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Gilded frame ready for burnishing with an agate stone tool
Application of gold leaf to a reproduction of a 15th-century panel painting
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Gilding is a decorative technique for applying a very thin coating of gold to solid surfaces such as metal (most common), wood, porcelain, or stone. A gilded object is also described as 'gilt'. Where metal is gilded, the metal below was traditionally silver in the West, to make silver-gilt (or vermeil) objects, but gilt-bronze is commonly used in China, and also called ormolu if it is Western. Methods of gilding include hand application and gluing, typically of gold leaf, chemical gilding, and electroplating, the last also called gold plating.[1]Parcel-gilt (partial gilt) objects are only gilded over part of their surfaces. This may mean that all of the inside, and none of the outside, of a chalice or similar vessel is gilded, or that patterns or images are made up by using a combination of gilt and ungilted areas.
Gilding gives an object a gold appearance at a fraction of the cost of creating a solid gold object. In addition, a solid gold piece would often be too soft or too heavy for practical use. A gilt surface also does not tarnish as silver does.
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Origins and spread[edit]
A gilded Tibetan Vajrasattva
Herodotus mentions that the Egyptians gilded wood and metals, and many such objects have been excavated. Certain Ancient Greek statues of great prestige were chryselephantine, i.e., made of gold (for the clothing) and ivory (for the flesh); these however, were constructed with sheets of gold over a timber framework, not gilded. Extensive ornamental gilding was also used in the ceiling coffers of the Propylaea. Pliny the Elder informs us that the first gilding seen at Rome was after the destruction of Carthage, under the censorship of Lucius Mummius, when the Romans began to gild the ceilings of their temples and palaces, the Capitol being the first place on which this process was used. But he adds that luxury advanced on them so rapidly that in very little time you might see all, even private and poor people, gild the walls, vaults, and other parts of their dwellings. Owing to the comparative thickness of the gold leaf used in ancient gilding, the traces of it that remain are remarkably brilliant and solid. Fire-gilding of metal goes back at least to the 4th century BC, and was known to Pliny (33,20,64–5), Vitruvius (8,8,4) and in the Early Mediaeval period to Theophilus (De Diversis Artibus Book III).
In Europe, silver-gilt has always been more common than gilt-bronze, but in China the opposite has been the case. The ancient Chinese also developed the gilding of porcelain, which was later taken up by the French and other European potters.
Processes[edit]
Modern gilding is applied to numerous and diverse surfaces and by various processes; those used in modern technology are described in gold plating. More traditional techniques still form an important part of framemaking and are sometimes still employed in general woodworking, cabinet-work, decorative painting and interior decoration, bookbinding, and ornamental leather work, and in the decoration of pottery, porcelain, and glass.
Mechanical[edit]
Regilding the statue Prometheus
Gilded page edges on a book.
Mechanical gilding includes all the operations in which gold leaf is prepared, and the processes to mechanically attach the gold onto surfaces. The techniques include burnishing, water gilding and oil-gilding used by wood carvers and gilders; and the gilding operations of the house decorator, sign painter, bookbinder, the paper stainer and several others.
Polished iron, steel and other metals are gilded mechanically by applying gold leaf to the metallic surface at a temperature just under red-hot, pressing the leaf on with a burnisher, then reheating when additional leaf may be laid on. The process is completed by cold burnishing.[2]
'Overlaying' or folding or hammering on gold foil or gold leaf is the simplest and most ancient method, and is mentioned in Homer's Odyssey (Bk vi, 232)[3] and the Old Testament. The Ram in a Thicket of about 2600–2400 BCE from Ur uses this technique on wood, with a thin layer of bitumen underneath to help adhesion.
Can I Paint Metal
The next advances involved two simple processes. The first involves gold leaf, which is gold that is hammered or cut into very thin sheets. Gold leaf is often thinner than standard paper today, and when held to the light is semi-transparent. In ancient times it was typically about ten times thicker than today, and perhaps half that in the Middle Ages.
If gilding on canvas or on wood, the surface was often first coated with gesso. 'Gesso' is a substance made of finely ground gypsum or chalk mixed with glue. Once the coating of gesso had been applied, allowed to dry, and smoothed, it was re-wet with a sizing made of rabbit-skin glue and water ('water gilding', which allows the surface to be subsequently burnished to a mirror-like finish) or boiled linseed oil mixed with litharge ('oil gilding', which does not) and the gold leaf was layered on using a gilder's tip and left to dry before being burnished with a piece of polished agate. Those gilding on canvas and parchment also sometimes employed stiffly-beaten egg whites ('glair'), gum, and/or Armenian bole as sizing, though egg whites and gum both become brittle over time, causing the gold leaf to crack and detach, and so honey was sometimes added to make them more flexible.
