ASTM D351-18

Standard Classification for Natural Muscovite Block Mica and Thins Based on Visual Quality

SIGNIFICANCE AND USE
4.1 The properties included in this standard are those required to control the visual quality, usable area, thickness, hardness, and stiffness.
SCOPE
1.1 This classification covers the determination of commercially available natural muscovite block mica and is intended to be independent of the basic color of the mica or its source.  
1.2 Muscovite mica is characterized by having an optical axial angle between 50 and 75° (see Appendix X1); and has a weight loss when heated for 5 min at 600°C not exceeding 0.2 % (based on the weight after drying at 120°C).  
1.3 The visual system of classifying the quality of natural muscovite mica covered by this specification is based upon relative amounts of visible foreign inclusions such as air bubbles, stains, and spots in combination with relative amounts and types of waviness, as well as other physical properties. In this system, a perfectly clear, transparent, flat specimen of mica is the visual standard of perfection. Increasing amounts of visual defects lower the visual quality, and a total of 13 levels of visual quality are covered by this standard. This method of classification, generally known as the Bengal India System, is purely qualitative and is entirely dependent on personal opinion and judgment.  
1.4 The standards for visual quality classification that are covered in this classification are the best commercially available concept of the various qualities and their relative positions. Variations in the methods of using and applying these standards from those herein defined are specified by the purchaser, or defined by agreement between the supplier and the purchaser.  
1.5 Standard size classifications are defined, based upon available usable rectangular areas and the minimum dimensions of the rectangles that the pieces will yield. Precautions to be taken in making thickness measurements are also described.  
1.6 This standard covers the following two definite forms of commercial preparation:  
1.6.1 Form 1—Full-trimmed natural block mica, 0.007 in. (0.178 mm) minimum thickness.  
1.6.2 Form 2—Partially-trimmed natural block mica, 0.007 in. minimum thickness.  
1.7 The basic color of mica, such as white, ruby, light green, dark green, brownish green, and rum, as well as other colors, and the method of controlling the color and other problems associated with the basic color, are not a part of this classification.  
1.8 The values stated in inch-pound units are to be regarded as standard. The values given in parentheses are mathematical conversions to SI units that are provided for information only and are not considered standard.  
1.9 Section 5 is technically identical to procedures specified in ISO 67-1981.  
1.10 Section 6 differs somewhat in procedure from ISO 5972-1978, but data obtained by either is expected to be identical.  
1.11 Section 7 is technically identical to procedures specified in ISO 2185-1972.  
1.12 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

General Information

• Status: Published
• Publication Date: 31-Oct-2018
• ICS: 73.080 - Non-metalliferous minerals
• Technical Committee: D09 - Electrical and Electronic Insulating Materials
• Drafting Committee: D09.01 - Electrical Insulating Products

Relations

• Replaces: ASTM D351-14 - Standard Classification for Natural Muscovite Block Mica and Thins Based on Visual Quality
• Effective Date: 01-Nov-2018
• Refers: ASTM D1711-24 - Standard Terminology Relating to Electrical Insulation
• Effective Date: 01-Mar-2024
• Refers: ASTM D1711-15 - Standard Terminology Relating to Electrical Insulation
• Effective Date: 01-Nov-2015
• Refers: ASTM D1711-14a - Standard Terminology Relating to Electrical Insulation
• Effective Date: 01-Nov-2014
• Refers: ASTM D1711-14 - Standard Terminology Relating to Electrical Insulation
• Effective Date: 01-May-2014
• Refers: ASTM D1711-13 - Standard Terminology Relating to Electrical Insulation
• Effective Date: 01-Nov-2013
• Refers: ASTM D1711-11a - Standard Terminology Relating to Electrical Insulation
• Effective Date: 01-Aug-2011
• Refers: ASTM D1711-08 - Standard Terminology Relating to Electrical Insulation
• Effective Date: 01-May-2008
• Refers: ASTM D1711-02 - Standard Terminology Relating to Electrical Insulation
• Effective Date: 10-Mar-2002
• Refers: ASTM D1711-99 - Standard Terminology Relating to Electrical Insulation
• Effective Date: 10-Oct-1999
• Refers: ASTM D374-99(2004) - Standard Test Methods for Thickness of Solid Electrical Insulation (Withdrawn 2013)
• Effective Date: 10-Mar-1999
• Refers: ASTM D374-99 - Standard Test Methods for Thickness of Solid Electrical Insulation
• Effective Date: 10-Mar-1999
• Referred By: ASTM F21-20 - Standard Test Method for Hydrophobic Surface Films by the Atomizer Test
• Effective Date: 01-Nov-2018
• Referred By: ASTM D2131-22 - Standard Classification for Natural Muscovite Mica Splittings
• Effective Date: 01-Nov-2018
• Referred By: ASTM D748-18 - Standard Specification for Natural Block Mica and Mica Films Suitable for Use in Fixed Mica-Dielectric Capacitors
• Effective Date: 01-Nov-2018

