CEMENT SURFACE FINISHES

Post 124 -by Gautam Shah

 Cement surface finishes occur in three different conditions:

• Finishes generated over the cast cement products,
• Rendered finishes of cements,
• Cement coatings’ finishes.

The major problem with cement finishes is the consistency of colour over the surface. Other problems happen due to differences in application (casting, rendering or coating), mixing water content and quality and curing processes. The colour differences arise due to many reasons: ingredients used in cement manufacturing, particulate size and its distribution, size and colour quality, proportionate quantity of other ingredients, quality-conditioning additives and mixing procedures.

● Cement cast products are plain or steel reinforced structures, pre cast blocks, units, etc. A cement finish of a cast product emerges due to the form-work, compaction, level of vibration (causing aeration), proportion of water and technique of casting (towards gravity, centrifugal or centripetal, short or long depth fall). 

Cement Concrete Surfaces can have finishes depending on several factors, such as:

1 Form work surface, joints and continuity, use of a release agent, absorbency of the form work surface and setting or hardening enhancer and retarders used.

2 Concrete mix proportions, ingredients’ colour, size, and texture (lighter toned aggregates and sands produce light-coloured concrete (colour of cement is variable not only from plant to plant but often batch to batch). Degree of mixing and air entrapment affect the colour. Free lime in water creates a soapy foam which also affects the colour.

3 Insufficient or uneven curing affects the hydration and eventually the colour of the concrete.

4 Inadequate vibration causes minor pockets of air bubbles, which affects the texture.

● Rendered finishes of cement include plasters, sprays, guniting, masonry pointing, screeds and daubing. These are comparatively of thinner mass. The compaction, if any is part of the application or rendering-levelling by trowels, plasters boards, etc. For rendered finish the surface quality is fairly consistent for small-extent surfaces. For very large surfaces, such as multi-storeyed buildings, the surface differences are noticeable. These anomalies can be reduced by dividing the surface with wide intervening elements of different colour, texture, projections or depressions. Same technique is used for pointing to the masonry faces of stones or bricks. Pointing is designed to enhance the joints’ pattern, as strongly horizontal, vertical, or both. The joint is usually of a contrasting colour and of smoother texture, then the main material surface. A raked joint or protruding joint looks much darker than a flushed joint or flat joint.  

 

Cement is used for creating in-situ and precast floor blocks. Use of fine sands increases the air entraining effect and reduces the work-ability. Angular or flaky sands are difficult to use in sand face plasters.

 

Cement plasters and concrete were new finishes in later part of 19th C and so, architects preferred to experiment its colour and texture. For this old forms were not suitable, and new forms, volumes were tried out. The new material was placed inside and outside, where the illumination, shadows and textures were the key expressions. Entirely a new vocabulary came into being. Brutalism was just one such expression.

 

The mixed mortars are affected by the colour of the aggregates. So it is very difficult to produce a perfectly white marble mosaic tile or washed chips’ plaster unless only pure white aggregates are used. Cement and aggregate flooring such as IPS - Indian Patent Stone, Red Madras floor and Ironite (cast iron milling waste) are all affected by the colour of the constituents. Coloured mortars have pigments of iron oxides (black, red and yellow). Green, blue and other colours (though not sun fast or long lasting) are achieved by use of chrome pigments.

Brick masonry pointing with cement

For vertical and ceiling surfaces ziki plaster formed with marble dust containing substantial amounts of fine mica and talc. Similarly pearl glow and a smooth surface can be achieved by including sea shell dust. Slow setting and engravable cement mortars require high workability, are achieved by addition of fully calcined gypsum or lime containing such compounds.

● Cement coatings’ finishes are in the form of cement paints and high viscosity or bodied rendering formulations. Cement paints have limited life of 3-5 monsoons and re-applicability of 5-7 coatings. Cement paints are alkaline materials and applied on similar substrates, and as a result the colour range is mostly of Oxides colours. Some blues and greens of darker range are available. The chief problem is the process of adding the water, which creates flocculation and aerated mass. Next problem is application on a dried out substrates with chances of poor adhesion and difficult brushing. Rendering formulations have high viscosity, and the success usage relies more on the craft of application. 

Cement Surface Finishes have few basic problems:

• fine hair cracks

• honey comb voids

• unbounded loose particles

• foreign particles stuck on the surface

• foreign particles deposited on the surface.