Other gilding processes involved using the gold as pigment in paint: the artist ground the gold into a fine powder and mixed it with a binder such as gum arabic. The resulting gold paint, called shell gold, was applied in the same way as with any paint. Sometimes, after either gold-leafing or gold-painting, the artist would heat the piece enough to melt the gold slightly, ensuring an even coat. These techniques remained the only alternatives for materials like wood, leather, the vellum pages of illuminated manuscripts, and gilt-edged stock.[citation needed]
Chemical[edit]
Silver gilt toilette set by Johann Jacob Kirstein (1733–1816) in the Musée des Arts décoratifs, Strasbourg
Chemical gilding embraces those processes in which the gold is at some stage of chemical combination. These include:
Cold[edit]
In this process the gold is obtained in a state of extremely fine division, and applied by mechanical means. Cold gilding on silver is performed by a solution of gold in aqua regia, applied by dipping a linen rag into the solution, burning it, and rubbing the black and heavy ashes on the silver with the finger or a piece of leather or cork.
Wet[edit]
Wet gilding is effected by means of a dilute solution of gold(III) chloride in aqua regia with twice its quantity of ether. The liquids are agitated and allowed to rest, to allow the ether to separate and float on the surface of the acid. The whole mixture is then poured into a separating funnel with a small aperture, and allowed to rest for some time, when the acid is run off from below and the gold dissolved in ether separated. The ether will be found to have taken up all the gold from the acid, and may be used for gilding iron or steel, for which purpose the metal is polished with fine emery and spirits of wine. The ether is then applied with a small brush, and as it evaporates it deposits the gold, which can now be heated and polished. For small delicate figures, a pen or a fine brush may be used for laying on the ether solution. The gold(III) chloride can also be dissolved in water in electroless plating wherein the gold is slowly reduced out of solution onto the surface to be gilded. When this technique is used on the second surface of glass and backed with silver, it is known as 'Angel gilding'.
Fire[edit]
Fire-gilding or Wash-gilding is a process by which an amalgam of gold is applied to metallic surfaces, the mercury being subsequently volatilized, leaving a film of gold or an amalgam containing 13 to 16% mercury. In the preparation of the amalgam, the gold must first be reduced to thin plates or grains, which are heated red-hot, and thrown into previously heated mercury, until it begins to smoke. When the mixture is stirred with an iron rod, the gold is totally absorbed. The proportion of mercury to gold is generally six or eight to one. When the amalgam is cold, it is squeezed through chamois leather to separate the superfluous mercury; the gold, with about twice its weight of mercury, remains behind, forming a yellowish silvery mass with the consistency of butter.
When the metal to be gilded is wrought or chased, it ought to be covered with mercury before the amalgam is applied, that this may be more easily spread; but when the surface of the metal is plain, the amalgam may be applied to it directly. When no such preparation is applied, the surface to be gilded is simply bitten and cleaned with nitric acid. A deposit of mercury is obtained on a metallic surface by means of quicksilver water, a solution of mercury(II) nitrate, the nitric acid attacking the metal to which it is applied, and thus leaving a film of free metallic mercury.
The amalgam being equally spread over the prepared surface of the metal, the mercury is then volatilized by a heat just sufficient for that purpose; for, if it is too great, part of the gold may be driven off, or it may run together and leave some of the surface of the metal bare. When the mercury has evaporated, which is known by the surface having entirely become of a dull yellow color, the metal must undergo other operations, by which the fine gold color is given to it. First, the gilded surface is rubbed with a scratch brush of brass wire, until its surface is smooth.
It is then covered with gilding wax, and again exposed to fire until the wax is burnt off. Gilding wax is composed of beeswax mixed with some of the following substances: red ochre, verdigris, copper scales, alum, vitriol, and borax. By this operation the color of the gilding is heightened, and the effect seems to be produced by a perfect dissipation of some mercury remaining after the former operation. The gilt surface is then covered over with potassium nitrate, alum or other salts, ground together, and mixed into a paste with water or weak ammonia. The piece of metal thus covered is exposed to heat, and then quenched in water.
By this method, its color is further improved and brought nearer to that of gold, probably by removing any particles of copper that may have been on the gilt surface. This process, when skillfully carried out, produces gilding of great solidity and beauty, but owing to the exposure of the workmen to mercurial fumes, it is very unhealthy. There is also much loss of mercury to the atmosphere, which brings extremely serious environmental concerns as well.
