry product, and have a good sales/delivery service and you’ll have a loyal customer base for years to come.

Question and Answer:

How do I know when the firewood is done ? We have two three proprietary methods to insure your firewood is dry. They are simple methods that will give you a standardized dry product.

Do I install the parts ? Yes, the kit is a do-it-yourself project. You may also wish to contract a plumber or electrician to install more technical items, and to be sure your project is to local code.

How is the kit shipped and when will it arrive ? After the final payment is made, the kit is drop-shipped from the parts distributors/manufacturers via UPS groundtrack. The entire process, from receipt of payment, takes about a week or two. Ordering boilers can take up to 3-4 months during peak season. It is best to plan ahead.

What is included in the kit and what does the buyer need to supply ? The kit includes all major components within the kiln chamber, such as fans, motors, venturies, vents, heating coils, controllers, water circulation pump, etc.

How is the firewood loaded into the kiln chamber ? If you have a hi-cube shipping container, or a insulated van, you can forklift in the baskets of firewood. Otherwise, you can install a simple track system and put the baskets on kiln carts. It’s important to have a material handling system for your firewood business, and this is just part of that overall system.

Do you recommend a shipping container or insulated van ? You can use any number of structures for the kiln chamber, even building one out of plywood or using a pre-fab metal building. The most expedient way to put together a firewood kiln is using a over-the-road insulated trailer (sometimes referred to as a reefer trailer). Used they are about $3-4,000 however they are already insulated and usually will have a higher clearance on the inside. Always choose a 9 ft 6 in height if possible. The 8 ft. high containers and vans don’t give much room for the interior fan rack. Even though the vans have wheels, you can simply knock them off so that the van lies flat on the ground.

What do you recommend for insulation ? If you purchase a container that is not insulated, you’ll need to insulate it with rigid foam, foil sided insulation (Thermax or similar brand names), that is a total of 2 inches thick. That amounts to about a R-14 which is sufficient. You do not need to insulate underneath the container or van. Always put in a vapor barrier if you plan to install the insulation yourself, and then have a top layer of 1/2 to 3/4 inch plywood to protect the insulation. Insulated vans sometimes have stainless steel interiors which are perfect for kilns. If you do install the insulation, vapor barrier and plywood, paint the plywood white so you can see inside of your kiln.

What are kiln baskets, and how are they made ? Kiln baskets usually measure 3x3x3 feet, but can be made to any dimensions, as long as they conform to your materials handling system to move the firewood around. To save money, weld these together yourself with angle iron (steel) and wire mesh (strong fencing material). We had one customer who actually thought about converting hog trailers to huge firewood holding pens and driving the entire trailer into a firewood kiln. As long as you can easily handle the basket, and it allows the heat and airflow to penetrate to the firewood, anything goes.

What temperatures does the kiln operation ? The kiln will operate up to 180 deg F. This is somewhat dependent on your heat source. Some waterstoves cannot get above 160 deg. F. Remember that the firewood needs alot of heat and airflow to dry.

Why do I need your kit and services – why can’t I just put this together myself ? We have experience in kiln design and operation. Just having a van, with a fan, and a heat source won’t dry your firewood professionally. We specialize in drying wood fast, efficiently, and to an optimal dryness to produce a superior product that your customer can trust (to burn clean, without insects, and uniformly dry).

I’ve seen other kiln sales groups out there with containers – are they copying your idea ? Yes, they are. But they probably don’t work quite as well and they certainly don’t have the experience (in both firewood processing and marketing) that we do.

Speaking of which – why do I care if you know how to sell firewood, all I want is a kiln ? It’s one thing to produce a premium product, but it’s another to market and sell that product. We have experience in both. Sometimes we can even provide you with buyer leads if you produce packaged firewood. We get calls from distributors all over the East Coast inquiring about quality kiln dried firewood.

What if I need assistance before, during and after fabrication ? Global provides unlimited consultation at any stage of your kiln project, after we are in receipt of an order.

Is there an official stamp for kiln dried firewood ?  There is no official stamp for kiln dried firewood.  I recommend you insert a pamphlet with your firewood delivery that signifies that it is KD.  You could also brand the wood with a KD stamp on the end of the firewood.

How do you store the firewood after it is kiln dried, and how do you package it ? After kiln drying, keep under cover so that it does not come into contact with rain.  Your best option is to package firewood by shrinkwrapping in two sizes (a) between .7-1 cubic foot, and (2) 4 cubic feet vertical.  There are several reasons for this packaging –

first, you can sell to convenience stores and customers who don’t want a face cord or full cord, second, you can charge more for the smaller packaging, and third, the larger 4 cubic feet can by moved via dolly for larger house or condo use.

Do firewood buyers know about kiln dried firewood ? Kiln dried firewood is not well known yet, but customers love it. It’s guaranteed dry – unlike seasoned wood that is often still wet or green.  People are willing to pay more for a consistently dry product. It’s heat treated, so there are no insects.  It also does not produce as much creosote as season wood does.

