2 ADVANTAGES AND DISADVANTAGES OF USING COMMERCIAL FILLERS

Working characteristics of commercial fillers offer some advantages over those of plaster and conservator-prepared fillers. Most obviously, the fillers are conveniently premixed and ready for use. The time-saving feature of ready-mixed products must be balanced with the ability to adjust the formulation for each object when making one's own fillers.

Timing is not critical when using ready-mixed fillers for gap filling since drying occurs slowly by evaporation rather than by the rapid chemical setting reaction of plaster. No heat is generated during the drying of commercial fillers. Multiple or adjacent applications of commercial fillers create a uniform surface, while with plaster it is difficult to control hardness and uniformity in more than one pour.

Vehicles or binders in the fillers increase adhesion to the artifact being treated. Plaster remains in place primarily by physical fit or interlock. Adhesion to the substrate may or may not be desirable depending on the specific treatment project. Filling of small cracks or chips in low-fire ceramics is made easier by the adhesive qualities of the commercial fillers. The vinyl and acrylic fillers generally do not adhere well to nonporous surfaces including porcelain, glass, and metal. An exception is BEVA Gesso, which sticks well to metal surfaces.

Due to the inclusion of the vehicle or binder, the vinyl and acrylic fillers are generally softer than plaster, making finishing and carving easier. The fillers are also resoluble in water or organic solvents such as acetone, depending on the product used, adding another method of shaping or finishing beyond abrasion and carving.

The major disadvantage is shrinkage caused by evaporation of water or solvents. Shrinkage ranges from a few percent up to 50%. On the other hand, plaster expands slightly (up to 0.5%), depending on the grade of plaster selected. Shrinkage prohibits the vinyl and acrylic fillers from being used to fill large losses without added structural support. Multiple applications are recommended to produce a uniform surface and to avoid shrinkage in deep losses. Applying large quantities in a single pass results in major cracks and unevenness. Casting into closed molds is not practical since drying is retarded or prevented. Therefore, the commercial fillers are relegated to filling small losses or to surfacing other structural fillers, such as bulked epoxies, polyesters, AJK or BJK dough strips, or plaster.

The commercial fillers are all opaque and range in color from pale cream to cold bluish white to light brown. Pigmented acrylic emulsion gessoes are also available. None of the fillers examined are suitable for use on transparent or translucent objects. If used on ceramics with a transparent glaze, the commercial fillers must be slightly recessed in the loss to permit application of overlying transparent varnishes or coatings.

As is true with plaster or conservator-prepared fillers, commercial fillers can cause ghosting on porous materials, especially low-fire ceramics and wood. Ghosting appears as a white haze surrounding a fill and is usually very difficult to remove. Ghosting results from the entrapment of the particulates or bulking agents (such as calcium carbonate or kaolin) in the pores, cracks, or recesses on the object surface. To prevent ghosting, coating the edges of the loss and adjacent surface to isolate the object is recommended. Plaster can introduce sulfate salts to porous bodies, but this concern is not as great with commercial fillers. Moisture will be absorbed by porous object materials unless a barrier is introduced. Dilute Paraloid B-72 in acetone is one effective barrier.