BURNISHING

Post 288 – by Gautam Shah

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Burnishing is a surface material finishing process. It polishes and hardens the surface, so that the endowed finish lasts longer. Surfaces that depend on the smoothness for reduced friction, and take lots of wear, need to be burnished. A burnishing rubs the rough surface texture and makes it shinier, but it is not intended as a polishing process. Polishing removes all excess (protruding) materials to level out the surface, whereas Burnishing removes minimum surface material and hardens the surface. It is true that a well polished or a smooth surface takes better burnishing.

Burnishing and Peening, are two most popular cold-work finishing operations. These are post-machining operations for improving the surface roughness, hardness, wear resistance, load-bearing characteristics, fatigue strength and resistance to corrosion. Burnishing machines press rollers or balls against a surface to mechanically deform and refine surface finishes. Burnishing and peening, are used for bearings, rotating shafts and turbine blades. Burnishing redefines micro sizing like the tolerances and geometry like quality of roundness.

Burnishing is mainly a Metal surface alteration process. It is used in various versions for Ceramics, Wood, Leather Paper Cement, Textiles and Artwork. Burnishing occurs on a surface, where another surface sliding on it creates a contact stress which locally exceeds the yield strength of the material. It induces plastic deformation of the surface component, hardens the surface by generating compressive stresses.

Burnishing is not always desirable process for all metal items. It affects the behaviour quality of the surface significantly and often unpredictably. A burnished face, visually seems smoother but with repeated sliding marks grooves over the surface in the sliding direction. Heavy burnishing forces separation of top layer causing peeling of it. Burnishing generates heat which is greater than rubbing or polishing. This excess heat deforms thin body parts. A part deformed due to heavy burnishing, takes greater friction, creating a ‘runaway’ situation where the part fails.

Ceramics burnishing are a treatment in which the green mass of the pot (before drying for baking) is polished and compressed. Compressing the mass and allows excess water to come to the surface, increases the density of the mass and provides a glossy surface. Burnishing is also done after coating the raw item with the slip. As part of surface compaction, sometimes patterns are embossed on the surface. Hard smooth surfaces like wood, bone, glass, metal, or ceramics are rubbed on the surface.

Wood Burnishing is done by rubbing hard grained wood piece along the surface of the wood. Burnishing generates heat, to dry out the surface, melt and fuse the resinous substances or additive substances such as oils, wax etc. Burnished surfaces retain the natural feel (grain and colour) of the wood, and is more natural looking then any coating treatment. Lacquer coated woods are burnished with wool fabrics to gain a natural sheen. Sometimes rubbing compounds that have very fine abrasive grains, wax, oils, lubricants like silicone oil and colouring dyes are used.

Leather Burnishing is used for top and under surfaces as well as edges of leather products. Hard wood pieces are rubbed over leather with or without rubbing materials like oil or wax to achieve a compressed mass and glossy face. The heat also facilitates penetration of rubbing material. The process is also conducted at leather sheet and product formation level. At a leather sheet level-heated roller with pattern compresses the leather. A process, reverse of burnishing is done to produce suede surfaces.

Paper Burnishing is a post paper forming process. It is done to compact the grain-mass and provide sheen, by heavy calendaring. Calendaring is accompanied by bodying with starch, minerals or resins. It is done to emboss textures or patterns. Photographic mount-boards have such ingrained textures.

Cement Burnishing is done to plasters and cast concrete surfaces. Cement plasters are re-trovelled after the initial setting of the cement. Trowels of wood or metal sheets are rubbed to compress the mass, bring the excess water to the surface and polish it. In case of Tri-mix concrete floors, post setting vibration compacts the surface bringing out the excess water, which is than suctioned out.

Textile Burnishing is a fabric finishing and texturizing process. Fabrics are hot pressed and passed through rollers. Sized and chemical treated fabrics get a sheen and smoother surface. Fabrics are singed during the process to burn standing or loose fibres and to compact the mass. Shrinking also reduces the mass.

Art-Work Burnishing involves applying colours and than rubbing them to level the surface. Frescoes =applying colours on fresh;y coated plaster, uses burnishing for two purposes 1, to press-in the surface with pigments in high saturation, and 2 to achieve a sheen on the surface.  The technique was used for Encaustic or wax colour painting. Wax colours were rubbed and polished to achieve a saturated effect. Tempera paintings were also treated or touched with same techniques. Modern day application uses wax crayons or pencils to fill in colours, which are then rubbed with smooth glass or stone. The surface gets warm to melt and fuse the colours.

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