What is the market for kiln dried firewood ?  The market is definitely for the middle to upper class.  You can do advertising directly to those customers via Google or

Who do I market my premium kiln dried firewood to ?  Market to a variety of groups, including, but not limited to condo/home owners, campgrounds, wood fired oven restaurants, and even retailers such as convenience stores. Focus on heat treated and insect-free firewood.

Wood Terms

Glossary of Wood Terms



A delivery by tractor-trailer originates from where the trailer is loaded, the load is delivered to a destination, then the trucker returns home. If the return is also a paying load to be delivered to the vicinity of the trucker’s home, that load is called a backhaul. If the trucker returns home empty, that run is called a “deadhead.”

Band sawmill

An evolution in sawmill technology that uses a thinner band saw blade (less kerf therefore less sawdust waste) than a circular saw. It also has teeth on both sides that allows cuts to be made in two directions instead of just one, improving efficiency and productivity.


The outer protective layer of the tree. Severely damaged bark on a tree is a defect that can lower the value of the its logs. At the sawmill, logs are first debarked, then slabs are cut off leaving a rectangular or square cant to be cut into lumber. There are two main types of debarkers: Rosserhead and Ring debarkers. Before raw bark is sold as bark mulch, it is ground in a tub grinder (hammermill) to give it the proper texture and consistency. Bark quality is a function of color.

Board foot

An volume of lumber that measures 1″ x 12″ x 12″. The number of board feet in a log is estimated using one of three log scales: Scribner, Doyle, or International Rule. The standard used in Massachusetts is the International Rule. The actual yield of a log after sawn into lumber is often greater than the estimated yield. Both logs and lumber are sold by the thousand board feet or MBF.

Bole wood

The lower section of the trunk of a tree from the ground to the first limb or branch. Some loggers and whole tree operations delimb trees leaving only the bole or stem portion of the tree. If chipped in a whole tree chipper, the result a “cleaner” chip with fewer leaves, sticks, or pine needles.


A log is first debarked then the rounded slab or outside portion of the log is cut off by the sawyer. The remaining square or rectangular portion of the log is called a cant. Lumber is cut from the cant. The more pieces of lumber cut, the more sawdust (waste byproduct ) is produced, reducing the log yield of marktable board feet.


The sawmill device on which a debarked logged is placed which moves the log back and forth through the saw blade creating slabs, cants and lumber. The log is also turned on the carriage before making the next cut.

Circular sawmill

The traditional sawmill that uses a circular saw (large version of hand held power saw). Circular saws are thicker (larger kerf) than band saws and produce more sawdust. Logs can be cut moving on the carriage in only one direction, then the carriage returns and turns the the log for the next cut.

Co-Gen Operation

Refers to the simultaneous production of steam and electricity.

Construction and demolition (C&D)

Though lumped together to refer to wood waste produced by construction or by demolition, the products can be quite different. Construction wood waste can be clean dimensionally cut lumber such as board ends or cutoffs. Demolition wood waste is often contaminated with nails, sheetrock, paint, etc. Markets for C&D are limited by how “clean” and free of contaminants the wood is. Some business specialize in processing and disposing of C&D.


Stacks of hardwood 4′ high by 4′ wide by 8′ long. It is the measure by which firewood is customarily sold , sawdust is sometimes sold, and small diameter logs sometimes bought. One cord is the equivalent of 128 cubic feet, 4.7 cubic yards. The weight of a cord varies if it is green (freshly cut), seasoned (partially air dried), or dry.

Cut to length (CTL)

New timber harvesting equipment allows loggers to fell trees, delimb them, and cut them to market length specifications before loading them on forwarders bound for the landing. CTL equipment is a recent trend in logging operations.

DBH (Diameter breast height)

The diameter of a tree at breast height (4.5 feet above ground) together with the estimated height of the usable logs in a tree is used to determine the volume of lumber likely to be yielded in a log depending on the log scale used (Scribner, Doyle or International Rule).

Doyle Log Rule

In use since about 1870, this scaling method deducts a full four inches for slabs. It grossly underestimates the yield on small diameter logs (less than 14″) . Every fourth Doyle load could be considered free in comparison to International rule, if the logs are within 14 to 20″ inches in diameter and the prices per MBF for both scales were identical. The variance between Doyle rule and other rules is based on diameters, rather than lengths. (Also see Scribner and International Rule.)

Flatbed trailer

Used to haul lumber. Flatbed operators may haul logs if they carry portable stakes. The number of stakes, stake height and distance between stakes determines the number of log tiers and length of logs in each tier a flatbed may carry. Flatbed operators will carry sawlogs before they will carry pulp logs, because the higher value of sawlogs ensures they are more likely to receive an acceptable rate. Lower value pulp logs may require quibbling over nickels and dimes in the rate.

