LOSS COMPENSATION METHODS FOR STONE
JOHN GRISWOLD, & SARI URICHECK
4 STONE REPLACEMENT
When part of a building stone is damaged to the extent that it cannot perform its function as a load-bearing element, stone replacement is often justified. Replacement is the system of compensation in which a piece or unit of stone (natural or imitation) is fitted to the area of loss in the original stone. Replacement may be “in kind,” with the unit made of the same exact stone, in “near kind” with a similar stone, or in an imitation stone. The process of using replacement stone pieces for localized damage is sometimes referred to as the dutchmen technique.2 The use of a dutchmen is practically suited to large-volume losses. It is often chosen in the case of marble and granite compensations, even indoors, because it is otherwise difficult to produce the appearance of these stone (Sourlis 1988).
A dutchmen repair is a labor-intensive process. Standard procedure for filling a loss with a replacement is first to identify the substrate through petrographic analysis or archival information and to match this original with the chosen replacement stone. The damage must be assessed and the original profile determined with or without a previous model (photographic or otherwise). When required, the damaged or deteriorated part of the stone will be excavated and in many cases the break surfaces planed to allow a good fit for the replacement. A model of the form is made to fit the loss, usually out of clay or plaster.
Based on this model, a copy is roughed out in the stone using pointing techniques. Sometimes the stone source for a dutchmen may be salvaged from an inconspicuous place on the monument, ensuring a proper match. With exacting measure the new piece must be tightly fitted into place. Supporting rods, usually stainless steel but sometimes titanium (Zambas et al. 1986) or polymeric composites, are used to dowel pieces together. The stone is adhered with epoxy, polyester, or mortar and clamped so the join may set. At this point, the roughed-out form is carved and refined to match the adjacent surface of the original. The process traditionally entails resurfacing the insert and the adjacent original stone to the same profile and finish. This last step is what often allows a virtually invisible replacement repair but is what commonly precludes the method's use in fine arts conservation.
The use of dutchmen is often a more effective and appropriate solution in architecture than sculpture because the forms being replaced are usually rectilinear. Also, the presence of mortar joints between the stones allows the replacements to be integrated as an original element of the building design. On sculpture, a replacement seam is an interruption of the form that must be hidden, best accomplished by following a natural contour.
The adhesive join between the replacement and the original is one of the most troublesome aspects of the dutchmen technique. This layer of epoxy or other adhesive could act as a moisture barrier, and its excessive strength may cause extraneous stress and possible delamination at the surface. Epoxy and polyester adhesives also weather poorly, darkening due to oxidation upon light exposure, which accentuates the seams of the compensations. To alleviate these problems, precautions such as bulking and pigmenting the adhesive, adding antioxidants, or “spot” adhering with adhesive and patching the rest with a cementitious grout may be employed. Despite their failings, these resins have not been superseded by other adhesives because they allow the thinnest possible seam to be made while successfully securing the replacement.
Fresh replacement repairs of losses made with natural, newly carved stone are often aesthetically unpleasing. When first applied, the freshly finished surface may not match the surrounding original stone, although after exposure the repair surface may gradually become better integrated through soiling and weathering. In some cases a new-looking replacement is desired as a clear statement of what is new and what is original (Papanikolaou 1994). However, as other cases in New York's Central Park demonstrate (Champe 1996), fresh dutchmen additions often invite vandalism, and disguising them is an important part of a comprehensive treatment. Texturing with tools and abrasive blasting, combined with various methods of pigmenting the stone may be employed. These methods include traditional recipes using iron in vinegar, pastes mixed with soil, clay, iodine, rust, magnesium oxide (MgO), boiled soot, and tea stains (Torraca 1986). At the Parthenon, ferrous sulfate (FeSO4) and iron oxide (Fe2O3) mixed in Paraloid B-72 were sprayed across the surface of replacement pieces to disguise them (Skoulikidis et al. 1993). Paints that may be used include acrylic-based, silicone resin-based, or alkaline silicate-based. Ideally, any paint applied is alkaline-resistant and water-permeable. In a study of the properties of different outdoor paint systems, ethyl silicate-based paints were found to be promising for permanent tinting with minimal reduction in water transport characteristics (Griswold 1994).
Another method of matching the stone more effectively is to use precast “artificial stone” that is manufactured to mimic the weathered surface in color and texture. Artificial stone can be cast in a mold to match the original, as utilized at the Metropolitan Club in Manhattan (Matero and Tagle 1995), or carved after casting to fit a repair and to match a surface, as at the reconstruction of the Basilica of St. Arbogast. This work is often performed off-site, made to order from plaster models by specialists (Murphy and Ottavino 1986).
The creation of artificial stone for original building elements, sculpture, and structural repairs is an ancient practice. Old inserts of artificial stone were found in a sandstone during restoration (Kralova et al. 1986). “Roman stone” is the Renaissance term used by Vasari for artificial stone made using natural hydraulic mortar and aggregate (Fitchie 1978). An 1850 treatise on artificial stone describes recipes for the latest modern stone substitutes (Fowler 1850). The development of glazed terracotta and other materials that imitate stone, such as Coade stone (Kelly 1978) was spurred by a growing market for elaborate building facades.
Replacement pieces can be made from a variety of other cast or pre-formed materials. These include modeled putties, laminated fiberglass shells (Fidler 1982; Rawson 1994), and other systems with underlying supports such as stainless steel armatures. Some complex treatments entail the use of fitted reversible or adjustable machined parts for reconstruction of monumental sculpture (Podany 1987; Garland and Rogers 1995). Painted wood has been used to reconstruct or fill large losses where stone replacement was not considered economically or structurally feasible (Phillips 1982). Care must be taken, however, that such nonstone replacement pieces do not merely cover the problem but eliminate sources of further deterioration. Such methods often involve the risk of exacerbating underlying problems, which can go undetected for long periods (Nelson 1992).