594 ANNUAL REPORT SMITHSONIAN INSTITUTION, 196 2 



responsible for the deterioration which an object may suffer during 

 its burial in the ground prior to excavation, or when it is exposed 

 to adverse environmental conditions in a museum, and the second 

 is to develop improved methods of conservation which will be based 

 upon an appreciation of the above factors and the use of new synthetic 

 materials. 



CONTROL OF ENVIRONMENT 



In the belief that prevention is better than cure, one of the first 

 studies undertaken in museum laboratories was devoted to the behavior 

 of archeological materials mider different environmental conditions, 

 so as to obtain a clear understanding of the factors likely to cause 

 deterioration. This knowledge could then be used to establish methods 

 of controlling the climate of a museum so that it would be best 

 suited for the preservation of its contents. The chief problem arose 

 in connection with museum objects of an organic nature, e.g., wood, 

 textiles, ivory, etc. Such objects are susceptible to variation in atmos- 

 pheric conditions, and the factor of paramount importance is con- 

 trol of the relative humidity of the museum atmosphere within pre- 

 scribed limits. In general, this means that the relative humidity 

 should lie in the range 50 to 65 percent, thus maintaining a reasonable 

 thermodynamic equilibrium between the moisture content of the air 

 and the moisture content of the organic material of which the object 

 is composed. In this way it is possible to avoid excessive dryness 

 which would lead to embrittlement by desiccation or excessive wetness 

 which would lead to mold growth, weakening of moisture-sensitive 

 adhesives such as glue, and, in the case of metallic objects, would 

 encourage corrosion. Another vital point which was realized was 

 the need to ensure adequate air circulation — particularly in storage 

 rooms — so as to guard against the possible formation of air pockets 

 where a high relative humidity beyond the safety limit of 65 percent 

 might be developed [2]. An interesting example of the need to guard 

 against this eventuality is shown in plate 1. A high relative humidity 

 had developed in a basement room where books were stored on steel 

 shelving, and resulted in mold growth becoming visible on the backs 

 of the books. This condition was rectified by adopting methods 

 whereby the ambient relative humidity was reduced to a desirable 

 level, namely 50 percent. Later, when a book adjacent to a hollow 

 steel shelf support was drawn out, it was found that mold was still 

 active on the inside edge of the spine of the book and was growing 

 in such a way as to reproduce the pattern of the holes in the hollow 

 support. This was clearly due to the fact that a pocket of moist 

 stagnant air had persisted inside the hollow support, because air 

 circulation had not been maintained at a sufficiently high velocity; 

 indeed, when the relative humidity of the air inside the support was 

 measured, it was found to be no less than 83 percent. The various 