Green Certification

Landowners who actively manage their woods can apply for green certification. Two agencies perform reviews and issue green certification: the non-profit Smartwood and the for-profit SCS (Scientific Certification Systems). Both agencies charge to ensure that land is properly and sustainably managed and that loggers employ best management practices (BMP) to cut wood on certified woodlots.

To maintain its green certification status from landowner to consumer, lumber mills must also be certified in chain of custody arrangements, that is, they ensure that certified logs are stored and milled separately from non-certified logs.

It was originally thought that certified logs would sell at a premium but that has not been the case in New England.

The Quabbin Reservoir managed by the Metropolitan District Commission in Massachusetts was the first publicly land to be green certified.

Ground wood chips

Ground wood is usually produced from a hammer mill or tub grinder and appears shredded and fibrous with irregular sizes, depending on the screen or grate used. Ground wood is easily distinguished by their geometry from wood chips produced from mill chipper or a whole tree chipper (WTC). WTC and mill chips appear square and evenly cut rather than fibrous and irregular.


Hardwood comes from deciduous trees that lose their leaves during the winter.


In a cross section of a log, the heartwood is the center and dead portion where growth rings appear. Also see bark and sapwood.


The practice of “high-grading.” is logging to cut only the higher value trees and leaving the lower value trees in the woods. It is frowned upon in this era of sustainability, and much effort is being made to find suitable markets for lower grade logs.

International Log Rule

In use since about 1906. Generally regarded as the most accurate of the three scaling methods, International log rule deducts only 2.12 inches for slabs and 1/4 inch for kerf. (Also see Scribner and Doyle Rules.) It is the standard rule used in Massachusetts.


The width of the sawblade (circular or band) and the source of sawdust. The more traditional circular sawblades are wider than the newer band saw blades and produce more sawdust, a waste byproduct of sawmills.


Freshly cut green lumber may be sold green or first dried in a kiln to accelerate removal of the moisture in the wood. Drying wood in a kiln is an art to ensure that the wood dries evenly to retain its strength and aesthetic properties. Different species dry at different rates. Kiln dried lumber commands a higher price than green or air dried lumber.

Laminated Veneer Lumber (LVL)

Structural grade timber veneers glued together under pressure to form a dimensionally stable and uniform product. An engineered (man-made) wood product that is a substitute for dimensional lumber. LVL can be made from small diameter logs thus creating a market for lower value trees.


That part of a wood lot to which fresh cut logs are skidded or forwarded, accumulated, cut to length (if not cut to length in the woods) , stacked, and loaded onto trailers for delivery or chipped and blown into trailers.

Log defects

Defects affect the log scale and value for which the log may be sold or bought. Deductions for defects are subjective depending on the scaler and a source of confusion in the buy-sell transaction. Defects may include red knots, black knots, rot, burned area of a log, crook, sweep, or doglegs.

Log scales

Determines the value of a log by estimating number of board feet of lumber it will yield (less allowances for bark, slab and kerf). Helps log sellers understand what they are getting for the product of their labor. Log buyers can usually predict the actual yield of board feet from a log depending on the log rule used.

Three major log scales are used: Scribner Log Rule, Doyle Log Rule and International Log Rule, although there are others (Maine and Roy). It is vital that sellers understand the differences between the scaling methods so as not to be taken advantage of. Savvy buyers are flexible in the rules they use.

A common rule of thumb is that International is always 25% better than Doyle, and Scribner is always 15% better. Doyle and International are dramatically different for small diameter logs, yet very similar for large diameter logs.

If the average diameter range of logs is 14″ to 20″, you can convert Doyle to International by multiplying 1.2 and Scribner to International by multiplying 1.11. For example, if logs for a particular site scaled about 5,000 board feet in Doyle, this would convert to 6,000 board feet in International and 5,500 board feet in Scribner.

Log trailer

Built with permanent stakes to carry sawlogs or pulp logs. Log trailers may have a grapple loader mounted front or rear in which case the operator can load himself. The increased cost of a loader on a log trailer means the trucker’s rate will likely be much more. Without a loader, the logger must use his equipment to load the log trailer.


See thousand board feet.

Mill chips

After debarking and before a sawmill cuts lumber, it saws off the outer four slabs to reduce the log to a square or rectangular cant. The slabs are mostly the sapwood portion of the log and may be resawn to save low quality boards (e.g., pallet boards), or the slabs are sent to the chipper. Most chippers pass their chips over a two-deck vibrating screen to separate the “overs,” “accepts” and “fines.” The “overs” are re-circulated through the chipper again and the “fines and sawdust” are blown into their own pile. The chip “accepts” are blown into a chip van trailer, blown into a pile on the ground to be loaded over the top of an open top trailer with a front-end bucket loader, or they are conveyed into an overhead bin which drops chips into an open top trailer.