• washable salts leached out from the surface

• salts and compounds formed over the surface by the constituents of the environment

• mould and fungi type bacterial growth

• disengagements from the substrate -peel off.

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CLEAR COATINGS

CLEAR COATINGS

 Post 119 - by Gautam Shah

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Clear coatings are primarily used to reveal the surface grain, colour and pattern, which however, are not always perfect or decent. Conditioning of the substrate surfaces is needed before application of a clear coating.

The most common problems with surfaces are like:

• uneven colour
• unsuitable tone
• uneven grain or pattern
• patchy absorbency
• uneven texture
• bleeding or soluble constituents
• waxy or oily deposits
• alkalinity
• acidity
• galvanic sensitivity
• moisture content and transfer

Shellac toning over pine wood stripe

Such problems with man-made surfaces are controllable, at industrial production level, but with natural surfaces like the Wood, have to be tackled on the site. To overcome the surface related problems in clear coatings, many types of surface treatments are used. Most of the surface conditioners or treatments are fast drying, easy to apply, non destructive for the substrate and insoluble in the subsequent coat material. Surface conditioners are of following types: Fillers (transparent or opaque), levellers, sealers, stainers (soluble and insoluble colourants), printed patterns, patterning tools, etching agents, bleaches, and reactants.

Fillers - Sealers are used to fill in pores so that coating material does not sink into it, and provide an even finish. Fillers generally consist of an extender, a binder and occasionally a colourant. Extenders are fine grade powder of materials like gypsum, chalk, china clay, precipitated calcium carbonate, lime, asbestine, colloidal silica, barytes and talc. Binders could be water, gums, oils, alkyds and poly vinyl emulsions. Solvent bound fillers are better compared with water bound fillers which raise the grain or fibres of the surface. Colourants provide the necessary tint, to white extenders. Fillers, made of pigment powders, serve the purpose of filling as well as staining. Transparent fillers (low viscosity NC lacquer, shellac, etc.) are used to fill the pores and provide a sealing coat to the decayable material in the grains and vessels filled with gum exhudents.

Stains provide a correct transparent tint to the surface. Stains are generally dye-materials, soluble in water, oil or solvents. Water soluble stains though of many different varieties raise the fibres and are difficult to penetrate. Oil soluble stains are heavy bodied, take longer to dry out and interfere in the subsequent coating application. Solvent soluble stains are costly, dry out immediately and may bleed residual gums and other exhudents. Stains have one important drawback that they darken the existing colour of the substrate. Where timber surfaces need to be of lighter colour, surfaces have to be bleached or toned with opaque materials.

Clear coating on Leather

Bleaching process include a treatment with hydrogen peroxide followed by an alkaline accelerator like lime, caustic soda, sodium silicate or ammonia. Bleaching affects adhesion and toughness of coating, it also provides an amber hue to the coating on aging. 

Imposed patterns: On surfaces where there are very irregular grains, patterns are screen-printed, pressed or embossed using stains. Such patterns may emulate a wood grain pattern, or are just very fine mesh or lines. Staining is also done by micro spray guns, singeing, burning or carbon deposition from flames.

Post application treatments: Clear finishes often require some post application treatments. These are mainly burnishing and waxing. Burnishing is mainly done to NC lacquer, acrylic and melamine coatings to provide a glossy surface. Burnishing is not done to slow drying coatings, because such coatings, though are dry on outer face, take days to thoroughly dry out. Burnishing is done with a Carborundum like rubbing material with a waxy or oily base. Waxing provides a dull sheen and a protective coating. Waxing is also done to renovate old coatings. Waxing compounds also include a small amount of oils and sometime silicone materials.

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FINISHING TECHNIQUES for MATERIALS

FINISHING TECHNIQUES for MATERIALS

Post 111 ⇒  by Gautam Shah ➔
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Large number of surface finishes evolve during the manufacturing of materials. These are self finishes, so integral and permanent. Surface-finishes are also obtained by treating Natural materials and secondary processing of Manufactured materials. Assemblies of materials require modification of their surfaces, or a new finish to overcome-equalize the surface quality, and manage the disorder caused by the joints.

There are essentially four methods for creating Surface Finishes. To achieve a finish for sensorial or technical requirements, one or several of these methods, are used.

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1 Techniques for surface finishing from the object itself.

2 Techniques for surface finishing with application of foreign materials.

3 Techniques of micro material deposition (same or foreign) for surface finishing.

4 Surface modification by secondary processing.

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It can also be said that finishes are created by physical or chemical alteration of the surface, by adding new materials, or by removing a part of the original cover.