This method of gilding metallic objects was formerly widespread, but fell into disuse as the dangers of mercury toxicity became known. Since fire-gilding requires that the mercury be volatilized to drive off the mercury and leave the gold behind on the surface, it is extremely dangerous. Breathing the fumes generated by this process can quickly result in serious health problems, such as neurological damage and endocrine disorders, since inhalation is a very efficient route for mercuric compounds to enter the body. This process has generally been supplanted by the electroplating of gold over a nickelsubstrate, which is more economical and less dangerous.
Depletion[edit]
In depletion gilding, a subtractive process discovered in Pre-columbianMesoamerica, articles are fabricated by various techniques from an alloy of copper and gold, named tumbaga by the Spaniards. The surface is etched with acids, resulting in a surface of porous gold. The porous surface is then burnished down, resulting in a shiny gold surface. The results fooled the conquistadors into thinking they had massive quantities of pure gold. The results startled modern archaeologists, because at first the pieces resemble electroplated articles. Keum-boo is a special Korean technique of silver-gilding, using depletion gilding.
Ceramics[edit]
Buddha, 16th Century, gilt on wood.[4]The Walters Art Museum.
The gilding of decorative ceramics has been undertaken for centuries, with the permanence and brightness of gold appealing to designers. Both porcelain and earthenware are commonly decorated with gold, and in the late 1970s it was reported that 5 tonnes of gold were used annually for the decoration of these products.[5] Some wall tiles also have gold decoration.[6][7] Application techniques include spraying, brushing, banding machines, and direct or indirect screen-printing.[8] After application the decorated ware is fired in a kiln to fuse the gold to the glaze and hence ensure its permanence. The most important factors affecting coating quality are the composition of applied gold, the state of the surface before application, the thickness of the layer and the firing conditions.[9]
A number of different forms and compositions are available to apply gold to ceramic, and these include:[10][11]
- Acid-etched gilding – developed in 1860s at Mintons, Stoke-on-Trent, and patented in 1863. The glazed surface, usually a narrow border, is transfer printed with a wax-like resist, after which the glaze is etched with dilute hydrofluoric acid prior to application of the gold, after which the design's raised elements are selectively burnished to give a bright and matte surface; the process demands great skill and is used for the decoration only of ware of the highest class.[12]
- Bright Gold or Liquid Gold is a solution of gold sulphoresinate together with other metal resinates and a bismuth-based flux. It is particularly bright when drawn from the decorating kiln and so needs little further processing. This form of gilding was invented or at least improved by Heinrich Roessler. Rhodium compounds are used to improve the binding to the substrate.
- Burnish Gold or Best Gold is applied to the ware as a suspension of gold powder in essential oils mixed with lead borosilicate or a bismuth-based flux. This type of gold decoration is dull as taken from the kiln and requires burnishing, usually with agate, to bring out the colour. As the name suggests it is considered the highest quality of gold decoration. One solvent-free burnish gold composition was reported to consist of 10 to 40% gold powder, 2 to 20% polyvinylpyrrolidone, 3 to 30% an aqueous acrylate resin and 5 to 50% water.[13]
See also[edit]
- Gilding metal—imitation of gold
References[edit]
- ^Sloan, Annie (1996) Decorative Gilding, Collins & Brown, ISBN978-0-89577-879-6
- ^Alexander E. Youman, A Dictionary of Every-day Wants, p. 420, New York: Frank M. Reed, 1872.
- ^'And as when a man overlays silver with gold, a cunning workman whom Hephaestus and Pallas Athena have taught all manner of craft, and full of grace is the work he produces, even so the goddess shed grace upon his head and shoulders' from this translation
- ^'Buddha'. The Walters Art Museum.
- ^Hunt, L. B. (1979). 'Gold in the pottery industry'. Gold Bulletin. 12 (3): 116–127. doi:10.1007/BF03215112.
- ^Etris, S.F. (1982). 'Gold And Lustres For The Ceramic Tile Industry'. Ceramic Industries. 119 (5): 36.
- ^Abt, K. (2008). 'Comeback Of Gold Decoration? Trends And New Materials For Tile Decoration'. Keram. Z. 60 (1).
- ^Groh, E. (1995). 'Precious Metal Preparations: Composition, Applications And Special Decorative Effects'. Ceramic Forum International. 72 (3).
- ^Gerasimova, L. V.; Ivanova, V. M.; Peskova, E. Yu.; Druzhinin, E. V. (1991). 'Improving gold decorating techniques'. Glass and Ceramics. 48 (11): 535. doi:10.1007/BF00676649. S2CID135923083.