Moisture Content

Weight of the water within a piece of lumber measured as a percentage of the weight of the dry wood. Typical moisture content for kiln dried construction lumber is 15%. Wood absorbs or gives off moisture depending on the ambient moisture in the air. The percentage of wood that is not moisture is referred to as “dry solids” that is dried construction lumber would be 85% dry solids.

Mud Season

The period of weeks between winter and spring or summer and fall when the ground in a forest is largely mud, thawing or freezing between warmer days and colder nights. During mud season, heavy logging equipment is not permitted in the woods nor are trees cut during this time. Industry professionals that depend on a continuous supply of logs must account for these seasons and stockpile sufficient quantities to process during mud season. Truckers are usually available during mud season to haul logs from distant concentration yards which may have accumulated logs for mud season.


Raw bark peeled from a tree and ground in a hammer mill (tub grinder) and sold as a landscape ground cover. Mulch functions to reduce weeds, retain moisture by reducing evaporation, and insulates the ground in cold weather, in addition to providing an aesthetic appearance for one’s garden.

Procurement forester

A forester employed by and accountable to a sawmill or paper mill and who is responsible for providing a continuous supply of logs for his/her employer. Also referred to as industrial foresters, they purchase trees on the stump or at the landing from landowners or loggers.

Pulp logs

Logs of lesser value (smaller in diameter with acceptable defects) than sawlogs, of greater value than cordwood. Pulp logs are usually bought by the ton to be debarked, reduced to chips, and used to manufacture pulp and paper.


The layer of new wood surrounding the denser, dead heartwood of a tree and under the cambium and bark layers.

Scientific Certification Systems (SCS)

One of two independent organizations that certify landowners engaged in active forest management. SCS is based in Oakland, CA and is a for profit operation. The other agency is Smartwood based in Manchester, VT.

Scragg mill

A special sawmill designed to saw small diameter logs. Not all sawmills have a scragg capability and so are limited to purchasing only larger diameter sawlogs.

Scribner Log Rule

In use since before 1846. This scaling rule is based on a diagram of the size and number of 1″ boards that could be sawn from it allowing for 1/4″ kerf.


Evergreen trees, conifers, cone-bearing trees or wood cut from these trees.

Stem Wood

See Bole Wood.

Stumpage fee

Price paid for trees before they are cut. University of Massachusetts publishes a stumpage fee report quarterly based on responses to a survey of local landowners and loggers.

Sustainable Forestry Initiative (SFI)

The program and polices formed by the pulp and paper industry members in response to criticism from environmentalists aimed at logging practices that did not promote forest sustainability. SFI is the industrial counterpart to programs promoted by Smartwood (non-profit) and Scientific Certification Systems (for profit) which promote and certify landowners engaged in proactive and sustainable forest management.

Tare Weight

Tare weight is the weight of tractor-trailer with no load. A tractor trailer typically weigh 30,000 to 35,000 pounds leaving a legal load weight of 45,000 to 50,000 (22 to 25 ton). The heavier overweight loads not only pay the trucker more, but are also more wear and tear on trucking equipment.

Thousand Board Feet (MBF)

One board foot is wood that measures 1″ x 12″ x 12″. Logs and lumber are measured by MBF or thousand board feet. MBF is determined by one of three major scaling rules: Scribner, Doyle, or International Rule. The board feet scaled for logs is an estimate only. The actual board feet yield depends on how the sawmill cuts the log. It is possible for a log to produce more board feet than was estimated producing an overrun.

Veneer Logs

Although veneer logs are sold by the board foot, they are never converted to lumber. Veneer logs are turned and rotary cut, that is, the wood is peeled off the log by turning it against a stationary knife. the sheets of wood may be laminated into plywood or laminated veneer lumber (LVL) products.

Whole tree chips (WTC)

Some mechanized loggers reduce trees that are not otherwise marketable as logs to whole tree chips to be sold to wood energy plants. Whole tree chips differ from mill chips in that they include the bark, sapwood, and heartwood of the tree, as well as branches and leaves (from deciduous or hardwood trees) or needles (from evergreen or softwood trees).

Wood energy plants

Electric generating plants that burn wood chips as fuel to produce steam and electricity. A number of these plants were built in the 1970’s subsidized by the federal government and electric utilities when the price of foreign oil rose dramatically.


Lumber Dry Kiln: Warp


William T Simpson of the Forest Products Laboratory maintained at Madison, Wisconsin by the Forest Service of the United States Department of Agriculture in co-operation with the University of Wisconsin, discusses the common forms of warp and what can be done to minimize them in the timber drying process.

Warp in lumber can cause significant yield losses and discourage the utilization of lumber from warp-prone species. All warp can be traced to two causes; differences in shrinkage within the wood as it dries, and growth stresses. Although little can be done to prevent shrinkage or growth stresses, we can identify ways to reduce warp during drying. First, however, we need to understand the forms of warp and bow they develop.

