Q.
Explain the preservation of Timber. How would preservation of timber affect its use?
A.
Please refer to Republic of Turkey Ministry of Culture and Tourism website here for a very detail write up on preservation of wood material by chemical technique.
Slide presentations on this topic is available here by N Kanagaraj, Kinds of Wood Preservatives and Its Characters. Coimbatore, India here.
PRESERVATION OF TIMBER
Timber has to be protected from the attack of insects, e.g. white ants etc., and from internal decay due to dry and wet rots.
Perfect seasoning is the most effective means of preservation. Timber should be used only when it is wholly dry and well ventilated or is wholly under water. It will not decay when kept under water but it will become soft and weak.
Proper damp proofing of the building and providing free circulation of air around the built in portions of timber are essential for the preservation of the timber used. However, when these conditions cannot be obtained then preservatives have to be applied for preservation.
Timber should be well seasoned before the application of preservatives as otherwise the preservatives would block the pores of timber thereby causing its decay due to the entrapped moisture.
Direct contact with lime mortar should be avoided while using preservative with masonry.
Methods of Preservation of Timber
Following are some of the common methods of preservation adopted
(i) Charring
(ii) Tarring
(iii) Painting
(iv) Creosoting
(v) Wolman salt
(vi) Ascu treatment
(vii) Fire proofing of timber
Charring
Lower ends of the posts that are to be embedded in ground are generally charred with a view to prevent dry rot and attack of worms. It is done by quenching the ends of posts in water after they are charred on wood fire to a depth of 1.5 cm.
Tarring
It consists in coating with tar or tar mixed with pitch. Embedded portions of timber fence posts, ends of door and window frames, battens and beams built in wall are usually tarred. Tarring is not done in case of those portions of structural members that are open to view, because of unsightly black colour.
Painting
A paint when applied to timber acts not only as a good preservative but also it enhances the appearance of the surface so treated. Only well seasoned timber should be painted as otherwise the moisture entrapped in the timber, because of the closing of timber bores by paint, would cause decay. Paints however, protect seasoned timber against moisture thereby prolonging its life.
(iv) Creosoting
Creosote is the portion of chemical products obtained by the distillation of a tar that remains heavier than water, notably useful for its anti-septic and preservativeproperties.[1] It is produced in some quantities from the burning of wood and coal in blast furnaces and fireplaces. Creosote is commonly found inside chimney flues when the wood or coal burns incompletely, producing soot and tarry smoke, and is the compound responsible for the preservation and the flavor of meat in the process of smoking. The name is derived from the Greek kréas (κρέας), meaning "meat", and sōtēr (σωτήρ), meaning "preserver".[2]
The two main types in industrial production are wood-tar creosote and coal-tar creosote. The coal-tar variety, having stronger and more toxic properties, has chiefly been used as a preservative for wood, while the wood-tar variety has been used for meat preservation, wood treatment, and for medicinal purposes as an expectorant, anti-septic, astringent, anaesthetic and laxative, though these have mostly been replaced by newer medicines. Coal-tar creosote was formerly used as an escharotic to burn malignant skin tissue and in dentistry to prevent necrosis. It is no longer used that way because of its toxic, carcinogenic properties and because better and safer treatments are now available. Varieties of creosote have also been made from both petroleum and oil shale called oil-tar creosote, when derived from the oil tar, andwater-gas-tar creosote when derived from the water gas tar. Creosote also has been made from pre-coal formations such as lignite, yielding lignite-tar creosote and peat, yielding peat-tar creosote.
Creosotes are commercially valuable and produced industrially on a large scale, either for direct use or as raw material for the production or extraction of various chemicals. There are several other names for such fluids, but most are not trustworthy, being regional, applying to only some variants, or to other fluids as well. For example, the term pitch oil can refer to either creosote-like fluids or kerosene.
(v) Wolman salt
RECOMMENDED SPECIFICATIONS FOR TREATING LUMBER WITH WOLMAN SALTS
AMERICAN LUMBER & TREATING CO 37 WEST VAN BUREN ST., CHICAGO, ILL.
(vi) Ascu treatment
ASCU is also called CCA - Chromated Copper Arsenate which is a type of pesticide.
Ascu (CCA) : Ascu (CCA) The ingredients of this proprietary preservative are: Arsenic penta-oxide 1 part by wt. Copper-sulphate CuSO4 5H2O 3 -do- Potassium or sodium- dichromate 4 -do- Combination of arsenic penta-oxide and copper-sulphate forms a good toxic preservative against wood destroying agencies like fungi, insects and borers.
Ascu : Ascu Function of potassium or sodium dichromate, is to fix the other two chemicals in the wood so as to make them unleachable. The degree of fixation attained with this preservative is very good; it is one of the best preservative. Additional advantage of being used cold which eliminates the necessity of heating apparatus in pressure treatments.
Ref:
http://www.authorstream.com/Presentation/kanagaraj007-166212-kinds-wood-preservatives-education-ppt-powerpoint/
(vii) Fire proofing of timber
Fire Proofing of Timber is largely by Fire Retardants:
Impregnation treatments: In 1905 a fire retardant called Minolith was introduced. This formulation consisted of triolith with a high concentration of rock salt to provide both a preservative and a fire retardant for use in the mines. Celcure F developed in 1930 contained boric acid instead of acetic acid and phosphates and zinc chloride. Minalith has the composition of 60% ammonium sulphate, 10% diammonium phosphate, 10% borax and 20% boric acid. Pyresote has also a composition of ammonium sulphate, boric acid and sodium sulphate, boric acid and sodium dichromate. Components of the typical fire retardants are leachable and hygroscopic.
The most prefered components are ammonium phosphates, ammonium sulphate, zinc chloride, boric acid and borates. An American product called as Non-Com Exterior consists of a preservative which polymerises within the wood to give a non-corrosive product with good resistance to leaching. The full-cell process is used for treatment of fire retardants. Halogenated compounds such as chloronaphthalenes, chlorinated paraffins and bromophenols can be used with incorporated catalysts, like antimony oxide.
Surface coatings: Surface coatings prevent flames spreading across the surface. These coatings are used in gymnasiums, hospitals, hotels, museums, restaurants, kitchens and laboratories.
Intumescent coatings: When exposed to fire, these coatings soften and generates non-flammable gases. The coating trapping the gases bubble and produce a foam. Then the fire retardant solidifies to insulate the surface from the fire.
Non-intumescent coatings: Some of them are formulated with materials which chemically interfere with the reactions of burning. Others based on silicates or borates melt in the fire and form a protective glassy film.
Ref:
http://www.kultur.gov.tr/EN,35286/preservation-of-wood-material-by-chemical-techniques.html