TOWARD STANDARDIZATION OF A HIGH-PERFORMANCE DISPLAY CASE

Abstract

Tokyo National Museum has introduced a new free-standing display case for its new extension which was opened to the public in the autumn of 1999. The 'Heiseikan Gallery' provides space for a permanent exhibition of archaeological objects on the ground floor and also for special exhibitions that will be held about four times a year on the first floor. About 80 free-standing display cases for the first floor and 20 for the ground floor were installed. Nearly half a million people have visited exhibitions at the Heiseikan Gallery in the six months since it was opened, and we are expecting more than one million visitors in a full year. Although we have focused on several points such as seismic isolation, air exchange rate of the cases, airflow, chemically neutral environment, space for humidity buffers, a lighting unit, a door unit, slender frame, durability, relatively light weight for ease of movement, security, the first four points have been especially stressed. A seismic disaster hit Kobe City in January 1995, with the result that a large number of cultural objects in museums suffered big losses. The maximum acceleration of the earthquake was in excess of 800 gal (about 0.8g). Afterwards, museums and manufacturers gave serious consideration to measures against earthquake damage employing seismic isolators. Generally speaking, two main methodologies exist in order to decrease input acceleration from the earthquake by changing the period of osculation. One method utilizes the osculation caused by the return force of a spring; the other, the osculation caused by the return force of rolling on a curved rail, the same as a pendulum. The period of osculation of a seismic isolator using the former method depends on the weight of the object placed on the isolator, while the latter method is independent of the weight of the object. We selected the second method. The seismic isolator is installed beneath the base of the case. As the inherent period of the isolator is four seconds, seismic force is decreased to below 100 gal on the isolator even if the attack is on the same scale as the Kobe earthquake. Moreover, it is unlikely that the inherent period of an earthquake will coincide with that of the isolator, so that no resonance would take place between them. The isolator does not require any regular maintenance. Museum staff want the display case to be as airtight as possible, to obtain a stable relative humidity and to prevent dust or air pollution from entering. However, there have been no clear guidelines on how to achieve such requests. We have introduced 0.1 air exchange per day as the exchange rate of the case, to reduce daily and weekly interior RH fluctuations to less than one tenth of ambient [I]. When a buffering materiul is pluced in the case, the annual change will be decreased to some extent. It seems that extreme airtightness, that is, an air change rate of less than 0.0 I, is directly connected with cost and is rather disadvantageous for conservation due to the retention of harmful gases emitted by objects and the materials of the case. Of course, extreme airtightness is useful if interior harmful gases are completely excluded. Therefore, the air exchange rate adopted by us works to prevent harmful gases originating from both interior and exterior causes from staying in the display cases. The air exchange rate was ascertained by the tracer gas method, using carbon dioxide [2]. To homogenize the distribution of relative humidity within the case as quickly as possible when the door is closed, a small electric fan has been provided to create air circulation in the case. A previous simulation using a 5m3 volume display case proved that 0.03m.sec-1 airflow would create uniform relative humidity within 10 minutes [3]. As a result, we have placed a 0.0Im.sec·1 airflow electric fan over a tray of humidity buffer beneath the platform in the case. The airflow does not affect the surface of objects. The materials of the display case should be examined before construction and should be excluded if they are identified as emitting harmful gases. The Oddy Test has been introduced to check materials, and an environmental monitor has been used to estimate the chemical environment within the case. Nobuyuki Kamba is head of conservation in the Curatorial Department, Tokyo National Museum, 13-9 Veno Park, Taito-ku, Tokyo 110-8712, Japan.