Many techniques of achieving surface finishes in use today are essentially the same as those employed in ancient times. These have been refined in terms of the tools used and rationalized in terms of procedures. Many processes are now highly mechanised saving time and energy, and some even are fully automatized, using robotics that allows faster, accurate and safer productions.

CUTTING: Cutting is the oldest of all techniques. Cutting is used for fast division of materials, and quicker removal of parts of materials (skinning, debarking, chopping, mining). Cutting is a crude, but primary technique of material finishing.

Wood cutting

CARVING: Carving is a controlled cutting technique, requiring use of a chisel and occasionally pounding by a hammer. Carving is associated with fine but soft grain materials like ivory, horn, bones and wood. Carving is also done to pliable metals like copper, silver-based alloys, and also hard but fracture-able materials like stones.

Quran inscriptions on wall of Lodhi Garden Delhi

ENGRAVING: Engraving is a delicate and shallower material removal technique than carving, using a chisel or sharp pointed tools with hand pressure, or very light pounding of the hammer. Engraving is today done by fine rotary tools similar to the dentist’s pneumatic drill. A computer controlled, diamond bit engraving is now also done to ultra thin materials. In Intaglio, or Gravure, printing, the image to be printed is etched or incised into the surface of the printing plate or cylinder. Gramophone records have engraved and embossed grooves.

 Glass engraving

CHASING: For chasing, the material is depressed or displaced by a fine tool as dots, small length linear-strokes or in continuous linear patterns. Wet ceramic pieces, and plastered-surfaces are pattern rendered by chasing. Braille writing on a thick paper sheet is a form of chasing. Engraving and chasing techniques are frequently used to provide a matt finish, onto normally very glossy stainless steel surfaces. Chasing techniques are also used for relieving as well as introducing stresses at the surface section, allow moisture and heat transfer, and improve the ductility of the material.

 The Treasure of Sânnicolau Mare in chasing technique

EMBOSSING: Embossing introduces a texture through pounding, beating or by pressing of the surface. The pressure may be applied from one face or both faces of a sheet, locally as spots, or continuously under a plate or roller, creating repeat patterns or random designs. Pounding or beating, compacts the surface-sections of the material, and thereby increases its density and integrity. Embossing techniques are used to reduce the gloss by matting the surface. Embossing is done to increase the thickness bulk of very thin surface materials and make them apparently stiffer. Synthetic fabrics and fibres are emboss-deformed and permanently set (perma-set and texturizing) through heat or chemical treatment.

 Archaeological museum ( Piombino ). Amphore of Baratti ( 4th C AD ).

REPOUSSE: Repousse is a method of raising a design in relief from the reverse side. The design is first drawn on the surface of the metal and the motifs outlined with a tracer, which transfers the essential parts of the drawing to the back of the plate. The plate is then embedded face down in an asphalt block and the portions to be raised are hammered down into the soft asphalt. Next the plate is removed and re embedded with the face uppermost. The hammering is continued, this time forcing the background of the design into the asphalt. By a series of hammering and re-embedding, followed finally by chasing, the metal sheet attains the finished appearance. There are three essential types of tools used: -for tracing, -for bossing, -for chasing. Ornaments in relief are also produced by mechanical means. A thin, pliable sheet of metal is pressed into moulds, between set of dies, or over the stamps. Embossed utensils of copper and brass, statuettes of gods formed of thin silver and gold plates are very much part of every Indian house. Today Aluminium craft pieces are similarly embossed and black anodized.

Traditional Indian Brass and copper utensils have hammered finish on the outside. The same is often copied on aluminium utensils but reducing the strength due to ‘cold working’ of the metal. Leather and paper surfaces are rolled embossed to create textured patterns. Timber veneered surfaces are pressed for texture creation. Rendering of a wet plaster face by variety of pressing and chasing tools is very common. Chasing is very common with copper and brass pots.

MATTING AND ETCHING: These are mainly used for creating textured surfaces. Matting is generally a mechanical technique compared to etching where a chemically active substance is used. Mating and etching, are also achieved by metal removal processes (reversing the metal deposition by changing the cathode charge) in the final stage of plating. Parallel, crossed, irregular, concentric, circular and other geometric configurations are carved or embossed on the surface. Line and spacing between them are often less than 1/100 part of a millimetre, depending on the compactness of the material mass. Another method of surface decoration is to impress it with repeating patterns of hatched lines (used on precious metals), thus matting or breaking up areas to contrast with other areas left polished and reflective.