Overview

ASTM D351-18: Standard Classification for Natural Muscovite Block Mica and Thins Based on Visual Quality provides a comprehensive framework for assessing commercially available natural muscovite mica, independent of its basic color or geological source. The standard establishes guidelines to control and classify key properties of block and thin mica, including visual quality, usable area, thickness, hardness, and stiffness. It serves as an authoritative reference for manufacturers, suppliers, and purchasers of electrical insulating materials where the visual and physical properties of mica are critical.

Key Topics

• Visual Quality Classification
  ASTM D351-18 introduces a qualitative, visually-based grading system – commonly known as the Bengal India System – that evaluates mica using up to 13 categories of visual quality. Criteria include relative amounts of visible foreign inclusions (such as air bubbles, stains, and spots), types and degree of waviness, and the presence of cracks or other imperfections.

• Size and Thickness Grading
  The standard defines conventional size categories based on the area and minimum dimensions of usable rectangular sections that can be obtained from each mica piece. It also specifies the recommended procedures for measuring thickness accurately, with emphasis on using calibrated micrometers and maintaining cleanliness during measurement.

• Forms of Commercial Preparation
  Classification includes two primary forms:

  • Full-trimmed block mica: Trimmed on all sides, with a minimum thickness of 0.007 in. (0.178 mm).
  • Partially-trimmed block mica: Also with a minimum thickness of 0.007 in, but not fully trimmed.

• Exclusion of Color as a Criterion
  The standard explicitly excludes basic color as a quality factor, focusing on visual clarity, physical defects, and dimensional attributes regardless of hues such as white, ruby, green, or brown.

Applications

• Electrical and Electronic Insulation
  Muscovite mica's high dielectric strength and resistance to heat make it a preferred choice for electrical insulation applications, including capacitors, commutators, and transformers. The classification provided by ASTM D351-18 ensures consistency and reliability in selecting suitable mica for these sensitive uses.

• Quality Control in Manufacturing
  Manufacturers use this standard to assess and certify batches of mica according to standardized visual and physical benchmarks, ensuring product suitability for end-users in critical industries.

• Procurement and Supply Chain Specifications
  The clear definitions for size, thickness, and defect tolerances support straightforward specification writing and compliance auditing between purchasers and suppliers.

• International Trade and Standardization
  A harmonized classification system facilitates global commerce in mica products, enabling clear understanding and agreement across different geographies and market participants.

Related Standards

• ASTM D374 – Test Methods for Thickness of Solid Electrical Insulation
  Provides methodologies referenced in ASTM D351-18 for precise measurement of insulation thickness.

• ASTM D1711 – Terminology Relating to Electrical Insulation
  Supplies standard terminology for definitions used within D351-18.

• ISO 67-1981 – Muscovite Mica Blocks, Thins, and Films-Grading by Size
  Procedures for size grading are technically identical to those in ASTM D351-18.

• ISO 2185-1972 – Muscovite Mica Blocks, Thins, and Films-Visual Classification
  Visual quality classification procedures closely align with ASTM D351-18.

• ISO 5972-1978 – Mica Blocks, Thins, Films, and Splittings-Measurement of Thickness
  The measurement method in D351-18 is expected to yield results identical to those in ISO 5972-1978.

Practical Value

ASTM D351-18 is essential for ensuring high-quality standards in muscovite mica used for insulation and electronic applications. The reliance on objective classification criteria fosters improved quality control, facilitates fair trade, and supports compliance with both U.S. and international best practices for mica evaluation. Its adoption assists stakeholders in minimizing disputes and achieving consistent end-product performance in demanding electrical and electronic environments.

