Need for drying a Wood
Get two planks of green wood, very moist, one in fir, the other in oak, and place them in an un-ventilated place. After a while, you will notice traces of heating
You therefore conclude that drying of the wood is necessary, whether it is soft or hard wood, and THAT THIS DRYING MUST BE RELATIVELY FAST.
Naturally, drying must take place BEFORE USE. Green wood from a recently felled log cannot be used; you understand that a work executed in green wood would not take long, as it dries, to deform, to warp.
Consequences of drying of Wood
The main consequence of drying is, as you already know, a MORE OR LESS IMPORTANT REMOVAL that the wood undergoes.
But it must be taken into account that A DRY WOOD CAN MOISTURE AGAIN, this humidification resulting in a SWELLING which can cause significant deformations.
As drying and swelling are irregular depending on the position in the tree of the flow considered, the degree of drying of a wood is therefore extremely important to know before use.
For example, for certain structural works: carpentry, shed, unpainted exterior carpentry, wood that has lost only part of its moisture is used. These woods are said to be only wiped; they give the touch of an impression of dryness.
In carpentry, semi-dry wood is also used for the openings: crossed, exterior beads.
Indeed, a window is exposed to the weather; if the wood is very dry, it will swell a lot and the window cannot be opened; if, on the contrary, the wood still contains water, it will swell less since, at a certain moment, it will no longer absorb the water and the clearance provided for the opening will be sufficient for the proper functioning of the structure . It is to curb this entry of moisture into the wood used that the works are covered with a protective coating, paint or varnish.
Note, however, that never re-wetted wood returns to its original dimensions: it takes smaller dimensions if it is re-wetted to the same degree of humidity.
Principles of drying
Have you wondered how moisture is evacuated? What were the different operations and the duration of drying?
Take again two green wooden boards, one in oak, the other in fir. Expose them to a fire. You will see that the boards have deformed:
- that in oak has cracks: the drying was too fast; heat, having absorbed surface moisture quickly, caused abrupt removal of surface tissue while the internal tissues did not vary in size,
- that in fir, more tender, therefore more porous, allowed the water to evacuate more easily and the withdrawal was carried out without risk of chapping.
It can therefore be said that HARDWOODS MUST DRY SLOWLY THAN SOFTWOOD.
Remember, moreover, that a soft wood generally contains less water before drying than a wood hard.
It should be noted however that the result would not have been the same if the surrounding air, very hot, had been also very humid. This is taken advantage of in the case of accelerated drying and in the manufacture of plywood. We will come back to it.
In practice, a wood is considered AIR DRY when it still contains a certain amount of water, when its MOISTURE RATE is around 15% of the weight that wood would have if it were completely without water.
Moreover, this moisture content of the wood is balanced with that of the surrounding air, generally more wet in winter than in summer.
Two drying methods are used:
- natural or slow drying,
- artificial or accelerated drying.
What is natural drying?
Natural drying is the evaporation of moisture by exposing the wood to the open air. It is slow because we generally admits one year per centimetre of thickness; so, for a board of 4 centimetres the drying would require 4 years, this rate can however be accelerated by rational stacking.
The moisture content of naturally dried dry wood, called AIR DRY, varies from 13 to 17%. He's never less than 13%, which is not enough for certain works.
What is artificial or accelerated drying?
Artificial drying allows moisture to be removed from the wood faster and more completely than by exposure to air. Currently, artificial drying is commonly used, to obtain wood having a degree humidity lower than the state of air dry wood, for example for certain works intended for very that require very dry wood to avoid warping.
Very dry wood has a humidity level of only 7 to 8%. But beware: placed in a workshop whose air is normally humidified it gradually absorbs moisture until balance is achieved by depending on the humidity level of the workshop. We must therefore hurry to implement it to avoid it regain its humidity.
Once used, this wood must be protected by a varnish; a cabinet table would be damaged if put for a long time in damp or unheated rooms. Thus, also thanks to bonding, its humidity not exceed 10% and there will be no deformation due to swelling.
Different processes are used for accelerated drying. We will study them in detail in a next lesson.
Slow or accelerated drying, which should we prefer?
The behavior of a wood depends on how it was dried.
Both dryings have their pros and cons; however, the former can be practiced by all craftsmen, while the second requires a large and expensive installation.
We will see in the following lessons how to conduct the drying of the different species and we will indicate the disadvantages and advantages of each method.
The degree of drying of the wood used must be given to the degree of humidity of the environment in which stay the work done: dry wood in a dry environment (interior); wood wiped outside.
Wood is said to be AIR DRY when its humidity is balanced with that of the air around it.
You saw in our last lesson that wood drying is done by two methods.
Let's study the oldest: natural drying.
How to conduct this drying
When you exposed your oak plank to blazing fire, cracking occurred; if you expose boards in the sun, you will get the same result. There will be a loss of wood in the flow as a result deterioration and disposal of a certain amount of material.
