Paint layer

Optical Techniques. The most important tool in a museum laboratory is the low power stereomicroscope. This instmment, usually used at magnifications of 3—50 x, has enough depth of field to be useful for the study of surface phenomena on many types of objects without the need for removal and preparation of a sample. The information thus obtained can relate to toohnarks and manufacturing techniques, wear patterns, the stmcture of corrosion, artificial patination techniques, the stmcture of paint layers, or previous restorations. Any art object coming into a museum laboratory is examined by this microscope (see Microscopy Surface and interface analysis). 

Structural Analysis. Some of the optical techniques are also used for stmctural analysis. Microscopic examinations of metallurgical cross sections or of sections through the paint layers of a painting are indeed stmctural examinations, as is ir reflectography. 

Paintings. Paintings are composed of a wide variety of materials. However, for the purposes herein, paintings may be characterized as a group by the paint layers which produce the image. 

Technological History (26,54—61). As a first approach, there are three groups of components supports, paint media, and pigments. The support is the substrate upon which the paint layers are laid down. This can be a specially prepared area on a wall for a wall painting, a wooden panel as in a panel painting, or a fabric in canvas paintings. Paper is a prevalent support in Oriental painting. Other supports are encountered less frequently, eg, metal panels such as copper sheet. 

A varnish is often appHed on top of the paint layers. A varnish serves two purposes as a protective coating and also for an optical effect that enriches the colors of the painting. A traditional varnish consists of a natural plant resin dissolved or fused in a Hquid for appHcation to the surface (see Resins, natural). There are two types of varnish resins hard ones, the most important of which is copal, and soft ones, notably dammar and mastic. The hard resins are fossil, and to convert these to a fluid state, they are fused in oil at high temperature. The soft resins dissolve in organic solvents, eg, turpentine. The natural resin varnishes discolor over time and also become less soluble, making removal in case of failure more difficult (see Paint and FINNISH removers). Thus the use of more stable synthetic resins, such as certain methacrylates and cycHc ketone resins, has become quite common, especially in conservation practice. 

A close inspection under normal illumination reveals many indications of the condition of the painting and previous repairs. Also, because oil paints become more transparent with age, pentimenti, which originally would have been invisible after the overpainting, can be observed. Raking light illumination is very useful to determine the extent of cracking, distortions of the support, delaminations of the paint layers, etc. This stage of the examination is often done in close cooperation with styHstic experts. Thus, obvious problematic areas can be identified before the other tests are started. 

Microscopic examination of cross sections through the paint layers gives definite information regarding the paint-layer sequence in the area from which the sample was taken (31,66). This information illustrates the artist s use of underlayers and glazes, superposition of compositional elements, and changes in composition. 

Deterioration. Paintings are composite objects that have high vulnerabiUty. The various materials are adhered to each other, especially in a laminated stmcture, to form a source of potential trouble. Any dimensional change in one of the components or between the components as a consequence of changes in environmental conditions results in a strain on the adhesion of the various parts. Strains can lead to failure of the adhesion. This is one of the principal causes of losses in panel paintings, where the dimensional changes in the wooden support cause losses in adhesion between the paint layer and the support. 

Conservation. Conservation problems in paintings can be considered according to the stratum in which these occur, ie, in the varnish, the paint layers, or the support (143—146). 

The most common problem in the paint layers, which can have a wide variety of causes, is loss of adhesion. Upon drying of the medium, the paint layers develop shrinkage cracks. In itself, this is not a particularly worrisome phenomenon, but, if through any cause the adhesion between paint layers and ground or between ground and support is lost, the paint begins to flake. First the flakes cud up, and finally become completely detached and lost. 

Wax lining, however, often causes a darkening of the painting, and the adhesive, if it penetrates through the paint layers, is almost impossible to remove. Consequently, much effort has been placed on the development of synthetic adhesives which can be used at lower temperatures and pressures, and use of these materials is commonly accepted as an alternative to wax lining (see Adhesives). 

Zinc arc spraying is an inexpensive process in terms of equipment and raw materials. Only 55—110 g/m is required for a standard 0.05—0.10 mm Zn thickness. It is more labor intensive, however. Grit blasting is a slow process, at a rate of 4.5 m /h. AppHcation of an adhesive paint layer is much quicker, 24 m /h, although the painted part must be baked or allowed to air dry. Arc sprayed 2inc is appHed at a rate of 9—36 m /h to maintain the plastic temperature below 65°C. The actual price of the product depends on part complexity, number of parts, and part size. A typical price in 1994 was in the range of 10—32/m. ... 

In misting areas, exercise special care in maintaining paint layers. Regularly inspect known problem areas. 

If the rf source is applied to the analysis of conducting bulk samples its figures of merit are very similar to those of the dc source [4.208]. This is also shown by comparative depth-profile analyses of commercial coatings an steel [4.209, 4.210]. The capability of the rf source is, however, unsurpassed in the analysis of poorly or nonconducting materials, e.g. anodic alumina films [4.211], chemical vapor deposition (CVD)-coated tool steels [4.212], composite materials such as ceramic coated steel [4.213], coated glass surfaces [4.214], and polymer coatings [4.209, 4.215, 4.216]. These coatings are used for automotive body parts and consist of a number of distinct polymer layers on a metallic substrate. The total thickness of the paint layers is typically more than 100 pm. An example of a quantitative depth profile on prepainted metal-coated steel is shown as in Fig. 4.39. 

Should an old bituminous paint layer have to be recoated, this should be done only with another bituminous paint, unless the surface is first insulated with one of the special primers which are available for the purpose. Bleeding and premature checking may otherwise occur. 

If an elastomer is bonded to a substrate such as steel, it is usual for the bond to have small areas of imperfection where the adhesive or the chemical preparation of the surface is defective. Such areas are known as holidays. In high-pressure gas environments, these holidays form nucleation sites for the growth of half-bubbles or domes, under conditions where gas has been dissolved in the elastomer and the pressure has subsequently been reduced. Gas collecting at the imperfection at the interface will inflate the mbber layer, and domes will show as bumps on the surface of the mbber-coating layer—just as a paint layer bubbles up in domes when the wood underneath gives off moisrnre or solvents in particular areas. 

The reactions and some physical phenomena occurring in the curing paint film have been modelled with special attention being paid to the formation of carbon dioxide (chemistry) and the transport of the carbon dioxide from the paint layer (physics). 

The model describes the kinetics of the chemistry in the paint layer and the transport of carbon dioxide from the paint (physics). 

Table I. Equilibrium water concentrations In an unplgmented paint layer at different relative humidities (22 C) one hour after spraying...

We have found that at 60Z R.H. the water concentration In the paint film has increased by l.OZ (m/m) in comparison with that In the wet paint (after correction for the Increase In solid content after spraying). The rate of water absorption by or desorption from the paint layer Is illustrated in Figure 2. 

Figure 1. Apparatus for the continuous determination of the carbon dioxide emission from a paint layer during curing.

Figure 2. Rate of water absorption by or desorption from an unplgmented paint layer.

Figure 8.8 Mean concentration profiles of Zn, Cu and Cd in multilayer acrylic paint, consisting of orange deposited on blue, both on yellow. The arrows define the three paint layer limits as they were evaluated through weighing. After Brissaud [299]. Reprinted from Nuclear Instruments and Methods in Physics Research, B117,1. Brissaud et al., 179-185, Copyright (1996), with permission from Elsevier...

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