Tuesday, July 22, 2014

WOOD FINISHES


by Gautam Shah

Woods are fashioned to a variety of finishes such as bark stripped, chopped, rough hewn, sawn, planned, sanded, etc. Finish for a wood depends on several factors.
1. Type of wood -soft or hard
2. Quality of wood -jungle, commercial or special, etc.
3. Portion of wood -sap inclusive or heart exclusive
4. Type of cut -quarter, rift, tangential or slab etc.
5. Section of cut -along, across or askew to the grain
6. Anatomical peculiarities -straight or cross grain, knots, shakes, pith
7. Presence of chemicals -aliphatic compounds, waxy and resinous substances
8. Environmental conditions
9. Type of feel (texture) required
10. Type of look (grain, pattern) required
11. Economics -cost and maintenance
12. Time available for process
13. Tools and technology available
14. Age of wood
15. Moisture content of wood
16. Proposed use

■ Rough finishes are cheaper, take less time to prepare and require simple tools and techniques. Some rough surfaces give better bondage to preservatives and coatings. Rough surfaces are good for moisture movement but are highly vulnerable to insect and bacterial growth. Rough finishes hide local defects such as stains, knots, ugly grains, fine cracks etc

■ Smooth finishes are costly, require finer tools and superior techniques. Smooth finish often give poor bondage to preservatives and coatings but one requires much lesser quantity for coverage. Such finishes do not collect dirt. Smooth finishes discourage bacterial growth as do the rough finishes. The timbers for smooth finish should have a fine grain pattern. Heart portions are much better for smooth finish then sap portions. Sap portions may however, be finished fairly smooth, provided are immediately covered with moisture proof coating. Hardwoods usually provide smoother finish and of permanent type then soft woods. Woods with resinous or oily substances generally have a natural smooth feel, however, if the substances are reactive or soluble in water or aliphatic solvents, may create problems during coating. Sisam and Rosewoods have oily or waxy face, which does not allow oil paints or varnish finish. Such woods need to be covered with very thin coatings based on solvent evaporation drying, like nitro cellulose lacquer. Timbers surfaces are flame charred or singed to provide slightly darker to black tone to selected areas. The flame is either `cool' capable of depositing carbon or `hot' to singe the surface.

Valence Parquet Wikipedia Image by Morburre

■ Colour of wood shows a wide range of variation. The colour, of the same kind of timber changes depending upon whether the surface is freshly cut or has been exposed. Both sapwood and heartwood change colour due to slight oxidation on exposure. Some timbers do show certain characteristic colours. Sapwood is generally lighter in colour than heartwood. This distinction may be well defined as in Chir. Sissoo and Kokko or somewhat less well defined as in Sal and Haldu. Some timbers like spruce, fir, etc. show such colour distinction between heartwood and sapwood. The colour of wood may be uniform. mottled or streaked. The colour may vary from creamy white (Deodar, Birch) to jet black (ebony, mahogany) through varying shade of grey (oak), yellow (Haldu), pink (Lali, Pali), red (Walnut, Sisam), brown (Teak) and purple (Rosewood). In general, a darker colour in wood indicates greater durability because of the presence of natural toxic substances. 


■ Grain in timber refers to the general direction or alignment of wood cells. Depending on the actual alignment, the grain may be straight, spiral, interlocked, wavy or irregular. The nature of grain considerably affects the strength, seasoning and other properties of timber. Grain if not straight is a defect in timber. Straight grains usually occur in normally grown or in planned plantation trees. Particular type of grains many commonly are found in some species, like interlocked grain in Sal. Spiral grain is a natural defect due to the irregularities in the formation of the fibres themselves. This may make conversion difficult and also reduce the strength of timber. Timbers in which the grain changes direction to left and right more or less regularly have erratic grains. When radially cut this may produce beautiful figures and is useful for veneering. Wavy grain is produced by undulations in the wood. This weakens the timber, but many times it is valuable for the beautiful figure on the cut surface.

 ■ Texture of a timber depends on the size and variation of cells, dimensions of vessels and the width and abundance of rays. Timbers with large vessels and broad rays have coarse texture. Trees growing in favourable conditions and having faster growth has larger cells and is less compact than trees growing in a difficult terrain and adverse climate. Softwoods have marked variation in rings due to severe seasonal climatic alterations, and consequently on drying show uneven texture. Softwoods generally have fine feel due to presence of resinous substances. A moist wood, if planned has coarse texture, due to the rise or uprooting of fibres on further evaporation. Similarly an old timber which is very dry tends to be brittle at edges. A fresh wood may provide smooth feel due to the presence of oils, waxes and moisture, compared to an old wood which may feel dry and less smooth due to micro pores on the surface

■ Performance of a timber is measured in terms of its strength, durability, stiffness, toughness, colour, texture, size, availability, weight and moisture characteristics. Timber has certain disadvantages in comparison with other structural materials. Its size is limited to the width of a tree trunk. Quality is rarely uniform even within a section. It is susceptible to decay. Its greatest strength lies in the longitudinal direction that is along the grain, whereas it is comparatively weak across the grain. Though some wood show excellent shear resistance across grain. Fast-growing trees which can provide an easy renewable source of supply provide timber of poor structural or aesthetic qualities. A substantial part of a tree is lost in making timber for appropriate use. 
.
Post a Comment

FIRES in SCHOOLS of ARCHITECTURE

Post 150 -by Gautam Shah  . A recent fire in Glasgow School of Art, designed by Charles Rennie Mackintosh has become hea...