Wednesday, January 5, 2011
Timber:
Defination of timber:
A single piece or squared stick of wood intended for building, or already framed; collectively, the larger pieces or sticks of wood, forming the framework of a house, ship, or other structure, in distinction from the covering or boarding.
Structure Of Wood:
The woods used for last-making grow from the outside - that is to say, the new wood is formed between the old wood and the bark. The newly produced wood is softer than the older wood, and in drying shrinks most. The old wood is termed heart-wood, and the new, sap-wood. Each season's growth of the tree may be distinguished by the markings of rings that in a section of a trunk would be concentric . These rings are termed annular rings, and the number of them gives the age of the tree. In the illustration, it will be noticed that there are other marks besides the concentric rings, and that they radiate from the centre, or pith, and form the radii to the circumference of the tree-section. These lines are termed medullary rays. As the timber dries or shrinks the direction of the shrinkage will be indicated by the annular rings, and in the "cracks" illustrated , it should be noticed that the shrinkage is greater nearer the bark than at the centre or pith. This fact will have an important bearing upon the way the wood is cut into lasts.
Spokes are the triangular pieces of wood, sawn from the trunks or boughs. They are usually either cut into lengths 36 to 40 ins., or into smaller sections about 12 ins. long. illustrates the former, and would provide timber for three lasts. There are three or four such wedgeshaped pieces to the length cut from the trunk, according to the size of the tree.
Seasoning the wood is an important operation in the preparing of the wood for last-making. The spokes are stacked in a dry, airy place, free from direct sunshine or heat, for a period of about two years at least. The imported blocks do not usually get such a lengthy drying, as they are supposed to be partially seasoned when received. With green wood, steaming is sometimes re-sorted to, being a shorter process and advocated because it prevents shrinkage. This steaming changes the colour of the wood, and makes it a little softer. The seasoning by exposure to the air seems to retain more of the nature of the wood.
Seasoning:
Wood Seasoning is the process of removal of moisture from Timber. This can also be termed as the drying process of timber. Fresh timber has a very high quantity of moisture and hence is not useful for use in construction or for manufacture of furniture.(or)Removal of moisture from wood to improve its serviceability, usually by air drying - drying by air exposure without artificial heat - or kiln drying - drying in a kiln with artificial heat. Plywood veneers are seasoned before lay-up and gluing into panels.
Air Seasoning :
The traditional method of seasoning timber was to stack it in air and let the heat of the atmosphere and the natural air movement around the stacked timber remove the moisture. The process has undergone a number of refinements over the years that have made it more efficient and reduced the quantity of wood that was damaged by drying too quickly near the ends in air seasoning.
The basic principle is to stack the timber so that plenty of air can circulate around each piece. The timber is stacked with wide spaces between each piece horizontally, and with strips of wood between each layer ensuring that there is a vertical separation too. Air can then circulate around and through the stack, to slowly remove moisture. In some cases, weights can be placed on top of the stacks to prevent warping of the timber as it dries.
Moisture loss from the side of the wood is at about the right rate not to cause collapse of the cells, but near the ends of the wood, the moisture loss can prove to be too fast. Often the ends are wrapped or painted to slow the moisture loss from the end grain.
While little additional energy needs to be supplied for this type of seasoning, the stacks of timber require a lot of land, represent a potential fire hazard, and the product is not able to be sold for a considerable time. The interest costs on holding stock for long periods can prove significant.
Air-drying of timber is really a more controlled facilitation of what happens to unseasoned sawn, timber, once it is placed into its “work” environment. The amount of drying that can occur is very much governed by the relative humidity of the drying environment and will often vary within individual boards as well as within the stack itself. The time taken for air-drying is a function of the thickness of the timber.
Air-drying is necessarily a slow process, particularly for hardwoods, typically taking 6 to 9 months to reach a moisture content in the range 20% to 25%.
Kiln Seasoning:
The process of kiln drying consists basically of introducing heat. This may be directly, using natural gas and/or electricity or indirectly, through steam-heated heat exchangers, although solar energy is also possible. In the process, deliberate control of temperature, relative humidity and air circulation is provided to give conditions at various stages (moisture contents or times) of drying the timber to achieve effective drying. For this purpose, the timber is stacked in chambers, called wood drying kilns, which are fitted with equipment for manipulation and control of the temperature and the relative humidity of the drying air and its circulation rate through the timber stack .