- ^Dodd, A.and Murfin, D. (1994) Dictionary Of Ceramics. The Institute Of Minerals.
- ^Rovinskaya, N. V.; Lapitskaya, E. V. (1998). 'Liquid gold and other components used in decoration of glazed porcelain and glass articles'. Glass and Ceramics. 55 (3–4): 98. doi:10.1007/BF03180905. S2CID136951654.
- ^Helena Hayward (ed.) (1960) The Connoisseur’s Handbook of Antique Collecting. Galahad Books, NY.
- ^'Burnish Gold Decorating Composition.' UK Pat.Appl.GB2216536 A, for Heraeus W.C., Gmbh.
This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). 'Gilding'. Encyclopædia Britannica. 11 (11th ed.). Cambridge University Press. pp. 13–14.
Further reading[edit]
- Carboni, Stefano; Whitehouse, David (2001). Glass of the sultans. New York: The Metropolitan Museum of Art. ISBN0870999869.
- Shretha, Sukra Sagar. 'Gold Gilding (A Traditional Craft in Kathmandu Valley).' Ancient Nepal – Journal of the Department of Archeology, Number 128–129, February–May 1992, pp. 5–9. [A detailed account of the complex traditional techniques of fire-gilding in Nepal.]
External links[edit]
Look up gilding in Wiktionary, the free dictionary. |
- Society of Gilders – art and science of gilding
Retrieved from 'https://en.wikipedia.org/w/index.php?title=Gilding&oldid=1008132138'
Paint is the most commonly used material to protect steel. Paint systems for steel structures have developed over the years to comply with industrial environmental legislation and in response to demands from bridge and building owners for improved durability performance. Previous five and six coat systems have been replaced by typically three coat alternatives, and the latest formulations have focussed on application in even fewer numbers of coats, but with increasing individual film thickness. Examples of this are epoxy and polyester glass flake coatings that are designed for high build thickness in one or two coat applications, and single coat high build elastomeric urethane coatings, up to 1000μm thick.
Modern specifications usually comprise a sequential coating application of paints or alternatively paints applied over metal coatings to form a ‘duplex’ coating system. The protective paint systems usually consist of primer, undercoat(s) and finish coats. Each coating ‘layer’ in any protective system has a specific function, and the different types are applied in a particular sequence of primer followed by intermediate / build coats in the shop, and finally the finish or top coat either in the shop or on site.
[top]Composition of paints and film formation
Paints are made by mixing and blending three main components:
(a) The pigments
Pigments are finely ground inorganic or organic powders which provide colour, opacity, film cohesion and sometimes corrosion inhibition.
(b) The binder
Binders are usually resins or oils but can be inorganic compounds such as soluble silicates. The binder is the film forming component in the paint.
(c) The solvent
Solvents are used to dissolve the binder and to facilitate application at the paint. Solvents are usually organic liquids or water.
Pigments are finely ground inorganic or organic powders which provide colour, opacity, film cohesion and sometimes corrosion inhibition.
(b) The binder
Binders are usually resins or oils but can be inorganic compounds such as soluble silicates. The binder is the film forming component in the paint.
(c) The solvent
Solvents are used to dissolve the binder and to facilitate application at the paint. Solvents are usually organic liquids or water.
Paints are applied to steel surfaces by many methods but in all cases this produces a 'wet film'. The thickness of the 'wet film' can be measured, before the solvent evaporates, using a comb-gauge. As the solvent evaporates, film formation occurs, leaving the binder and pigments on the surface as a 'dry film'. The thickness of the 'dry film' can be measured, usually with an electro-magnetic induction gauge. The relationship between the applied 'wet film' thickness and the final 'dry film' thicknesses (dft) is determined by the percentage volume solids of the paint, i.e. dft = 'wet film' thickness multiplied by the % vol. solids.
In general the corrosion protection afforded by a paint film is directly proportional to its dry film thickness.
- Paint constituents and their function
- Schematic diagram of a paint system
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[top]Classification of paints
Since, in the broadest terms, a paint consists of a particular pigment, dispersed in a particular binder, dissolved in a particular solvent then the number of generic types of paint is limited. The most common methods of classifying paints are either by their pigmentation or by their binder type.
Primers for steel are usually classified according to the main corrosion inhibitive pigments used in their formulation, e.g. zinc phosphate primers and metallic zinc primers, etc. Each of these inhibitive pigments can be incorporated into a range of binder resins giving for example, zinc phosphate alkyd primers, zinc phosphate epoxy primers, etc.