Six common forms of warp are: cup, crook, bow, twist, diamond, and oval. Cup is caused by greater shrinkage parallel (rather than perpendicular) to the growth rings. The greater the difference between this tangential (parallel) and radial (perpendicular) shrinkage, the greater the cup. It occurs in flat sawn boards, where the growth rings are parallel to the wide face. The concave surface of the cup is toward the face that was closest to the bark. Boards cut from the outer portion of large tees will have relatively little growth-ring curvature and thus less cup than boards from small logs or the inner portion of large logs where growth ring curvature b pronounced.

Two other forms of warp, crook and bow, are caused by a difference in longitudinal shrinkage on two opposite faces of the board. Juvenile wood, which is wood formed near the centre of a tree, shrinks more longitudinally than mature wood.

Reaction wood, which is formed in parts of Ieaning or crooked stems and in branches, also shrinks more longitudinally than normal wood. When one edge or face of a board contains normal wood and the opposite edge or face contains either of the two forms of abnormal wood, the difference in shrinkage along the length of the board causes crook or bow. Grain deviations on the edges or faces of boards can also cause unbalanced longitudinal shrinkage. Cross grain, where the wood fibres are not lined up with the long axis of a board, can also lead to unbalanced shrinkage. Knots are another source of localized grain deviation that can cause crook or bow.

Twist is a form of warp caused by a combination of unequal longitudinal shrinkage und grain deviations such as spiral, wavy, diagonal, locally distorted, or interlocked grain.

Diamond is a form of warp found in squares. It results from the difference between radial and tangential shrinkage in squares where the growth rings run diagonally from corner to corner. Oval occurs in rounds, and is also caused by the difference between radial and tangential shrinkage.

The other cause of warp, growth stresses, is not related to drying but can nevertheless be serious. Longitudinal growth stresses can cause crook or bow to develop immediately after sawing boards from logs. The growth stresses develop in a tree as it grows; causing tension in the outer periphery of the stem and compression in the inner portion. When logs containing growth stresses are sawn into lumber, the internal stresses become unbalanced. Release of the stresses after cutting causes crook, bow, and twist. The bark side of these boards shortens relative to the pith ride, and distortion results.

Warp in lumber can be reduced by careful sawing, stacking, restraining, and drying.

The first operation in which warp can be controlled is sawing. This controls orientation of annual rings, juvenile and reaction wood, and localized grain deviations relative to board edges and faces. In manufacturing lumber, planned positioning of these characteristics can reduce the effect of their different shrinkage rates and thus reduce warp. If cup becomes a serious problem, lumber can be quarter sawn instead of flat sawn. If growth rings are oriented perpendicular to the wide face of a board, cup will be reduced greatly. Wood near the pith, which may contain juvenile or reaction wood, often has abnormally high longitudinal shrinkage. This is a good reason for “boxing” the heart in sawing so that the pith does not fall near one edge or face. Localized grain deviations, such as knots, can also be placed (or eliminated) so that they do not cause unequal shrinkage on opposite faces.

After careful sawing, good contact between lumber and stickers can further reduce warp, and uniform lumber thickness is necessary to ensure good lumber-sticker contact. Any sawmilling or maintenance procedures that can reduce variation in lumber thickness are a good first step in ensuring that good stacking procedures will be effective in reducing warp. A light surfacing or blanking, is sometimes necessary to improve thickness uniformity.

Uniform lumber thickness is only the first step in ensuring good sticker contact to reduce warp. In a well-stacked lumber pile, every piece is held flat during drying and shrinking. The pile foundation, kiln floor, or kiln trucks must provide a firm, flat bearing surface for the lumber pile. A crooked or uneven surface will cause twist or bow in lumber during drying. Properly sized and placed stickers are important, with uniform lumber thickness a prime requirement. Broken or distorted

stickers can increase warp and should be discarded . Sticker spacing of 40 to 60 cm is satisfactory for many tropical hardwoods, provided all tiers of stickers are fully supported by bolsters and their alignment is perfectly vertical. Highly valuable or especially warp-prone lumber may benefit from spacing as close as 30 cm.

Random-length lumber should be box piled. The ends of lumber piles should be even with no overhanging boards, and board ends should be supported by stickers wherever possible. Piles should be covered so that the top layer is not exposed to sun and rain that will cause alternate shrinking and swelling and thus aggravate warp.

End racking  should not exceed 3 to 4 days for 2.5 cm thick lumber or one week for 5 cm lumber. End racking during rainy periods should be done under shed roofs. After the end racking period, the lumber should be stacked in stickered piles or immediately bundled if dry enough for shipment.

Boards at the bottom of a pile develop less warp than those near the top because weight of the pile prevents distortion. Some form of restraint, such as spring-loaded clamps anchored lower in the pile or added weights (concrete blocks for example), applied to these upper layers can reduce warp.

