3 ACRYLIC MODELING PASTES: GENERAL DISCUSSION

The vehicles for modeling pastes are acrylic polymer emulsions. The exact identification of the emulsions or dispersions in commercial fillers is generally a trade secret and is only generically listed by the manufacturers. The polymers are often combinations of methyl, ethyl, butyl acrylates, and methacrylates. The bulking agents are frequently forms of calcium carbonate, but calcium sulfate, barium sulfate, talc (French chalk), kaolin, and other clays may be present. Additional ingredients may be ammonia or ammoniated compounds, ethylene glycols, pigments, and emulsifying or foaming agents. The ammonia or ammoniated compounds present in most acrylic fillers may serve a role as biocides. The ammonia or ammoniated compounds, which may cause corrosion on some metals, are present in extremely small quantities. No problems of corrosion related to the fillers have been located in conservation publications.

A few commonly used modeling pastes available in the United States are: Liquitex Modeling Paste, Golden Molding Pastes, and Utrecht Modeling Paste. Introduced in 1958, Liquitex Modeling Paste probably finds the most widespread use in objects conservation in the United States. Modeling pastes have been extensively used for gap filling on ceramics, especially low-fire earthenware, plaster sculpture or gesso, and, to a lesser extent, on wood and metal artifacts. Flexibility of the dried films varies considerably. The lesser flexibility of some, especially Liquitex Modeling Paste, limits potential use on checks or cracks in wood or on hygroscopic materials, such as ivory. Modeling pastes are intended for indoor applications only.

The pastes are available in many grades, labeled light, heavy, or hard. The manufacturers' instructions recommend building forms or creating textures with the modeling pastes. Only Liquitex Modeling Paste is recommend as carvable. Golden Light Molding Paste is intended to help build depth without adding weight. Many of the modeling or molding pastes form very flexible and soft films which may not be suitable for gap filling on rigid materials, objects that will be handled or displayed in the open.

The pastes are suitable for application with spatulas. Multiple thin applications are preferred to avoid shrinkage or evaporation cracks. Evaporation cracks are formed by the varying rate of escaping water from the thick and thin areas of the fill. The cracks can be avoided by applying thin layers and slow drying. The cracks can be filled with no effect on the final strength and bond of the filler (Wall 1997). The modeling pastes can be thinned with water to be made brushable or modified with compatible media supplied by the manufacturers. Extenders, retarders, gels, and acrylic paints are available. Tools can be cleaned with water before drying. Once opened, the container should be tightly closed to prevent drying out. The manufacturers recommend working above 50� F. The products must be kept at temperatures above freezing during shipment and storage.

When dry, the acrylic fills can be sanded or rubbed down with solvent. Acetone (ketone) or toluene (aromatic hydrocarbon) are suitable solvents for removing excess filler or smoothing the surface. Liquitex Modeling Paste is the most easily sanded of the acrylic products mentioned. The others feel more plastic, elastic, or “rubbery,” tend to grab abrasives, and peel up. The fillers are thermoplastic and can be slightly modified using heat. Acrylic emulsion paints, resin-based paints, watercolor, and gouache are suitable for inpainting.

There are limited conservation references regarding acrylic fillers. Reports of testing (Barov and Lambert 1984) indicate that the thermal expansion of Liquitex Modeling Paste, and by inference the other acrylic fillers, makes it compatible with ceramic objects under extreme thermal fluctuations possibly experienced during transport. No major problems of incompatibility, rapid deterioration, decrease in reversibility over time, or other conservation problems regarding acrylic fillers are noted in the literature. Recently, however, turbidity and the formation of crystals of poly(ethylene glycol) compounds in Liquitex acrylic emulsion paint films have been documented (Whitmore et al. 1996), but no extension of the phenomena or problem has been made yet in regard to the modeling paste.

3.1 ACRYLIC COMMERCIAL FILLERS: PRODUCT-SPECIFIC INFORMATION

Liquitex Modeling Paste—Shrinkage of Liquitex Modeling Paste is about 20% upon drying. Of the acrylic modeling pastes examined, Liquitex Modeling Paste is the hardest, least plastic, and most easily worked when dry. 2-Amino-2-methyl-1-propanol (AMP, isobutanolamine), which is used in industry as an emulsifying agent, is listed as a hazardous component, but no Threshold Limit Value (TLV) has been set. The quantity present is extremely small trace amounts. No corrosion on metal due to its presence is reported in the literature or by the manufacturer's representative. Bonding to smooth surfaces may be problematic (Wall 1997).

Golden Molding Pastes—Golden produces a series of molding pastes: Extra Heavy/Molding Paste 03110, Molding Paste 13570, Hard Molding Paste 03571, and Light Molding Paste 03575. As far as can be determined, the ingredients are similar. All are softer, more plastic, and more flexible than Liquitex Modeling Paste. None are easily sanded. Golden Hard Molding Paste comes the closest to Liquitex in terms of handling and the final surface. The Light Molding Paste does not shrink as much as the others. When the dried film is held up to light, numerous small air bubbles are visible, suggesting the use of foaming agents. Attempts to finish the surface of the Light Molding Paste expose air bubbles resulting in a rough and uneven surface.

Utrecht Modeling Paste—The MSDS for Utrecht Modeling Paste provides little information, claiming no hazardous materials. The pH of the undried paste is approximately 9. The solid material is partially soluble and effervesces in dilute mineral acid, indicating some carbonate-containing bulking agent. The color is a cold opaque white, suggesting that a pigment has been added, probably titanium dioxide. The dried film or fill is soft, rubbery, and picks up dirt easily. Carving and abrasion are difficult and not successful. Comparing the weights of an equal volume of Utrecht to Liquitex Modeling Paste, the Utrecht is much lighter, suggesting that less bulking agent may be present in the formulation.

Liquitex Acrylic Gesso—The Liquitex Acrylic Gesso has lower viscosity and will not hold peaks or forms without additional support or additives. The shrinkage rate is approximately 40%. It is highly recommended that thin layers be applied to avoid formation of cracks. The evaporation cracks do not weaken the bond or strength of the final fill. For an equal volume, its drying time is one of the longest of all the commercial products discussed. The dried fills can be sanded.

TABLE 1. PRODUCT SPECIFIC INFORMATION FOR COMMERCIAL ACRYLIC FILLERS

Golden Acrylic Gesso—The Golden Acrylic Gesso is also a lower viscosity liquid that is not self-supporting. The dried fills can be sanded.

Liquitex Acrylic Colored Gessoes—The Liquitex Acrylic Colored Gessoes were introduced in 1990. The colored gessoes offer a rich, matte surface. The factory-prepared gesso cannot be duplicated by mixing of other available media, pigments, or agents. The gesso has low viscosity and will not hold peaks or forms without additional support. The shrinkage rate is 50% (Wall 1997). For an equal volume, the drying time was the longest of all the commercial products tested. The specific gravity is approximately the same as the white-colored gesso. The black pigment has a very small and uniform particle size. The fill is rubbery and difficult to finish by sanding. A smooth surface can be obtained by wiping with acetone or ethanol. Ghosting is hard to avoid, even with isolating barriers.

Golden Acrylic Black Gesso—The Golden Acrylic Black Gesso has low viscosity, but not as low as Liquitex Black Gesso. It still will not hold sharp peaks or forms without additional support or additives. The filler dries faster than Liquitex Black Gesso. The fill is also rubbery and difficult to finish by sanding. A smooth surface can be obtained by wiping with acetone. Ghosting tends to be a problem, as the pigment size is very fine.