Radiant Heat for Vacuum Tables

Abstract

The construction of a 1-80 x 3-00 m vacuum table is described. Instead of fabricreinforced neoprene rubber type blankets, electric radiant heat panels are used to reach a maximum of 950C. Heating takes approximately one hour. Recently the author encountered an unusual number of large canvases, all of which required lining as part of their conservation program. Many of these were lined, in sections, on a 1-20 x 1-80 m vacuum table. This, however, proved to be time-consuming, and finally induced the author to design and construct a larger table in order to facilitate matters. It was planned with a 1-80 x 3-00 m top and was 80 cm high. For framing, perforated metal angles which could be cut, constructed, and bolted together easily were used. Plywood c. 1-3 cm thick was bolted into place to form the bottom, and the entire frame was supported on six heavy-duty, ball-bearing casters with hard rubber wheels (Fig. 1). When this metal FIG. 1 Perforated metal frame for table, mounted on heavy-duty ball-bearing casters. frame had been completed, it was sheathed in wood. Eight sets of doors were installed along the sides and ends to allow easy access to the storage area and shelves. Selection of satisfactory heating units posed a major problem. In two tables formerly constructed by the author, heat units of the fabric-reinforced neoprene rubber type which have foil strips to carry the current through the blanket were used. Such a unit, for a table of 1.80 x 3-00 m, would have to be ordered under special contract since it exceeded standard stock sizes, and would draw approximately 8000 watts. It was also disconcerting that prices on these items had doubled in recent years. This content downloaded from 157.55.39.231 on Wed, 05 Oct 2016 04:34:11 UTC All use subject to http://about.jstor.org/terms Radiant heat for vacuum tables 171 Exploring numerous heating units, the author selected electric radiant heat panels constructed by the Electro-Products Division of the 3M Company. Each panel is a 0-7 cm rigid, high-pressure laminate of non-metallic construction in which the heating element is completely sealed. Materials used are high-quality electrical insulators, impervious to moisture, which retain excellent physical properties at elevated temperatures. Three 240 x 60 x 0-7 cm, 1000-watt panels were laid side by side running the length of the table. A fourth panel, 180 x 60 x 0-7 cm, 750 watts, was placed across the end of the table to fill out the area to be heated (Fig. 2). By having the normal margins along the tangent edges trimmed at the factory, the strips of unheated surface are so minute that they are of no concern. Thus, since these panels use a non-metallic continuous-sheet type heating element, and are perfectly smooth, the entire table top is uniformly heated. The combined panels use only 3750 watts. The maximum suggested operating surface temperature for these panels is 95'C, well above that normally used in most conservation projects involving a vacuum table. For wax-resin adhesives, heat-up time takes approximately one hour for complete infusion with about the same amount of time for the surface to cool. If fans were employed, cooling time could be cut considerably. A double thickness of 1-2 cm Homasote board was selected as a base for the heat panels. FIG. 2 Table with radiant heat panels in place and wooden framing applied. This material allows easy cutting for recessing the electrical connections on the back of the panels and leading the supply conduits to a central thermostatic control on the side of the table. It also possesses good insulation qualities. Table construction was completed with a piece of 180 x 300 x 0-7 cm aluminium alloy sheet placed directly on top of the radiant heat panels (Fig. 3). A vacuum pump, operating from a single opening in the 210 x 330 cm rubber sheet, completes the combination. Many of the features of this table were incorporated after experience with other models, examining tables used in other laboratories, and from reading literature previously Studies in Conservation, 14 (1969), 170-173 This content downloaded from 157.55.39.231 on Wed, 05 Oct 2016 04:34:11 UTC All use subject to http://about.jstor.org/terms