Coatings accomplish several functions when used in conjunction with wooden objects. First, and probably foremost in terms of the fabricator’s intent, is that coatings alter the appearance of the surface. That is, coatings serve some aesthetic purpose. Second, coatings offer protection to the object’s surface (spills, abrasion, etc.) and structure (relative humidity [RH] shifts and ensuing dimensional-change-caused deterioration). Finally coatings provide scholars, including conservators and historians, information regarding the practices and technologies of the past.
As with other considerations when evaluating historic artifacts of all kinds, these factors must be integrated with the precept that all materials which exist as part of an object contribute to the integrity and uniqueness of that object. Both historic and contemporary fabricators of wooden objects generally consider(ed) coatings, particularly “non decorative,” to be a potentially sacrificial element of the whole. However, for the reasons enumerated above, conservators do not consider any portion of an object to be routinely expendable, including finishes and coatings.
Contrary to the “strip and dip” approach to dealing with coatings so prevalent in many commercial refinishing and restoration shops, conservators and sensible restorers attempt to preserve the coating on the object whenever possible. This is not to suggest that coatings are never altered or replaced in the course of conducting a conservation treatment on an object. In many cases this intrusion must take place, but the ideal is to intercede minimally and to leave the surface as undisturbed as possible while assuring stability for, and preservation of the artifact. Preserving finishes and treating degraded coatings requires a broad base of knowledge and diverse skills, including the knowledge of coating materials and their deterioration, as well as craft skills necessary to manipulate the films, whether in preserving existing films or applying new coatings.
The manifestation of coating deterioration depends entirely on the kind of film forming materials used, additives used to modify that material, and the various environment conditions to which the coating has been subjected. A beginning point in the treating and preserving of coatings is the most specific description of the coating system possible. Coating systems can range from simple single component applications to sophisticated preparations applied in an exacting and complex procedure.
This will be an overview of the field of coatings and their preservation as a whole. Any of the areas of discussion here, coating materials and techniques, coating deterioration, and treatment of degraded coatings, could and have filled volumes. In addition to existing literature, dozens of conservators and other scholars are continuing to prepare articles, monographs and books on the subject of furniture coatings. With that in mind the reader is reminded of the superficial nature of the information presented here (in general), with particular emphasis on the temporal pertinence of the treatment section. The conservation treatment of damaged furniture finishes is a relatively new discipline, and it is likely that much of the framework discussed for such treatments will be superceded by new techniques and approaches in relatively short order.
The scope of materials used to form finishes on furniture and wooden objects is a broad one encompassing ingredients from several categories. The most general distinction delineating coatings is whether they are transparent (varnishes), opaque (paint or polychromy) or metallic (leaf). Among these are waxes, gums, and oils, plus natural and synthetic resins, all of which can be used as transparent coatings. By the addition of dyes or pigments to transparent materials, paints are formulated. Paints are also formed by the addition of colorants to liquids which are not usually employed as transparent coatings, such as casein or “milk paint.” There are also coatings of applied metal sheet, such as gold and silver leaf, which are adhered to a wood or mineral substrate with protein or resinous binders. Finally, there are film formers which do not fit neatly into any of the aforementioned categories, such as urushiol, or Oriental lacquer, which is a reactive latex with vague similarities to both oils and resins, and which can be used as a transparent or opaque coating. Within the organization of this document urushiol is included with natural resins.
A second broad category of definition for coating materials involves the drying mechanism of the film, which in turn may reveal chemical and physical properties such as solubility, thermoplasticity, rheology, and others. Drying mechanisms are separated into two broad categories; solvent release and polymerization. In solvent release or “spirit varnish” systems the coating film is a residue remaining after the evaporation of the volatile solvent from a solution containing the solvent and the involatile film former. Polymerizing or “reactive varnish” systems harden rather than dry. This is accomplished by the polymerization of the mono or oligomeric constituents of the formulation. In most cases the exact chemical reaction process of the polymerization is relatively unimportant for either the creating craftsman or the conservator/restorer.
In addition to film forming materials, coating formulations frequently contain additional materials to enhance certain properties of the coating. These additives can alter working characteristics of a liquid coating during application, or visual or physical properties of the dried films. Common additives to coating systems are solvents, plasticizers, gloss suppressants, retarders, colorants, and chemical degradation inhibitors.
There are several ways of organizing reviews of materials. Here, the information about the coatings is loosely grouped according to the respective material’s functionality as a coating on wood. It must also be noted that many, if not most formulations of coatings for wooden objects contain components from several of the groupings. Reviews of coating materials by chemical constituency can be found in several of the references listed after the conclusion.
Original article posted on Smithsonian Institute