PROPERTIES OF MATERIALS
Materials may be broadly categorized into two groups namely metals and non-metals. Metals may further be divided into two classes, namely; ferrous metals and nonferrous metals. The former group consists of those, which have iron as one of their constituent elements. An example of this is steel. Metals of the latter group do not have iron in their compositions. An example is silver.
Non-metals may also be classified into two, namely; natural materials and artificial (also called synthetic) materials. Clay and wood belong to the former class while plastics and rubber are part of the latter class. A synthetic material is produced by the combination of certain other naturally existing substances.
The most often used materials are wood, metals, ceramics, plastics, and rubber. These are the materials, which will be studied in this subject. We begin with a discussion of their properties.
Identification of Wood
Wood is the solid portion of a tree stem found below the outer covering of the stem. After the wood is prepared for use it is referred to as timber. It is not every tree that can supply timber. Only those trees whose stems get wider in cross-section, roughly on a yearly basis produce timber. Baobab, Casuarina, Iroko and others belong to this group. Certain other trees whose stems stop increasing in cross-section after some period of growth, do not supply timber. To this group belong palms, bamboos, canes and so forth.
Parts of a Tree
The main parts of a tree are as follows.
1) Roots: These hold the tree to the ground.
2) Stem or trunk: This holds the branches and leaves up for sunlight. This function is vital to photosynthesis; which is the process through which the plant manufactures food.
3) Leaves: The manufacture of food takes place in the leaves.
4) Bark: This is the outer covering of the stem. Its function is to protect the tree from harsh weather and physical injury.
The cross-section of a tree stem is shown below.
- Bark or Cortex: This covers the tree trunk. Most trees have peculiar colour and texture of bark. This implies for example, that the barks of Mahogany trees are similar but of course different from those of Obeche trees.
- Bast: This is the moist layer underneath the bark along which the plant food travels.
- Cambium layer: This part is responsible for growth of the tree. It is situated next to the bast.
- Sapwood: The living part of the stem, underneath the cambium layer, through which water and mineral salts (collectively called sap) travel from the roots to the leaves, through the bast, to the cambium layer:
- Medullary rays: These are cells arranged horizontally in a radial fashion to form passages from the middle of the cross-section of the stem to its outer parts. Medullary rays also store food.
- Heartwood: This is the dead, matured central portion of the stem, usually darker than the sapwood. It supports the stem. Heartwood is the most useful part in woodwork.
- Annual or growth rings: These are concentric layers of cells produced by the cambium layer. This causes the girth (i.e. circumference) of the stem to increase. Because one ring is produced roughly every year, they are also called annual rings. The number of growth rings is indicative of the age of the tree.
- Pith: The central part of the tree trunk. Quality of seeds, soil, climate and conditions of cultivation are some of the factors, which determine the quality of timber grown.
Classification of Wood
Timber is broadly classified into Hardwoods and Softwoods.
This is wood obtained from a flowering tree belonging to the group of angiosperms. An angiosperm has enclosed seeds in pods. Hardwood trees have broad leaves, and because most of them shed their leaves sometime during each year, they are collectively called deciduous trees. Note that although the leaves are broad, they may be of any size.
Hardwood trees grow extensively in both the tropical and temperate climatic regions. Because they grow very slowly, they are usually much denser and by implication stronger than the faster-growing softwoods. The structure of hardwood stems is made up of fibres and vessels. The former provides mechanical support for the stem while the latter provide passages for the conduction of sap. Hardwood trees have relatively short trunks and branches with bushy heads.
This is wood obtained from a conebearing, naked-seeded tree belonging to the group of gymnosperms. The seeds of a gymnosperm lie enclosed on cone-shaped scales. Consequently they are also called coniferous trees or simply, conifers. Softwood trees have needle-like leaves, and because most of them retain a full covering of leaves all year round, they are collectively called evergreen trees. They flourish more in the temperate regions than the tropical regions. The structure of softwood stems is made up of tracheids, which both conduct sap and provide mechanical support. Softwood trees have erect stems with coneshaped crowns. Crown refers to the branches and leaves.
Timber from some hardwood trees e.g. Basswood and Poplar is much softer than timber from some softwood trees. Likewise timber obtained from some softwood trees for example Yew and Larch is much harder than timber from some hardwood trees. However, generally timber from deciduous trees are hard and those from conifers are soft.
Common names of some Nigerian hardwoods as well as their scientific names (included in brackets) are the following:
- Afara (Terminalia superba)
- Agba (Gassweilerodendron balsamiferum)
- Danta (Nesogordonia papaverifera)
- Ebony (Diospayros species)
- Ekki (Lophira alata). Ekki is particularly hard
- Iroko (Chlorophora excelsa)
- Locust bean (Parkia species)
- Mahogany (Khaya species)
- Obeche (Triptochiton cleroxylon)
- Omo (Cordia platythyrisa)
- Opepe (Nuclea diderrichii)
- Shea – butter (Butyrospermum species)
A fairly common softwood tree in southern Nigeria is Casuarina popularly called the Christmas tree.