Intermediate and finish coats are usually classified according to their binders, e.g. epoxies, vinyls, urethanes, etc.
[top]Paint coatings
Paints are usually applied one coat on top of another and each coat has a specific function / purpose. These are described as follows.
[top]Primers
The primer is applied directly onto the cleaned steel surface or, in the case of duplex systems, the sealed metal coating. Its purpose is to wet the surface and to provide good adhesion for subsequently applied coats. For primers applied directly to steel surfaces, these are also usually required to provide corrosion inhibition. There are two basic types of primer.
(a) Primers pigmented with metallic elements anodic to steel
These primers are formulated so that, when a break in the coating (due to damage or local corrosion) exposes the steel substrate, the anodic metal corrodes sacrificially in preference to the steel. This effectively stifles steel corrosion and under-rusting of the primer until the anodic metal is exhausted. Zinc-rich primers are the most commonly used of this type.
These primers are formulated so that, when a break in the coating (due to damage or local corrosion) exposes the steel substrate, the anodic metal corrodes sacrificially in preference to the steel. This effectively stifles steel corrosion and under-rusting of the primer until the anodic metal is exhausted. Zinc-rich primers are the most commonly used of this type.
(b) Primers relying on the high adhesion and chemical-resistance properties of the binder
With these primers, good adhesion is obtained (provided that the surface is very thoroughly cleaned) and it is sufficient to prevent under-rusting at any break in the coating (due to damage). Two-pack epoxy primers are typical of this type. These primers may contain inhibitive pigments to interfere with the corrosion process. Zinc phosphate, for example, is a mildly inhibitive pigment and is widely used in modern primer formulations.
With these primers, good adhesion is obtained (provided that the surface is very thoroughly cleaned) and it is sufficient to prevent under-rusting at any break in the coating (due to damage). Two-pack epoxy primers are typical of this type. These primers may contain inhibitive pigments to interfere with the corrosion process. Zinc phosphate, for example, is a mildly inhibitive pigment and is widely used in modern primer formulations.
[top]Intermediate coats
Intermediate coats are applied to ‘build’ the total film thickness of the system. Generally, the thicker the coating the longer the life. Intermediate coats are specially designed to enhance the overall protection and, when highly pigmented, decrease permeability to oxygen and water. The incorporation of laminar pigments, such as micaceous iron oxide (MIO), reduces or delays moisture penetration in humid atmospheres and improves tensile strength. Modern specifications now include inert pigments such as glass flakes to act as laminar pigments. Undercoats must remain compatible with finishing coats when there are unavoidable delays in applying them.
[top]Finish coat
The finish coat provides the required appearance and surface resistance of the system. Depending on the conditions of exposure, it must also provide the first line of defence against weather and sunlight, open exposure, and condensation.
[top]Stripe coats
Stripe coats are additional coats of paint that are applied locally to welds, fasteners and external corners. Their function is to build a satisfactory coating thickness at edges and corners where paint has a tendency to contract and thin upon drying. Specifications should indicate the type and number of stripe coats required and state when they are to be applied.
[top]The paint system
The various superimposed coats within a painting system have, of course, to be compatible with one another. They may be all of the same generic type or may be different, e.g. chemical resistant types, such as a recoatable polyurethane finish coat, may be applied onto epoxy primer and intermediate coats. However, as a first precaution, all paints within a system should normally be obtained from the same manufacturer and used in accordance with the manufacturer’s recommendations.
An important factor in the coating system is the definition and measurement of the dry film thickness (dft). My beloved world sonia sotomayor. Dry film thicknesses are generally checked on the complete paint system, although individual films may be checked separately. Usually, nominal dry film thicknesses are specified but sometimes minimum values are quoted.
For nominal dry film thicknesses, individual values less than 80% of the nominal thickness are not acceptable. Values between 80% and 100% are acceptable provided that the overall average (mean value) is equal to or greater than the nominal.
Specifications for minimum dry film thicknesses require careful paint application to avoid excessive film thickness. The ‘over application’ of paints can result in the formation of high stresses and may cause premature failure of the system. Wet film thickness (wft) checks may also be required during the application of the coating to check that a subsequent satisfactory dry film thickness will be achieved.
[top]Main generic types of paint and their properties
(a) Air drying paints
For example alkyds
These materials dry and form a film by an oxidative process, which involves absorption of oxygen from the atmosphere. They are therefore limited to relatively thin films. Once the film has formed it has limited solvent resistance and usually poor chemical resistance.