Research has shown that weights effective in reducing warp vary from 250 to 1,000 Kg/m2, depending on species, lumber thickness, and degree of warp reduction accomplished. Regardless of added restraint, if wide, flat-sawn boards make up part of the board mix, they should be placed at the bottom of the pile.

Strategic sawing, good stacking, and restraining are the most effective ways to reduce warp, but drying procedures can also be helpful. When kiln drying, schedule modifications that provide initial conditions of lower temperature and lower relative humidity than those recommended in drying manuals can reduce warp. For example, cup in American elm can be reduced by 50 percent when a schedule starting at 38 C and 65 percent relative humidity is used instead of one starting at 54 C and 81 percent relative humidity. Lower initial relative humidity causes a large amount of tension set to develop in the outer shell, which helps to hold lumber flat in later stages of drying. Equalizing and conditioning to relieve this stress following kiln drying is essential for lumber that will be resawn or machined in any way that removes unequal amounts of material from opposite faces of a board. This machining creates an imbalance in the drying stresses, and distortion is the response that restores balance.

Though many opportunities exist to help control warp, the characteristics of wood that cause it are beyond our control. Nevertheless, recognizing and understanding them can lead to processing strategies that will minimize their effect. An example of one such strategy is Saw-Dry-Rip (SDR), a procedure developed at the Forest Products Laboratory to reduce crook caused by longitudinal growth stresses in lumber from small hardwood logs.

The SDR method uses both sawing and drying procedures to reduce warp. First, the log is live sawn through and through on the same plane. This sawing pattern retains the balance between the longitudinal tension growth stresses at the edges of the flitch and the compression growth stresses at the centre of the flitch. During high-temperature kiln drying, compression stresses develop at the edges to counteract the tension growth stresses, and tension stresses develop at the centre to counteract the compression growth stresses. When lumber is then cut from the dried flitches, the stresses are reduced and the result is a significant reduction in crook compared to lumber cut before drying.

By using established warp-reducing procedures such as those discussed in this paper, and by developing and using new procedures such as SDR, mill owners can increase their lumber yield and profits. Failure to do so results in an unnecessary waste of natural resources.

Tropical Hardwoods


Also known as the “King of Woods”, Teak is amongst the most precious in the world. Naturally rich in oils, its color is golden to dark brown, sometimes reddish brown. It is easily worked, and dresses to a very smooth finish. Teak is very durable and resistant to moisture and the drying effects of exposure to weather and is unique in that it does not cause rust or corrosion when in contact with metal. Teak is highly sought after in the boat industry and is also used for flooring, carving, joinery, fine furniture and cabinetry, doors, paneling, turnery and veneer, trim and detail work on boats and decks. Due to consistent reforestation, by the year 2000 Costa Rica will be the #1 supplier of Teak in the world!


Almendro is one of the hardest woods in the world, also known familiarly as Stone Wood. It is used widely for flooring, stairs and rails, tool handles, bridge building and other applications where strength matters. Light Brown in color, with a uniquely delicate grain pattern that’s very appealing. It finishes well and is of the highest durability.


Also known as Royal Mahogany, (Mahogany is widely regarded as the world’s premier wood for fine furniture and cabinetry, musical instruments, sculpture, joinery, decorative veneer, interior trim, doors, turnery, carving and boat trim) Caobilla polishes to a high luster, with excellent working and finishing characteristics. It responds well to hand and machine tools, has good nailing and screwing properties, and turns and carves superbly. The Caobilla tree is one of the largest in Central America.


One of the most distinctive woods in the world, a uniquely beautiful purple wood, prized for fine inlay work, turnery, parquet and strip flooring, decorative and figured veneer and many special items such as handles, billiard cue butts and carving and fine furniture and cabinetry. Brown when fresh cut, turning deep purple to purplish brown over time. With a hardness rating double that of Hard Maple, Purpleheart is one of the hardest woods in the world.  It turns smoothly, is easy to glue, takes finishes well providing a lifetime of durability with an exotic flair. Purpleheart is prized as one of the most exotic woods in the world.


Gavilan is a very popular hardwood throughout Central and South America, prized for its beauty and finishing properties.  It is used in door construction, furniture and cabinets, turnery, carving, paneling, interior trim, decorative veneer and musical instruments, and more. It is one of the most useful hardwoods in Central America.


Laurel is a strong yet lightweight lumber that’s very easy to work, turn, carve and glue. Its used in door construction, furniture and cabinetry, musical instruments, sculpture, joinery, decorative veneer, paneling, interior trim, and turnery. Laurel finishes well with an appealing grain pattern and high durability.

Southern Yellow Pine Stain

Drying Thick Southern Yellow Pine Export Lumber to Minimize Brown Stain

Joseph Denig, Assistant Professor and Wood Products Extension Specialist

Stephen J. Hanover, Associate Professor and Wood Products Extension Specialist

North Carolina State University

Southern yellow pine flitches and joinery exported to Europe are remanufactured and sometimes finished with clear coatings. Brown stain may detract from the desired appearance. Thus, presence of brown stain in export stock may decrease its value and be reason for a claim against the exporting company.

