Coating

Coated wood soldier
MFA# 55.801

Description

A thin layer deposited or applied on the surface of an object (substrate). The primary purpose of applying a coating is to improve an objects critical properties and to create a protective barrier against deterioration of the surface. Secondary purposes may be aesthetic. Coatings may be applied as liquids, gases or solids e.g. powder coatings.

Examples of the types of materials are used for coatings include:

• Fabrics, Metals, Paints, Varnishes and Polymer films.

Often coatings are used to achieve multiple goals. For example, coatings can be formulated to:

• Seal a surface, form a barrier, change color, produce or minimize texture, maintain surface appearance, be anti-corrosive, be anti-reflective, decrease friction, increase wearability, minimize light transmission, provide adhesion, prevent adhesion (non-stick), provide flame resistance, and to provide identification.

A major consideration for most coating processes is to control coating thickness and adhesion. Methods of achieving this range from a simple brush to expensive precision machinery.

• Application processes include: draping, brushing, rolling, dipping, spraying, baking, shrink-wrapping, vapor deposition, electroplating, anodizing, printing and laminating.

Synthetic coatings	Process	Application	Advantages	Disadvantages
Acrylic	Premixed solutions of acrylic polymer bits dispersed in water; cures by drying, multiple options for clear or colored coatings.	May emit acetic acid vapors (allow 4 weeks cure); cleans with soap and water while wet	Cost effective, dry quickly easy to use, resists weathering and fading, environmentally friendly	Surface must be well-prepared for optimal adhesion
Alkyd	Oil-modified polyester that cures through oxidation	May release peroxides and carboxylic acids; cleans with solvent based products	Glossy finish, robust; resistant to weathering and wear; glossy finish	Slow drying, strong odor during application
Ceramic coating	Resin containing silica and titanium dioxide, applied using a spray gun, that chemically bonds to paint	Primarily used as a high gloss top coat for autos	Improves durability and wear resistance; has low surface tension;	Cannot be used directly on a substrate; provides little protection from tensile or compressive stress
Epoxy	Two component solution (epoxy resin and hardener) carefully mixed prior to used; water-based options available	Careful mixing and curing required	Excellent for metal surfaces, high moisture resistance; resistant to corrosion and damage	Surface must be well-prepared; coatings are sensitive to UV light and temperature; low flexibility
Laminate	A multilayer material or a film used to provide a protective layer	Plastic or aluminum sheets adhered to the substrate	Provide a barrier to moisture or volatile organic	The film must be uniformly sealed without any holes or gaps
Polyaspartic	Polyurea coatings formed by reaction with polyisocyanates and aspartic esters	Buildings, bridges, automobiles	Cures rapidly; highly durable, resistant to weathering and wear; maintains high gloss, does not yellow	Surface must be well-prepared to ensure adhesion
Polyester	Synthetic resin that hardens with a catalyst		Strong, durable surface; withstands abrasion, weathering and wear; flexibility minimizes cracking	Application requires precise control over temperature and humidity to fully cure.
Polyurethane (aliphatic)	Pre-mixed solution of a polyol with an isocyanate; cures with moisture in air; water-based options available	Must be cured completely at optimal conditions of moisture and temperature; isocyanate is a carcinogen	Often used as a top coat, highly durable, resistant to wear and weather; excellent gloss and color retention; low VOC formulations available
Powder Coating	An electrostatic process to adhere powder-based substance onto a charged metal surface followed by heat curing.	Metals, household appliances, automobile parts	Coatings are durable with many color options; no solvents are used thus no VOCs emissions; typically used on metals	Requires spray booths and ovens, difficult to achieve thin coatings
Silicone	Oligosiloxanes dispersed in water that dry into a flexible silicone resin	Spray, dip or paint; clean with soap and water; does not need primer; non toxic	Heat and moisture resistant, good adhesion, highly flexible, may be used on multiple surfaces even fabric and concrete	Requires up to 48 hours to fully cure; heat and UV light can speed curing
Polysiloxane	Option that combines silicone and polyurethane properties, without using isocyanates	Applied directly to primed steel or previously painted substrates by brush, roller or spray; low VOCs	Durable, glossy finish; highly resistant to UV, weather, and chemicals; retains color and gloss	Surface must be well-prepared to ensure adhesion; low flexibility

