Name: Melvin Lim & Mohd. Syafiq Subject: Building Material

Student ID: 0315772, 0314702 Question: 13

13. (a) Various processes of application of preservatives are:

Brushing and Spreading

Brushing is the simplest and most readily available method of applying a wood preservative, and is particularly useful to the general public and for on-site treatments in building.

Spraying and fogging

Spray application of wood preservatives is generally comparable in its results to brush application, and can be carried out as an alternative to brush treatment under appropriate circumstances.

Immersion

Immersion treatment is used with timbers which are to be subjected to a wide variety of hazards but, because the preservative absorption and therefore the effectiveness of the treatment is related to the time for which the timber is immersed, it is most important that a suitable immersion period is specified and adhered to.

Hot and cold stepping in open tanks

Most commonly, the timber is immersed in cold preservative and the bath and timber are heated up together to around 80 C until the timber is hot throughout, and the whole then allowed to cool down together. Treatments may be carried out more rapidly by using two baths, one hot and one cold. Timber is put into the hot bath until it is heated throughout and then transferred quickly to the cold bath.

Diffusion

This process is used to treat unseasoned timber, particularly Canadian hemlock, Baltic redwood and home-grown spruce, the preserved timber being used in building.

Pressure Processes

In most of the simpler treatments so far described - (i.e. brush, spray, deluge and immersion) - the absorption of preservative fluid is mainly by capillary attraction under atmospheric pressure.

Double Vaccum

This process depends primarily on creating a vacuum inside the timber and then using this to draw preservative fluid into the timber; the impregnation period is usually at atmospheric pressure, although in some variations it may still be under vacuum. With refractory timbers, or to obtain high preservative loading, a positive pressure may also be used, usually around 1 x 10^5 N/m^2.

13. (b) Define seasoning of timber. How is seasoning done on large scale

Seasoning is the controlled process of reducing the moisture content (MC) of the timber so that it is suitable for the environment and intended use. We need to reduce the MC of timber for the following reasons:

Every time the MC reduces the timber shrinks especially tangentially.

Consequently it will show fewer tendencies to warp, split or shake.

Seasoned timber although lighter will be stronger and more reliable.

The sap in timber is a food for fungi and wood parasites. Remove the sap and the wood will be less attractive to these dangers.

Dry well-seasoned timber is stronger.

Dry well seasoned timber is easier to work with and consequently safer especially machine working.

Timber with higher moisture content is difficult to finish i.e. paint, varnish, etc.

There are two main ways of seasoning timber, Natural (Air) and Artificial (Kiln) drying.

Air Seasoning

Air seasoning is the method used with the timber stacked in the open air.

Kiln Seasoning

There are two main methods used in artificial seasoning, compartmental, and progressive. The amount and duration of air, heat and humidity again depends on species, size, quantity, etc.

Compartmental

A compartment kiln is a single enclosed container or building, etc. The timber is stacked as described above and the whole stack is seasoned using a programme of settings until the whole stack is reduced to the MC required.

Progressive

A progressive kiln has the stack on trolleys that ‘progressively’ travel through chambers that change the conditions as it travels through the varying atmospheres. The advantage of this system, although much larger, has a continuous flow of seasoned timber coming off line.

13. (c) Why is it necessary to provide odd numbers of veneers in a plywood?

It is necessary to provide odd numbers of veneers in a plywood because that way the surface layer on either side will be pointing in the same direction.

Reference Links

1) Wood Protection Association. “Methods of applying Wood Preservatives” Retrieved from: http://www.wood-protection.org/newpubs/files/T3Methodsapplyingpreservatives10_05.pdf (26 May 2014).

2) http://wiki.answers.com/Q/Why_is_plywood_always_made_from_odd_numbers_of_veneers?#slide=3

3) http://www.geoffswoodwork.co.uk/seasoning.htm4) http://www.nzdl.org/gsdlmod?e=d-00000-00---off-0hdl--00-0----0-10-

0---0---0direct-10---4-------0-1l--11-en-50---20-about---00-0-1-00-0-0-11-1-0utfZz-8-10&a=d&cl=CL1.1&d=HASH70c81f6386a2600bdfdd3f.3.14