What is Brown Stain?

Brown stain is a chemical discoloration within the wood that ranges from light brown to deep chocolate color. This stain is not caused by fungus. It should not be confused with the so-called “kiln burn” that often shows as a charred discoloration on the board surface after high-temperature kiln drying. The stain may develop on the surface, slightly under the surface or throughout the piece. More typically the stain concentrates directly under the surface. Areas under the sticker are generally not stained. Sometimes it is very difficult to detect the stain as the normal latewood (summerwood) color blends with the brown stain colors. Heartwood is generally not affected.



Important facts are: (1) the stain is caused by the precipitation of water soluble extractives; (2) these extractives are carried along with the free water within the wood as it dries; (3) the extractives precipitate at the location in the wood where liquid water turns to vapor. This is usually at or just beneath the surface. Water in the wood directly under the sticker moves laterally and/or into the sticker. Thus the stain is not generally found directly under the sticker; and (4) the solubility of the extractives increases with higher temperatures. Any drying practice that increases solubility of the extractives (e.g. higher temperatures) usually increases the depth and color of the stain.



Literature from the 1930’s to the present has discussed brown stain in the white pines (both east and west) but not in Southern yellow pine. No foolproof solutions have been developed but several processing variables that can be controlled at the mill and some dip solutions have shown promise in reducing the intensity of brown stain in the white pines. Processing variables discussed in the literature include:


Processing For Improved Results

Length of log storage Quick turnover

Time between sawing and stacking Stacking immediately

Time between sawing and kiln drying Kiln drying within 1-2 days

Kiln temperatures Lower dry bulb temperature and lower humidity

Pre-dipping Mixed results


Brown Stain Control Study

North Carolina State University Wood Products Extension personnel conducted a study to develop processing recommendations for the Southern pine industry that would minimize and/or eliminate the development of brown stain. The study included two phases. The first phase consisted of studying processing variables and their effect on brown stain at several mills. The second phase was testing kiln pretreatments and schedules at the Wood Products Laboratory, North Carolina State University and at cooperating mills.


Results from Phase I – Visitations

Six Southern pine mills producing export material were visited by North Carolina State University Wood Products Extension personnel during the spring of 1985. The following observations were noted during the visits.


Length of Log Storage

The length of log yard storage varied from zero to two years. Mill personnel interviewed believed that logs stored under water sprinklers for several months exhibited more brown stain than fresh logs.


Time between Sawing and Kiln Drying

The time between sawing and kiln drying varied greatly. Practices ranged from dead piling at the green chain for several days and then stickering the material for either air or kiln drying to immediate stickering after sawing followed by kiln drying within one to three days. The effect of the length of time between sawing and drying was not clear.


Kiln Drying Schedules

Kiln drying schedules ranged from low temperature to high temperature. Generally good results were observed with low temperature schedules; bad results were observed from high temperature (above 2120F) schedules. Very good results were obtained from a dehumidification kiln with a maximum dry bulb temperature of 1400 – 1500 F.


Mill Productivity and Kiln Size

Most mills have difficulty in cutting a sufficient volume of export stock to fill a dry kiln within a relatively short period of time. As a consequence, loads of mixed thicknesses, and/or moisture contents and partial loads were often dried. Some mills dried both dimension (on one track for example) and thick stock (on the other track) within the same kiln, keeping the thick stock in the kiln for 1* to 2 runs. The consequences of these variables with regard to brown stain are not known. An alternative observed was a small kiln. This small kiln dried a full charge of similar thicknesses soon after sawing.


Results from Phase II – Kiln Tests

Two kiln tests were conducted to evaluate the effect kiln schedules and pretreatments have on the degree and depth of brown stain. The results of the kiln tests were as follows:

1. 12/4 Effect of Dry Bulb Temperature

The effect of dry bulb temperature on the degree of brown stain was evaluated using a high temperature, conventional, and low temperature schedule.

12/4 – High Temperature versus Low Temperature Schedule

Number of Final


Kiln Schedule Days M.C.% Results

High Temp (2120+) 2 6 medium-heavy brown stain

Conv. Temp (l700-1800) 5 11 medium brown stain

Low Temp (1400 -1500) 7 15 slight brown stain

2. 10/4 – Pretreatment by Dipping Versus Non-Dipping Followed by Low-Temperature Versus High-Temperature Drying

Sixteen foot fresh cut flitches were designated to be either low or high temperature dried. Each flitch prior to kiln drying was cut in half. Half of each flitch was pretreated by dipping in sodium azide (pH=10). The other half was not dipped. The end-matched sample set from each flitch was then either low- or high-temperature dried according to the following schedules:

