Defects due to Conversion

• Chip mark: Marks or signs placed by chips on the finished surface of timber.
• Diagonal grain: This defect is formed due to improper sawing of timber.
• Torn grain: This defect is caused when a small depression is formed on the finished surface of timber by falling of a tool or so.
• Wane: This defect is denoted by the presence of original rounded surface on the manufactured piece of timber.

Defects due to Fungi

Fungi attack timber only when the following two conditions are satisfied simultaneously:

• The moisture content of timber is above 20 percent.
• There is presence of air and warmth for the growth of fungi.
  If any of the above condition is absent, decay of wood due to fungi would not occur. Hence, dry
  wood having moisture content less than 20 per cent will remain sound for centuries.

 Following defects are caused in timber by fungi:

• Blue Stain: Sap of wood is stained to bluish colour by the action of certain types of fungi.
• Brown Rot: The term rot is used to indicate decay or disease of timber. Certain types of fungi remove cellulose compounds from wood and hence, wood assumes the brown colour due to removal of cellulose.
• Dry Rot: Dry rot occurs at places where there is no free circulation of air such as improperly ventilated  basements, rooms, etc. Unseasoned soft woods and sap wood are easily attacked by dry rot. Dry rot is also caused by charring, painting and tarring the unseasoned timber.
• Heart rot: This is formed when a branch has come out of a tree and fungi is able to pentrate deeper upto the heart wood or timber. The tree becomes weak and it gives out hollow sound when struck with a hammer.
• Sap Stain: Certain types of fungi do not bring about the complete decay of timber. But they feed on cell contents of sap wood. In doing so, sap wood loses it colour. It generally occurs when moisture content goes beyond 25 percent or so.
• Wet Rot: Fungi cause chemical decomposition of wood or the timber and in doing so, they convert timber into a greyish brown powder. The important facts to be remembered in connection with wet rot are:
  • Alternate dry and wet conditions favour the development of wet rot.
  • If unseasoned or improperly seasoned timbers are exposed to rain and wind, they become easily liable for the attack of wet rot.
• White Rot: In this case, certain types of fungi attack lignin of wood and wood assumes the appearance of a white mass consisting of cellulose compounds.

Defects due to Insects

Following are the insects which are usually responsible for the decay of timber:

(i) Beetles 

• They form pin-holes of size about 2 mm diameter in wood. They attack the sap wood of all species of hard woods.
• The timber is converted into fine flour-like powder. They usually do not disturb the outer shell or cover. Hence, timber piece attacked by beetles may look sound till it completely fails.

(ii) Marine Borers

• These are generally found in salty water. Most of the varieties of marine borers do not feed on wood. But they make holes or bore tunnels in wood for taking shelter.
•  The diameter and length of these holes may go as high as 25 mm and 60 mm respectively.
•  No timber is completely immune from the attack of marine borers.

(iii) Termites

• These are popularly known as white ants and they are found in abundance in tropical and subtropical countries.
• These insects live in a colony and they are very fast in eating away the wood from core of the cross-section. They make tunnels inside the timber in different directions and usually do not disturb the outer shell or cover.
• Very few good timbers such as teak, sal, etc. can resist the attack of white ants.

<< Previous | Next >>
Must Read: What is Construction Material?