Should we then stack the boards on top of each other to slow drying?
You have already seen drying hay that you want to keep for the winter: we are starting to spread it in the open air so that it loses its humidity, then it is brought in when it is very dry; if it was brought in wet, it would heat up and mold. It is the same for wood: if we did not take the same precautions, it would heat up.
These two examples clearly show that the drying of wood must be carried out with caution.
What are the different operations?
The NATURE OF THE WOOD (hard or soft) and the DEBIT mode (in studs or in planks).
Natural drying takes place in three stages:
- a) Wiping which is the temporary exhibition at the sawmill of soft and resinous wooden boards, freshly sawn.
- b) Stacking, which consists of placing the wood to be dried in the open air; the wood remains in piles until it has become air dry, until it no longer risks damage on wood on wood.
This degree of drying can be assessed by empirical methods, such as sound, but it is worth make better use of scientific processes, such as the electric hygrometer.
- c) Storage which aims to remove the wood from external humidity; the wood is stacked at the shelter in the stores for final drying.
A wood is wiped when it no longer causes a feeling of humidity when the hand passes.
In fact, wiping is an artisanal process and generally lasts only a short time. Good stacking done is actually enough
Stacking of softwoods
When the saw falls, the planks are stacked so as to reduce the surfaces as much as possible. contact. It is a lame stack, on the ground (fig. 1) or against any support (fig. 2)
When the surface of the wood is relatively dry, the boards are mounted in hollow, triangular stacks (fig. 3) or square (fig. 4); this is the stack used by artisan sawmills
The piles are established on construction sites which are large pieces, in scrap wood but healthy, that is to say free from worms, so as not to contaminate the sawn timber.
Why are the sites of large section? They must be thick to avoid any contact of the wood with moist soil. The distance between the bottom plank and the floor should be 30 to 50 cm if possible.
- Care must be taken to ensure that the sites are level.
- Note that sometimes the sites are replaced by masonry dice.
- Woods containing more than their water constitution are stacked in grids (fig. 5).
- The structural parts, planks, bastaings, are piled up by beds (fig. 6).
- The first bed rests on well aligned, well planed sites, spaced at a maximum of 1.50 m.
Why so many precautions? If the sites were not aligned, the wood, when drying, would deform; suppose the middle site lower than the other two; once dry, all the planks would be hollow.
The various beds are separated by soft wooden rods 15 to 20 millimetres thick by 30 millimetres in width, called a pin.
The pins, all of the same thickness, are always placed directly above the work sites to avoid deformations. They allow air circulation between the beds. Thick pins increase the drying speed; thin pins are used to slow drying.
The spacing between the pins varies from 30 to 60 centimetres depending on the thickness of the boards: the more boards are thin, the more the pins must be brought together.
Stacking of hardwoods
We know that hardwoods require slow drying. The stacking of hardwoods is different depending on the wood is cut into plots or planks.
- a) The logs cut into studs are reconstituted on well aligned sites. The first board, the lower back, is removed to give more stability to the stack.
The boards are separated by pins so that the air is renewed well (fig. 7).
Note the arrangement of pins at the end of the boards. Why ? By that a board dries faster at its end than in the middle, and you have seen that drying causes cracks.
The purpose of the pins is therefore to slow down the drying at the end, therefore partially avoid the drying slots.
Also note that the pins generally protrude a few millimetres at the end of the boards (fig. 7); this is certainly so that the drop shadow produced by the pins somewhat slows down the end drying.
For the same purpose, we also nail cleats at the end of the boards. Sometimes in the trays, we push pieces of iron in the shape of an “S” (fig. 7) in order to limit the drying slots.
Finally, the ends of the trays can also be coated with ordinary paint or paint containing emulsion waxes; this process completely removes the slots, saves nails and allows save a lot of time in manpower.
- b) The hardwoods aligned or edged are stacked by plank beds between which one spares space of 2 to 3 centimetres (fig. 8).
13-Fig. 8 Each bed is separated by pins placed directly above the construction sites. The stacks must only contain boards of the same gasoline, of the same thickness and cut at the same time to protect them from heavy rain and the sun, the batteries will be covered with a roof made of scrap boards.
Periodically, the boards can be un-stacked, the wood having stayed in the upper part passing to the lower part; we thus regulates the drying and one can notice in time alterations that may occur.
Wood partially dried in the open air is stored in a store, which must be dry and ventilated, however well closed, on paved or cemented ground; the openings must be oriented, as far as possible possible, so as to receive dry winds blowing in the region.
Hardwoods are stacked and pinned just like on the job site. However, when the woods are perfectly dry, it is sometimes advantageous for thin woods to remove the pins and stack flat. We can thus correct certain deformations occurring during drying.