For example alkyds
These materials dry and form a film by an oxidative process, which involves absorption of oxygen from the atmosphere. They are therefore limited to relatively thin films. Once the film has formed it has limited solvent resistance and usually poor chemical resistance.
(b) One pack chemical resistant paints
For example acrylated rubbers, vinyls
For these materials, film formation requires only solvent evaporation and no oxidative process is involved. They can be applied as moderately thick films though retention of solvent in the film can be a problem at the upper end of this range. The formed film remains relatively soft and has poor solvent resistance but good chemical resistance. Bituminous paints also dry by solvent evaporation. They are essentially solutions of either asphaltic bitumen or coal-tar pitch in organic solvents.
For example acrylated rubbers, vinyls
For these materials, film formation requires only solvent evaporation and no oxidative process is involved. They can be applied as moderately thick films though retention of solvent in the film can be a problem at the upper end of this range. The formed film remains relatively soft and has poor solvent resistance but good chemical resistance. Bituminous paints also dry by solvent evaporation. They are essentially solutions of either asphaltic bitumen or coal-tar pitch in organic solvents.
(c) Two pack chemical resistant paints
For example epoxy, urethane
These materials are supplied as two separate components, usually referred to as the base and the curing agent. When these two components are mixed, immediately before use, a chemical reaction occurs. These materials therefore have a limited 'pot life' before which the mixed coating must be applied. The polymerisation reaction continues after the paint has been applied and after the solvent has evaporated to produce a densely cross linked film which can be very hard and has good solvent and chemical resistance. Liquid resins of low viscosity can be used in the formulation thereby avoiding the need for a solvent. Such coatings are referred to as 'solvent less' or 'solvent free' and can be applied as very thick films.
For example epoxy, urethane
These materials are supplied as two separate components, usually referred to as the base and the curing agent. When these two components are mixed, immediately before use, a chemical reaction occurs. These materials therefore have a limited 'pot life' before which the mixed coating must be applied. The polymerisation reaction continues after the paint has been applied and after the solvent has evaporated to produce a densely cross linked film which can be very hard and has good solvent and chemical resistance. Liquid resins of low viscosity can be used in the formulation thereby avoiding the need for a solvent. Such coatings are referred to as 'solvent less' or 'solvent free' and can be applied as very thick films.
Binder | System cost | Tolerance of poor surface | Chemical resistance | Solvent resistance | Water resistance | Overcoating after aging | Comments |
---|---|---|---|---|---|---|---|
Black Coatings (based on Tar products) | Low | Good | Moderate | Poor | Good | Very good with coatings of same type | Limited to black or dark colours. May soften in hot conditions. |
Alkyds | Low – Medium | Moderate | Poor | Poor – Moderate | Moderate | Good | Good decorative properties. High solvent levels. |
Acrylated Rubbers | Medium – High | Poor | Good | Poor | Good | Good | High build films that remain soft and are susceptible to sticking. |
Epoxy (Surface Tolerant) | Medium – High | Good | Good | Good | Good | Good | Can be applied to a range of surfaces and coatings. |
Epoxy (High performance) | Medium – High | Very Poor | Very Good | Good | Very Good | Poor | Susceptible to ‘chalking’ in U.V. light. |
Urethane and Polyurethane | High | Very Poor | Very Good | Good | Very Good | Poor | Can be more decorative than epoxies. |
Organic Silicate and Inorganic Silicate | High | Very Poor | Moderate | Good | Good | Moderate | May require special surface preparation |
[top]Prefabrication primers
Also referred to as blast primers, shop primers, temporary primers, holding primers, etc., these primers are sometimes used on structural steelwork, immediately after blast cleaning, to maintain the reactive blast cleaned surface in a rust free condition until final painting can be undertaken. They are mainly applied to steel plates and sections before fabrication. The main requirements of a prefabrication primer are as follows:
Stainless Steel Paint Brush
- The primer should be capable of airless spray application to produce a very thin even coating. Dry film thickness is usually limited between 15-25μm. Below 15μm, the peaks of the blast profile are not protected and 'rust rashing' occurs on weathering. Above 25μm, the primer affects the quality of the weld and produces excessive weld fume.
- The primer must dry very quickly. Priming is often done in-line with automatic blast cleaning plant which may be handling plates and/or sections at a pass rate of between 1-3 metres/minute. The interval between priming and handling is usually of the order of 1-10 minutes and hence the primer film must dry within this time.
- Normal fabrication procedures (e.g. welding, gas cutting) must not be significantly impeded by the coating, and the primer should not cause excessive weld porosity. A welding certificate should be available from the paint manufacturer.