ETCHING: Etching is usually done by an active substance that will either eat away part of the surface or change its colour quality. Acid and alkali treatments also provide etched surfaces. Etching is also an artwork technique. Glass surfaces are etched with Hydrofluoric acid.

SURFACE LEVELLING: Surface levelling is a major field of surface finishing. Surfaces are levelled by chipping away very thin sections off the surface. The material must have layered formation (e.g. layered stones, wood, bamboo, cane) or fracturable or brittle constitution (e.g. stones). Surfaces are ground and polished for a levelling.

GRINDING: Grinding removes material from the surface to roughen a normally glossy surface like glass, or polishes a rough surface like stone. Grinding requires material of higher hardness than the surface material, and is done by rubbing down with a graded series of coarse to fine abrasives, such as Carborundum, sandstone, emery, pumice, sand, glass and diamond powders. Where a material constitution permits, very fine grinding may polish the surface. Grinding is a cutting operation in which each grit that comes in contact with the material cuts out a minute chip, or swarf. Grinding wheels usually consist of particles of a synthetic abrasive, such as silicon carbide or aluminium oxide, mixed with a vitrified or resinoid bonding material. Grinding can be coarse or fine, depending on the size of the grit used in the grinding wheel. Metal and glass can be ground to a mirror finish and an accuracy of 0.0000025 cm.

Metal Grinding

POLISHING, HONING, LAPPING, BUFFING: Polishing uses extremely fine abrasive substances, such as jewellers rouge, Tripoli, whiting, putty powder and emery dust to rub or burnish an extremely smooth and brilliant finish on the surface of a material. The polishing materials are coated on the surface of cloth, felt, leather, rubber pr polymer wheels or as belts. Metal surfaces are levelled and finished by honing and lapping. Honing removes less than 0.0125 millimetres of material from the surface to eliminate micro scratches and machine marks from ground machine parts. It is done with bonded abrasive sticks or stones that are mounted in a honing head. Lapping is a process in which a soft cloth (wool, linen and chamois-leather) impregnated with abrasive pastes (rubbing compounds), is rubbed against the surface of a work-piece. Honing and lapping, are essentially metal finishing techniques.

BUFFING is a term used for polishing of metals. Buffing is done with polishing compounds and brushes of various shapes, and abrading materials, like: (animal hair, synthetic fibres, plant fibres -coir), flex, wool and leather. Barber polishing the razor on a leather stripe is a buffing process that levels out small nicks on the blade. Utensils are buffed for a polished surface.

LAPPING is used to produce a high-quality surface finish or to finish a workpiece within close size limits. Dimensional tolerances of 0.00005 millimetres can be achieved in the hand or machine lapping of precision parts such as gauges or gauge blocks.

SHAVING AND SPLITTING: Shaving is done to remove material’s components such as outward hair or fibres, layers, etc. Leather surfaces are shaved for thinning and to remove the surface hair. Leathers are also surface split to separate leather suitable for uppers and soles. The palm leaves are shaved to remove the stems and make them smoother for writing. Tree-barks are removed by axes and choppers to retard insect attack and increase moisture removal. Timbers are re-cut or planned with finer tools to achieve a smoother surface. Timbers are split very finely to create veneers. Wood planning is also a shaving technique. Carpets and rugs require close shearing by scissors to shave of protruding fibres.

 

SINGEING: singeing is a controlled burning (or a heat treatment) at the surface section to remove part of the material and to change the colour or texture properties of the surface. Singeing is both a process of surface finish and surface cleaning. Most of the organic materials can be surface-treated directly with fire or indirectly with high heat to achieve a singed or ironed effect. Textiles, paper, leather, leaves, wood, etc. are some materials that can be burnished. High temperature singeing removes the surface fibres and hair, and chars or burns (sinter) the top part of the surface, creating a burnt colour + texture effect. Textiles are Ironed, for de creasing or perma-set, or creased with pressurized heat treatment. Synthetics or composite textiles are selectively or locally singed to fuse the fibres or filaments, to create texturized effects, and also alter the transparency, opacity, etc. Wood surfaces on flame treatment attain a dehydrated or shrivelled-shrunk surface, similar to an old wood. Metal surfaces also singed not only to harden or anneal the top surface but to burn the oily residues, dehydrate, and descale the surface. Singed metal surfaces often attain peculiar colour and pattern effects.

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