Without a doubt the heated vacuum table is the heart of the modern conservation studio. It is an invaluable tool that allows the conservator to apply even heat and pressure to a painting during various treatments. After moving to a new studio the table built by Agass Baumgartner in 1980 failed. During the repair process Julian reverse engineered the 40-year old electronics and updated them with modern digital equipment. After that exercise and armed with an intimate understanding of how the table worked Julian embarked on building a new, larger table.
The new table measures 10' x 7' and has a surface of two slabs of thixtropic epoxy-bonded 84" x 120" x 1/2" 6061 aluminum. Six 1500 watt silicone pads below the aluminum provide the heat and are wired in three groups of two pads each allowing for zoned heating. Three discreet digital temperature controllers are used to monitor and adjust the heating pads and provide precse heating control. A panel controled 1/4 HP rotary vacuum pump with two quickconnect hose ports allow for air extraction and even pressure via flexible tubing with quickconnect thruports. The table base is built from 4040 extruded aluminum tubing. 1/4" luan plywood atop the base provides the platform for 1" aluminum coated polyisocyanurate insulation below the heating pads. The heating pads run on 240 volts and draw approximately 9000 watts of power at 40 amps. The vacuum pump operates on 120 volts with a full load of 4.5 amps.
The base was constructed of 4040 aluminum T-slot extrusion. The aluminum was securred with 90 degree aluminum corner gussets, bolts and T-slot nuts. Any material would suffice for the base including wood, steel angle iron or welded tubing provided that it is stiff, resists deflection and can support the 540 pounds of aluminum slabs. Aluminum extrusion was chosen for the low weight, ease of machining and strength. Atop the base a layer of 1/4" luan plywood acts as a platform for 1" aluminum foil faced polyisocyanurate insulation. The insulation is necessary to ensure that the heat is directed to the aluminum slabs and not lost to the table below.
The table has three discreet circuits that control the heating pads. Each circuit utilizes both 120v and 220v and is composed of a 120v toggle switch, a 120v/240v relay switch, a temperature controller, a high voltage solid state relay switch, a type-J thermocouple and two silicone heating pads. The toggle switch is wired to a 120v/240v relay switch that sends power to the temperature control unit. As the controll unit does not have an on/off switch and will continue to consume power even when not in use it was wired to the toggle switch and relay so that it could be powered on and off and not only reduce power consumption but extend the life of the unit as well. The temperature control unit is powered by 240v and takes input from a type-J thermocouple mounted to the underside of the aluminum slabs. The temperature controller output is a low voltage DC current which is delivered to a high voltage solid state relay that switches the 240v current to the pads on and off. The silicone heating pads were speciffically designed for this application and consume 1500w each. They can reach maximum temperature of about 200F in aproximately 20-30 minutes depending on the starting ambient air temperature. Each circuit contains two heating pads.
The vacuum suction is provided by a rotary vacuum pump capable of delivering 30 mercury inches of consistent pressure. The vacuum pump operated on 120v and is wired to a panel-mounted toggle switch. The pressure is regulated both at the pump and with an inline needle valve that has been mounted into the control panel as well. The inline valve limits the flow of air and is connected to an analog pressure gauge that has been panel mounted. From the inline needle valve the hose is split and directed to two quick release air hose connections. Two thru-bag vacuum connectors commonly used for vacuum bagging in fibergalss and carbon fiber production enable the extraction of air through a PET film layer.