The quality classification of muscovite mica being based on visual tests depends on individual opinion. Also products of different mines vary in physical characteristics to such an extent that the development of a single standard with reasonable limits of tolerance becomes an acutely difficult task. Added to these difficulties are the facts that quality classification has to be carried out by a large number of individual workers and that it is highly influenced by such circumstantial factors as lighting conditions etc. Under these circumstances any standard for quality classification of mica can give at best an approximate idea of a particular quality and its reactive position with respect to other qualities.”

The quality classification of muscovite mica being based on visual tests depends on individual opinion. Also products of different mines vary in physical characteristics to such an extent that the development of a single standard with reasonable limits of tolerance becomes an acutely difficult task. Added to these difficulties are the facts that quality classification has to be carried out by a large number of individual workers and that it is highly influenced by such circumstantial factors as lighting conditions etc. Under these circumstances any standard for quality classification of mica can give at best an approximate idea of a particular quality and its reactive position with respect to other qualities.”