Kiln drying provides a means of overcoming the limitations imposed by erratic weather conditions. In kiln drying as in air drying, unsaturated air is used as the drying medium. Almost all commercial timbers of the world are dried in industrial kilns. A comparison of air drying, conventional kiln and solar drying is given below:
Timber can be dried to any desired low moisture content by conventional or solar kiln drying, but in air drying, moisture contents of less than 18% are difficult to attain for most locations.
The drying times are considerably less in conventional kiln drying than in solar kiln drying, followed by air drying.
This means that if capital outlay is involved, this capital is just sitting there for a longer time when air drying is used. On the other hand, installing an industrial kiln, to say nothing of maintenance and operation, is expensive.
In addition, wood that is being air dried takes up space, which could also cost money.
In air drying, there is little control over the drying elements, so drying degrade cannot be controlled.
The temperatures employed in kiln drying typically kill all the fungi and insects in the wood if a maximum dry-bulb temperature of above 60 °C is used for the drying schedule. This is not guaranteed in air drying.
If air drying is done improperly (exposed to the sun), the rate of drying may be overly rapid in the dry summer months, causing cracking and splitting, and too slow during the cold winter months.
The significant advantages of conventional kiln drying include higher throughput and better control of the final moisture content. Conventional kiln and solar drying both enable wood to be dried to any moisture content regardless of weather conditions. For most large-scale drying operations solar and conventional kiln drying are more efficient than air drying.
Compartment-type kilns are most commonly used in timber companies. A compartment kiln is filled with a static batch of timber through which air is circulated. In these types of kiln, the timber remains stationary. The drying conditions are successively varied from time to time in such a way that the kilns provide control over the entire charge of timber being dried. This drying method is well suited to the needs of timber companies, which have to dry timbers of varied species and thickness, including refractory hardwoods that are more liable than other species to check and split.
The main elements of kiln drying are described below:
( a) Construction materials: The kiln chambers are generally built of brick masonry, or hollow cement-concrete slabs. Sheet metal or prefabricated aluminium in a double-walled construction with sandwiched thermal insulation, such as glass wool or polyurethane foams, are materials that are also used in some modern kilns. Some of the elements used in kiln construction. However, brick masonry chambers, with lime and (mortar) plaster on the inside and painted with impermeable coatings, are used widely and have been found to be satisfactory for many applications.
( b) Heating: Heating is usually carried out by steam heat exchangers and pipes of various configurations (e.g. plain, or finned (transverse or longitudinal) tubes) or by large flue pipes through which hot gases from a wood burning furnace are passed. Only occasionally is electricity or gas employed for heating.
( c) Humidification: Humidification is commonly accomplished by introducing live steam into the kiln through a steam spray pipe. In order to limit and control the humidity of the air when large quantities of moisture are being rapidly evaporated from the timber, there is normally a provision for ventilation of the chamber in all types of kilns.
( d) Air circulation: Air circulation is the means for carrying the heat to and the moisture away from all parts of a load. Forced circulation kilns are most common, where the air is circulated by means of fans or blowers, which may be installed outside the kiln chamber (external fan kiln) or inside it (internal fan kiln). Throughout the process, it is necessary to keep close control of the moisture content using a moisture meter system in order to reduce over-drying and allow operators to know when to pull the charge. Preferably, this in-kiln moisture meter will have an auto-shutoff feature.
Solar Seasoning:
Solar seasoning offers a compromise between the low energy requirement of air drying and the speed of kiln drying. A number of different configurations have been tried, including a double skinned inflatable kiln. Because the energy input is variable, the kilns often have effective insulation to hold the heat inside at night time. Some kilns developed overseas can generate temperatures of over 40C inside even though the temperature outside is – 20C.
Solar-drying, which can take nearly twice the time required for kiln-drying, is well suited to high-grade applications such as furniture. Solar-drying is really a slower and gentler form of kiln drying. Some seasoned hardwood producers use air-drying down to fibre saturation point (FSP) prior to stacking the timber in the solar kiln.
Chemical Seasoning:
Chemical seasoning, which is sometimes used for very high value applications such as rifle butts, golf club heads and carvings, where the timber is required to be completely free of surface checks. The process involves soaking the green timber (as soon as it is cut) in a hydroscopic chemical for about a day, which slows down the rate of moisture loss and minimises the risk of inducing the internal stresses which cause checking to occur during the subsequent air-drying.
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