ASTM D748-14

4.1 The properties included in this specification are those required to identify the types of natural block mica and mica films (cut and uncut) suitable for use in the manufacture of fixed mica-dielectric capacitors. It is possible that other requirements will be necessary to identify particular characteristics. These will be added to the specification as their inclusion becomes generally desirable, and the necessary test data and methods become available. It is possible that natural block mica and mica films that do not conform to the requirements of this specification for capacitor use are capable of meeting the requirements for other critical electrical insulation purposes.

4.2 The system of classifying electrical quality of natural block and mica films (cut and uncut) covered by this specification is based on a combination of electrical and physical properties, and visual qualities specified herein, which the mica must possess. This system differs radically from past practices and previous concepts of mica quality for capacitor use. The electrical classification system does not discriminate against the presence of spots and stains in even first quality electrically selected mica, provided the mica conforms to specific and physical requirements. Appreciable amounts of air inclusions and waviness also are permitted in all electrical quality classes, provided the mica meets specific electrical and physical requirements. Mica meeting these requirements is acceptable without regard to color or origin. However, mica meeting these electrical and physical requirements but having lower visual quality than that meeting the requirements for the visual quality classification is not considered generally as desirable.

4.3 In capacitor fabrication, one or more pieces of cut film or block mica having lower than required electrical and physical properties will possibly prevent meeting the end requirements of the capacitor. It is therefore required that each piece of block (cut) or film (cut or uncut), or both, be tested for the electrical requirements and inspected for the visual requirements listed in this specification.

1.1 This specification covers natural block mica1.2 The values stated in inch-pound units are to be regarded as standard.