SOFT AND SOFTWOOD TIMBER is often pricked upright. To facilitate the choice of planks, we can prepare them in such a way as to handle them pages of a book (fig. 9).
As, unfortunately, the ends of the boards are rarely square, their lower end is made to rest on a batten placed on the ground.
The smell from a store must be pleasant. One should never keep worm-eaten wood in store as it will contaminate healthy wood.
Drying requires a lot of care. It includes different operations: wiping, stacking, storage.
The stacking and storage of hardwoods and softwoods are different, the latter having to dry faster.
After natural drying, let's study the different drying acceleration processes that make up this also called artificial drying.
One of the advantageous methods consists in replacing the sap of green wooden logs with water: this is the desiccation, in other words the elimination of the sap.
In particular, its advantage is to eliminate the starch from the sap which would subsequently be eaten by insects and destroyed by fungi: the wood therefore keeps better.
Desiccation is done by floating before flow or by steam after flow.
By float before flow
This process is used especially for light woods, softwoods in particular. It is more difficult to use with heavy woods, oak for example; logs may sink.
Two methods can be used:
- a) Floating with hot water: the sap is soluble in hot water; however the temperature does not must not be too high, otherwise the sap coagulates. This process is little used.
- b) Floating in a river or pond: the wood is immersed in running water for a period of 15 days to 1 month or more, or in standing water for 1 to 2 months. The wood is then stacked for drying.
The water evaporates faster than the sap, which results in a certain acceleration of drying. But the practice shows that there is only an acceleration at the beginning of drying, but more thereafter, on the contrary. The gain of drying time is therefore illusory.
But a softwood log kept by floating has the advantage of being wetter when sawing; it is therefore easier to saw; then just use a saw with a large enough pitch.
Steam after flow: This is called TUBING.
The boards are stacked without pins in a well-closed chamber in masonry called an oven. We introduces steam so that the temperature is around 80 °. Wood softens, fabrics distends and is penetrated by the vapor which carries the severe water. The wood is dried up when the water, leaving the oven, no longer has any color.
The boards are taken out of the oven and exposed to air for 1 to 3 months: this is wiping; we stack them then in stores.
In conclusion, desiccation makes it possible to have woods which dry more quickly at the start and which are rid of a large amount of nutrients that normally attract insects and cause the development of fungi. Thus, they keep better than dried wood naturally. Remember that saving drying time is not appreciable if you want to obtain by this process very dry woods.
Drying With Hot and Humid Air
With desiccation, there is another artificial drying process: drying with hot and humid air.
You have observed that wet soil dries quickly in summer. The reason is very simple: dry air and warm absorbs moisture. On the other hand, if the air is hot and humid, after a storm for example, the soil dries slower. Also, you may have noticed that a detergent can dry very quickly on certain days without sun on the condition that the air is dry, certainly faster than when the air is warmer but strongly wet. It is on these remarks that this drying process is based: the speed of drying depends not only heat, but the hygrometric state of the air.
The woods are stacked on pins in a dryer. A stream of hot, humid air passes between the boards and absorbs moisture from the wood. Thus, the rate of evaporation is tempered by the high humidity of hot air and wood does not tend to split as it would if the air was dry (remember the experience of boards subject to rapid evaporation). In practice, we start the ventilation with air that is almost cold and becomes all the more humid the warmer it becomes; then at as the wood becomes drier, the warm air is humidified less and less. We will only study neither does this process, for you will agree that it is very delicate; it is the work of skilled workers knowing their job thoroughly.
Advantages and disadvantages of different drying
Slow drying can be practiced by all, but it has the disadvantage of immobilizing important capital due to the huge amount of wood remaining on the site; moreover, it cannot provide sufficiently dry wood for use in interior carpentry, furniture.
On the contrary, accelerated drying is used by wood merchants and those who need more wood dry than natural drying allows because, building an expensive oven or dryer, it must be amortized by constant use.
Does wood experience different drying without damage?
Until recent years there has been a justified distrust of artificially dried wood; some had been mistreated in the dryers, had not been monitored during drying by staff insufficiently aware of the technique used, resulting in internal slits which adversely affected the flow solidity of the works and their conservation. The wood was not healthy enough.
BUT THIS CHALLENGE MUST NOW DISAPPEAR. Constant improvements in installations and the training followed by drying technicians and workers, the care and attention with which they are surrounded all drying operations are such that we can now, without second thought, use wood artificially dried.
You now know LE BOIS. You know that the tree is a living being, subject to faults and diseases. There are immense forests in the world which give various species in density, in color, in resistance.
The wood is cut according to different processes which vary with the species and its destination. In addition, in the In most cases, the wood must undergo appropriate drying before being used. The characteristics that have been given, the knowledge you have acquired will allow you to better study the material which was the first used by men.
The following lessons will be devoted to the study of tools for woodworking.