- Weld fume omitted by the primer must not exceed the appropriate Occupational Exposure Limits. Proprietary primers are tested and certified by the Newcastle Occupational Health Agency. A health and safety certificate should be available from the paint manufacturer.
- The primer coating should provide adequate protection until either overcoated or placed within a non corrosive environment e.g. indoor storage or within a building construction. In practice, such periods are rarely met except in the least arduous conditions, e.g. indoor storage. In aggressive conditions, durability can often be measured in weeks rather than months. Zinc rich and zinc silicate primers provide the highest order of protection of all prefabrication primers.
Steel Paintbrush Hamilton Mt
- The primed surface, after weathering, should require the minimum of re-preparation for subsequent painting and must be compatible with the intended paint system. Many proprietary prefabrication primers are available but they can be classified under the following main generic types:
[top]Etch primers
These are based on polyvinyl butyral resin reinforced with a phenolic resin to increase water resistance. These primers can be supplied in a single pack or two pack form, the latter providing better durability.
[top]Epoxy primers
These are two pack materials utilising epoxy resins and usually have either polyamide or polyamine curing agents. They are pigmented with a variety of inhibitive and non-inhibitive pigments. Zinc phosphate epoxy primers are the most frequently encountered and give the best durability within the group.
[top]Zinc epoxy primers
These primers can be either zinc rich or reduced zinc types. Zinc rich primers produce films which contain about 85% by weight of metallic zinc powder and the reduced zinc type as low as 55% by weight. When exposed in either marine or highly industrial environments, zinc epoxy primers are prone to the formation of insoluble white zinc corrosion products which must be removed from the surface before subsequent overcoating.
[top]Zinc silicate primers
Zinc silicate primers produce a level of protection which is comparable with the zinc rich epoxy types and they suffer from the same drawbacks, e.g. formation of zinc salts and production of zinc oxide fume during welding. There are currently different categories of zinc silicate primers based upon the binder (organic or inorganic) and the zinc content. Low zinc primers in this group have been developed to improve their weldability and to minimise weld porosity, however their durability is also reduced. The organic silicate primers are the most suitable as prefabrication primers.
[top]Application of paint coatings
[top]Methods
The method of application and the conditions under which paints are applied have a significant effect on the quality and durability of the coating. Standard methods used to apply paints to structural steelwork include application by brush, roller, conventional air spray and airless spray.
Airless spraying has become the most commonly used method of applying paint coatings to structural steelwork under controlled shop conditions. Brush and roller application are more commonly used for site application, though spraying methods are also used.
[top]Brushing
This is the simplest method and also the slowest and therefore most expensive. Nevertheless it has certain advantages over the other methods, e.g. better wetting of the surface and can be used in restricted spaces, be useful for small areas, with less wastage and contamination of surroundings.
Brush application of paint
(Video courtesy of Corrodere/MPI)
(Video courtesy of Corrodere/MPI)
[top]Roller
This process is much quicker than brushing and is used for large flat areas, but demands suitable rheological properties of the paint.
Airbrushing Metal
Roller application of paint
(Video courtesy of Corrodere/MPI)
(Video courtesy of Corrodere/MPI)
[top]Air spray
Paint coatings for structural steelwork are usually spray applied. The paint is atomised into fine droplets and projected onto the surface to be protected where the droplets join together to form a continuous film. The atomisation can be accomplished in a number of ways.
In air spraying, the paint is atomised by mixing it with a stream of compressed air in a conventional spray gun. The paint can be either sucked into the air stream (as in the simple suction-cup gun used for application to small areas) or fed to the spray gun under pressure from a pressure pot. For ideal application, careful adjustments of the spray nozzle and air pressures must be made by a skilled operator, according to the consistency and composition of the paint product and the film thickness required. The application rates for air spray are quicker than for brushing, or rolling, however paint wastage by overspray is high.
[top]Airless spray
Airless spray application of paint
(Image courtesy of Mabey Bridge Ltd.)
(Image courtesy of Mabey Bridge Ltd.)
For airless spraying, the paint is hydraulically compressed and, on release through a small orifice in an airless spray gun, it is atomised and projected onto the surface. By changing the orifice size and shape and by varying the hydraulic pressure, atomisation can be accomplished for a wide range of paint consistencies from thin to thick, to give a wide range of rates of deposition. The equipment required is much more expensive than for conventional air spraying, because it must withstand the much higher pressures involved. However, the application rates are higher than for air spray with overspray wastage greatly reduced. For conventional air spraying, the maximum air pressure will normally not exceed 100 psi (6.9 bar); for airless spraying, hydraulic pressures up to 4,000 psi (280 bar) may be required.