Natural coatings	Examples	Application	Advantages	Disadvantages
Drying oil (unbaked)	Linseed oil, Tung oil, Tall oil, Poppyseed oil, Perilla oil; Sunflower oil; Hempseed oil; Safflower oil; Lumbang oil; Oiticica oil; Stillingia oil; Walnut oil	After application to a substrate, drying oils auto-oxide in ambient condition forming a thin non-tacky tough film. The curing time for each type of oil is depentent on its degree of unsaturation. Additives can speed the drying time. A solvent is required for cleanup.	Efficient provides antifungal and antibacterial properties, water repellancy, protection from UV radiation.Typically used for wood protection and preservation	Known to release peroxides for the first few days and carboxylic acids for many weeks or many months. Many oils will discolor with age, especially with exposure to UV light; difficult to remove
Carbohydrate	Starch, Cellulose, Pectin, Gums, Agar, Carrageenan,	Manufactured by plants, most of these materials swell in water to form a gel that dries by releasing water; Additives, such as lignin, can decrease biological attacks	Most are typically used as adhesives, consolidants, emulsifiers, and thickeners; Films are thin, flexible and matte; Non-toxic; Biodegradable.	Films usually shrink on drying and are not optically transparent; they are susceptible to biological attack; Rarely used as a durable coating
Mineral	Whitewash (Calcium carbonate/Calcium hydroxide); Water glass (Potassium silicate, Sodium silicate, or Calcium silicate)	Appropriate for concrete, stucco, brick, plaster or any adobe-type of surfaces; Some have been used on glass, porcelain, leather and textiles; Brushes and rollers, while still wet, should clean easily with water.	Coatings typically penetrate and fill pores and cracks; good durability; Non-flammable providing passive fire protection	Process is not reversible
Protein	Casein, Collagen, Gelatin, Albumin (egg white), Keratin, Whey, Soy protein, Zein, Animal glue	Ensure formula does not contain a drying oil in the formula, Brushes and rollers, while still wet, should clean easily with water.	Suitable for most objects and outside of case; Non toxic; Biodegradable; Most often used as adhesive and not a coating	Most are hydrophilic; Hot water will cause breakdown to smaller components; Additives are need to produce a durable film;
Resin (Varnish)	Shellac, Dammar, Mastic, Sandarac, Rosin, Amber, Copal, Myrrh	Resins from coniferous plants contain terpenes and derivatives that are soluble in hydrocarbon solvents (such as turpentine) and sometimes in alcohol; drying occurs with evaporation of solvent; Solvent required for clean-up	Suitable in enclosures after complete drying; Avoid direct contact with object in a humid environment as surface may become sticky; Safe for most collections;	Poor vapor barrier unless many layers are used; dry film becomes sticky in moist environments; Solvent is the main VOC; May discolor with age especially with exposure to UV light.
Wax (plant or mineral)	Beeswax, lanolin, spermaceti, lac wax, carnauba, candelilla, bayberry, Japan wax, paraffin, ceresin, Ozocerite, montan mineral wax	Waxes are thermoplastic and melt at low temperatures (between 40 and 100C). soluble in organic solvents; As a coating, they often need repeated depositions for a noticeable layer can gradually build up.	Easy to apply;produce a smooth, glossy surface, they are hydrophobic and are often used on wood

Metal Coatings	Process	Substrates	Advantages	Disadvantages
Anodized	Formation of protective oxide layer; immersion in electrolytic solution	Non-ferrous metals, primarily aluminum	Long-lasting, chemically stable,, non-toxic, no dangerous by-products	Unsuitable for ferrous metals; limited options for colors
Galvanized	Immersion of metal in molten zinc bath to produce protective zinc carbonate layer	Steel, bolts and nuts, air ducts, trash cans	Highly stable, adheres tightly; zinc coating actively covers scratches, cuts and dents after they occur	Expensive for large items; color is dull gray
Electroplated (electrodeposition)	Deposition of a thin layer of one metal on the surface of another; both metal are placed in electrolytic solution	Copper, nickel, gold, silver, chrome, zinc tin, jewelry, ornaments;	Excellent corrosion resistance; often increases in mechanical properties, can produce aesthetic appearance	Expensive; produces non-uniform coating thicknesses; potentially toxic; solutions contain environmentally harmful chemicals

Synonyms and Related Terms

coat; surface coat; Anstrich (Deut.); enduit (Fr.); revêtement (Fr.); recubrimiento (Esp.); rivestimento (It.); revestimento (Port.)

Collection Risks

(from Tetreault 2017) "Coatings formed by oxidative polymerization, such as oil-based paints, oil-based urethanes, alkyds, melamine and epoxy esters (one-component or one-part systems), should be avoided in museums because they release acids and peroxides that can harm objects, especially papers, metal objects (lead, copper and their alloys) and black and white photographs. Fortunately, with new volatile organic compound (VOC) regulations in Canada and the United States, these coatings are becoming less and less available. The coatings that are generally acceptable in archival and museum contexts are acrylic or acrylic-urethane emulsion paints (latex) and, for special applications, two-part epoxy and two-part urethane paints."

Resources and Citations

• Jean Tétreault, Products Used in Preventive Conservation CCI, December 2017. Link
• Corrosionpedia: Coatinghttps://chansmachining.com/types-of-industrial-coating/ Accessed October 2024
• Wikipedia: Coating Accessed Oct 2024
• Chans Machining: Industrial coatings Accessed October 2024
• Richard S. Lewis, Hawley's Condensed Chemical Dictionary, Van Nostrand Reinhold, New York, 10th ed., 1993
• ASTM, "Standard Terminology Relating to Paint, Varnish, Lacquer and Related Products", Annual Book of ASTM Standards, Section 6, Paints, Related Coatings and Aromatics, ASTM, D16, 7-Jan, Jul-96
• Random House, Webster's Encyclopedic Unabridged Dictionary of the English Language, Grammercy Book, New York, 1997
• The American Heritage Dictionary or Encarta, via Microsoft Bookshelf 98, Microsoft Corp., 1998
• Art and Architecture Thesaurus Online, http://www.getty.edu/research/tools/vocabulary/aat/, J. Paul Getty Trust, Los Angeles, 2000