10/4 – Kiln Schedule, Time, and Final M.C. %

Temperature Time Final M.C. %

Schedule D.B. 0F W.B. 0F (Hours) Ave. Range

High Temp 230 170 29 15 10-23

Low Temp 140 120 126 15 12-19

rising to


After drying, the flitches were surfaced to various depths and evaluated for brown stain. A four point brown stain visual grading system was used. Results of the study were:


10/4 – Degree of Stain by Depth and Treatment

Treatment Degree of Stain by Depth (inches)

1/16 2/16 3/16

High-Temp-Dipped 2.4 1.4 0.1

High-Temp-Not Dipped 2.7 1.5 0.1

Low-Temp-Dipped 1.7 0.2 0.0

Low-Temp-Not-Dipped 1.9 0.1 0.0



The kiln temperature had a significant effect on the degree of brown stain in both test charges. Temperatures above 2120F resulted in considerable brown stain. Lower temperatures gave much better results. A very mild schedule, keeping temperatures at or below 1500F produced the best results, but the drying time was increased by 40% over a conventional schedule. For all test schedules most stain was removed by surfacing at a depth of 3/16-inch. High temperature dried flitches still had slight to medium stain at a depth of 2/16-inch, while nearly all stain was eliminated at this depth in flitches dried in lower temperatures. Dipping in sodium azide proved not effective in reducing brown stain.


To minimize brown stain in yellow pine flitches and joinery, firms should (1) use low temperature or conventional kiln schedules, (2) avoid dipping in azide as it offers no advantage, and (3) keep the length of log storage to a minimum.

For quality control practices, firms should (1) document processing procedures including length of log storage, time between sawing, stacking, air/kiln drying, and kiln conditions, (2) determine from each kiln load if indeed brown stain exists. This is not easy because most of the time the stain is just under the surface. A hand block planer, or a portable hand power planer can be used to remove the surface within a small area of selected pieces to determine both degree and depth of the stain, and (3) correlate procedures with absence or presence and degree of brown stain. Adjust procedures if possible to reduce the staining problem.

Evaluate your long term commitment to the export market. Study the feasibility of installing a smaller, low temperature kiln if you intend to pursue this market.


Quality Control in Lumber Purchasing: Lumber Storage

Quality Control in Lumber Purchasing: Lumber Storage

Daniel L. Cassens Department of Forestry and Natural Resources

Once lumber is kiln dried to the specified moisture content, it must be properly stored or it will continue to gain or lose moisture in response to the relative humidity of the surrounding air. The wood is attempting to come to equilibrium with the moisture in the surrounding air; this is called the “equilibrium moisture content” or EMC of the wood. The moisture content to which lumber is kiln dried should be the same as the average moisture content it will equalize to in service. For woods used indoors in most parts of the United States, this is 6 to 8 percent. In the dry desert regions of the southwest the figure is somewhat lower, while it is higher in the damp coastal areas. If lumber is kiln dried to the 6 to 8 percent moisture content suitable for interior applications and then exposed to the outdoor atmosphere, it will gain excessive moisture and swell in most regions and seasons of the United States. If manufactured into a product and exposed to conditions inside a plant or house, for example, it will again lose moisture and shrink. End splits, open glue joints, and warping will likely result. These problems are particularly severe during the heating season because indoor relative humidities are reduced. To further complicate the situation, lumber is normally stored in bundles. In this case the outside boards and especially the ends will be the first to pick up or lose moisture, while the interior of the bundle will be slow to change. The moisture content between and within boards will no longer be uniform, and end-use problems are nearly certain. Table 1 shows the equilibrium moisture content by month for wood exposed to outdoor atmospheric conditions for selected areas of the United States. To illustrate the significance of these numbers, Table 2 presents the equilibrium wood moisture content for different relative humidities at 70o F. The dimensional changes for a six-inch-wide northern red oak board with an initial moisture content of 7.1 percent are also given. Note that as the board dries from 7.1 to 4.5 percent, it shrinks 0.05 inches or well over 1/32 inch. If the moisture content of the six-inch-wide board increases to 11.8 percent, it will have expanded in width by 0.08 inches or well over a 1/16 inch. Light-weight woods generally do not shrink and swell as much, whereas heavier ones, such as hickory and beech, will generally shrink and swell more. The point is that if kiln-dried lumber is to be maintained at the desired 6-8 percent moisture content level, it must be stored in an enclosed heated shed for all of the United States except the dry southwestern states.

FNR-131 Forestry & Natural Resources Purdue University Cooperative Extension Service West Lafayette, Indiana

Summary: Lumber is kiln dried to the moisture content consistent with its intended end use. This practice limits the amount of shrinkage and swelling which will occur under normal circumstances. Unfortunately, kiln-dried lumber, when exposed to unheated storage conditions, will regain moisture and swell. Defects will likely result in products manufactured from this material and exposed to indoor conditions. Thus, lumber once kiln dried must be properly stored while in inventory or during the manufacturing process.