A variant of the above involves heating to reduce the consistency of the paint rather than adding diluents. In this way greater film thickness per application is achieved. This method can be used for the application of solvent-free materials such as two-pack products, which can be mixed at the spray gun nozzle at the moment of application. The use of expensive equipment and highly skilled labour is necessary for the achievement of optimum results but may be justified for the protection of large and important structures.
Airless paint spraying
(Video courtesy of Corrodere/MPI)
(Video courtesy of Corrodere/MPI)
As in Levi's book, a major theme of Eyes of the Tailless Animals (and indeed, the source of the book's title) is how a labor camp actively erodes a person's humanity, and how survivors of such an experience fought this erosion every single day of their lives in the camp. Naturally tailless dogs.
[top]Conditions of application
The principal conditions that affect the application of paint coatings are temperature and humidity. These can be more easily controlled under shop conditions than on site.
(a) Temperature
Air temperature and steel temperature affect solvent evaporation, brushing and spraying properties, drying and curing times and the pot life of two-pack materials, etc. Where heating is required, this should only be by indirect methods.
Air temperature and steel temperature affect solvent evaporation, brushing and spraying properties, drying and curing times and the pot life of two-pack materials, etc. Where heating is required, this should only be by indirect methods.
(b) Humidity
Paints should not be applied when there is condensation present on the steel surface or the relative humidity of the atmosphere is such that it will affect the application or drying of the coating. Normal practice is to measure the steel temperature with a contact thermometer and to ensure that it is maintained at least 3°C above the dew point.
Paints should not be applied when there is condensation present on the steel surface or the relative humidity of the atmosphere is such that it will affect the application or drying of the coating. Normal practice is to measure the steel temperature with a contact thermometer and to ensure that it is maintained at least 3°C above the dew point.
However, moisture cured paints are available. These paints are specifically formulated for application in damp and humid conditions; reference should be made to the manufacturer’s data sheets for details of limiting conditions of application.
[top]Coating applicator training and certification (ICATS)
With modern high performance coatings, correct application has become increasingly important to achieve the intended performance. Industry has recognised this and established a training and certification scheme for paint applicators (ICATS – Industrial Coating Applicator Training Scheme). ICATS registration (or to an equivalent scheme) has subsequently become a mandatory requirement for work on Highways England and Network Rail bridges.
ICATS is owned by the Institute of Corrosion and operated through Correx Ltd., a wholly owned subsidiary of the Institute. The scheme has a mandatory Basic Unit 'Industrial Coating Applicator' that comprises 6 modules:
- Health & Safety
- Site Access
- Plant & Equipment
- Paint Types & Application
There are also 2 optional specialist modules:
- Spray Painting
ICATS is a registered training scheme that meets the requirements of NHSS 19A[1] and enables contractors to provide 'best value' service to maximise coating performance over structure life and to reduce whole life costs.
[top]Additional education
For engineers and inspectors wishing to further their knowledge of corrosion control in the context of structural steelwork, there are web based training programmes available from a specialist training provider, Corrodere:
- Traintheinspector
- Trainthecoatingspecialist
Trainthepainter is an internationally recognised and certified training programme that provides trainee coating applicators with a comprehensive package of training modules that ensure professionalism and give an insight into methods of surface treatment and application of protective coatings. As with ICATS, this is a registered training scheme that meets the requirements of NHSS 19A[1].
[top]Relevant Steel for Life sponsors
This website is maintained by Steel for Life with funding provided by a number of BCSA’s Industry Members. Those sponsors relevant to this article are as follows:
Silver
Silver
Stainless Steel Paint Brush Cleaner
Bronze
[top]References
- ↑ 1.01.1NHSS 19A: National Highways Sector Schemes for Quality Management in Highway Works, 19A. For corrosion protection of ferrous materials by industrial coatings. Issue 6 (9001:2015), 30 June 2020
[top]Resources
- Hendy, C.R.; Iles, D.C. (2015) Steel Bridge Group: Guidance Notes on best practice in steel bridge construction (6th Issue). (P185). SCI
- Steel Buildings, 2003, The British Constructional Steelwork Association Ltd.
- Chapter 12 – Corrosion Protection
[top]Further reading
- D.Deacon & R.Hudson (2012), Steel Designer’s Manual (7th Edition), Chapter 36 - Corrosion and corrosion prevention, The Steel Construction Institute.
- D.A. Bayliss & D.H.Deacon (2002), Steelwork Corrosion Control (2nd edition), Spon Press.
[top]See also
[top]External links
[top]CPD
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