Timber. Wood formation. Wood structure. Нeat conductivity of wood, страница 5

4.   Why is it important that the logs taken from the wood should be sawed into lumber as soon as possible?

5.   Where is it better to store the logs before   sawing: in the piles or in the pond?

6.   What can protect the freshly sawed lumber against the attack by fungi and insects?

7.   How many per cent of lumber is lost due to splitting wood?

8.   What can protect lumber from splitting?

9.   How are the cold coatings applied to lumber?

10. How is lumber treated with the hot coatings?

Timber drying methods.

There are many ways of seasoning timber. But only two of them are in common use: air - drying and kiln - drying.

The choice of seasoning method depends on local conditions. For instance, if the climate is humid an accelerated kiln-drying method is likely to be cheaper than air-drying. However, for poles which need specially designed kilns air - drying may be the most economic method.

Air - drying is a traditional method of drying. Air current transfers its heat to the wood. Under the influence of heat moisture in wood evaporates, air current then carries away the water vapour. That is why individual logs and boards are placed in a pile so that air could freely circulate between them and the maximum surface area would be exposed.

Drying rate depends on the climate. Drying will be quicker in spring and summer months. However, rapid drying is not always an advantage. Most hardwoods suffer less splitting and checking when drying is begun in winter when evaporation of water is slow. Drying time also depends on timber thickness: thin boards will dry more quickly than thick ones. In a temperate climate wood can be air seasoned to a moisture content of 23-24 per cent in winter and 16 per cent in summer.

Kiln - drying allows timber to be seasoned to any desired moisture content. Kiln is a chamber designed so that the temperature, humidity and the flow of air could be controlled.

Two types of kiln are known to be used in Great Britain: the progressive kiln and the compartment kiln.

In the progressive kiln timber is moved through a chamber with different zones of temperature and humidity. The advantage of this design is the continuous discharge of dry timber. On the other hand, the necessity of a continuous supply of wet timber of the same species, thickness and initial moisture content is considered to be the main disadvantage.

Compartment kilns have a longer history of use than progressive kilns. The principle of drying is similar. But in this type of kilns timber remains stationary throughout the drying period. The advantage of this design is that the kiln is simple to construct.


Ex.1: Translate the following words and remember them.

seasoning, drying, air - drying, however, thick, chamber, area, progressive kiln, compartment kiln, wet,

similar, simple.

Ex.2: Translate the words given below, paying attention to different suffixes and prefixes: special, specially, specialist, speciality;   differ, different,   differently, difference;   circulate,  circulation, circulating; quick,   quickly, quicker; thick, thickness; charge, discharge; humid, humidity;   evaporate, evaporation; continue, continuous; necessary, necessity; advantage, advantageous,   disadvantage; local, locality.

Ex.3: State the part of speech of the underlined words and translate the sentences.

1.   The choice of seasoning method depends on local conditions.

2.   Drying time depends on timber thickness; thin boards will dry more quickly.

3.   The advantage of the progressive kiln is the continuous discharge of dry timber.

4.   The main disadvantage of the progressive kiln is the necessity of a continuous supply of wet timber of the same       species, thickness and initial moisture content.

Ex.4: Answer the following questions.

1.   What methods of drying timber are in common use now?

2.   Does the choice of seasoning method depend on local conditions?

3.   How are the logs placed   in the pile for seasoning?

4.   How does the drying time depend on timber thickness?

5.   When can the boards be air seasoned to a moisture content of 16 per cent?

6.   What is a kiln?

7.   What types of kilns are used in Great Britain?

8.   What is the advantage of progressive kilns?

9.    What is the disadvantage of progressive kilns?

10.  What kilns are simple to construct?

Mastering timber.

From the earliest times man has used wood for tools and weapons, for buildings and furniture. Man quickly learnt, that the wood of one species served his needs better than that of another. Tough and elastic ash, for example, has been used for making axe handles for thousands of years.

Valuable qualities were discovered in other timbers as well. Elm was hard to split. It made good chair - seats. The heartwood of oak would endure even in contact with damp ground. Therefore it was chosen for fence posts.

The first settlers in new England were quick to make comparisons between the native American trees and those of Europe. Many species proved to be similar, but others were new and strange. While exploring the tropics,  Europeans  found that there, too, grew many woods with

exceptional qualities. Teak and mahogany were soon exported to Europe. No species could match them for strength, attractive appearance and workability.

Until recent times man had to use hand tools to fell trees and work up the wood. Machines that can work up the wood are nearly all twentieth - century inventions. If the machinery had come into general use earlier, the large amounts of timber would have been utilised more effectively and a great number of valuable species would have been saved for future generations of people.

It should be noted that properties of different species of timber are thoroughly studied. Hardwood species, for example, have a tendency to shrink more than softwoods. Therefore softwoods are used primarily in house construction, whereas hardwoods are more popular for furniture.

The principal hardwood species, for example, are birch, alder, walnut and mahogany. Alder is without doubt one of the most important and generally used hardwoods. Its wood is known everywhere for its strength, durability and good appearance. It has always been regarded as a shipbuilding wood and is highly valued by   railroads for ties and car construction.