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation:D351 −18
Standard Classification for
Natural Muscovite Block Mica and Thins Based on Visual
Quality
This standard is issued under the fixed designation D351; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
This standard has been approved for use by agencies of the U.S. Department of Defense.
1. Scope 1.6.1 Form 1—Full-trimmed natural block mica, 0.007 in.
(0.178 mm) minimum thickness.
1.1 This classification covers the determination of commer-
1.6.2 Form 2—Partially-trimmed natural block mica, 0.007
ciallyavailablenaturalmuscoviteblockmicaandisintendedto
in. minimum thickness.
be independent of the basic color of the mica or its source.
1.2 Muscovite mica is characterized by having an optical 1.7 Thebasiccolorofmica,suchaswhite,ruby,lightgreen,
axial angle between 50 and 75° (see Appendix X1); and has a
dark green, brownish green, and rum, as well as other colors,
weight loss when heated for 5 min at 600°C not exceeding and the method of controlling the color and other problems
0.2 % (based on the weight after drying at 120°C).
associated with the basic color, are not a part of this classifi-
cation.
1.3 The visual system of classifying the quality of natural
muscovite mica covered by this specification is based upon
1.8 The values stated in inch-pound units are to be regarded
relative amounts of visible foreign inclusions such as air
as standard. The values given in parentheses are mathematical
bubbles,stains,andspotsincombinationwithrelativeamounts
conversions to SI units that are provided for information only
and types of waviness, as well as other physical properties. In
and are not considered standard.
thissystem,aperfectlyclear,transparent,flatspecimenofmica
1.9 Section 5 is technically identical to procedures specified
is the visual standard of perfection. Increasing amounts of
in ISO 67-1981.
visual defects lower the visual quality, and a total of 13 levels
of visual quality are covered by this standard. This method of
1.10 Section 6 differs somewhat in procedure from ISO
classification, generally known as the Bengal India System, is
5972-1978, but data obtained by either is expected to be
purely qualitative and is entirely dependent on personal opin-
identical.
ion and judgment.
1.11 Section 7 is technically identical to procedures speci-
1.4 The standards for visual quality classification that are
fied in ISO 2185-1972.
covered in this classification are the best commercially avail-
able concept of the various qualities and their relative posi- 1.12 This international standard was developed in accor-
dance with internationally recognized principles on standard-
tions. Variations in the methods of using and applying these
standards from those herein defined are specified by the ization established in the Decision on Principles for the
Development of International Standards, Guides and Recom-
purchaser, or defined by agreement between the supplier and
the purchaser. mendations issued by the World Trade Organization Technical
Barriers to Trade (TBT) Committee.
1.5 Standard size classifications are defined, based upon
available usable rectangular areas and the minimum dimen-
2. Referenced Documents
sions of the rectangles that the pieces will yield. Precautions to
be taken in making thickness measurements are also described.
2.1 ASTM Standards:
D374 Test Methods for Thickness of Solid Electrical Insu-
1.6 This standard covers the following two definite forms of
lation (Metric) D0374_D0374M
commercial preparation:
D1711 Terminology Relating to Electrical Insulation
This classification is under the jurisdiction of ASTM Committee D09 on
Electrical and Electronic Insulating Materials and is the direct responsibility of
Subcommittee D09.01 on Electrical Insulating Products. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved Nov. 1, 2018. Published November 2018. Originally contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
approved in 1932. Last previous edition approved in 2014 as D351 – 14. DOI: Standards volume information, refer to the standard’s Document Summary page on
10.1520/D0351-18. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D351−18
2.2 ISO Publications: 4. Significance and Use
ISO 67-1981 Muscovite Mica Blocks, Thins, and Films—
4.1 The properties included in this standard are those
Grading by Size
required to control the visual quality, usable area, thickness,
ISO 2185-1972 Muscovite Mica Blocks,Thins, and Films—
hardness, and stiffness.
Visual Classification
ISO 5972-1978 Mica Blocks, Thins, Films, and Splittings—
5. Grading for Size
Measurement of Thickness
5.1 Full-Trimmed—All full-trimmed mica blocks and thins
shall be fully trimmed to remove all cracks, holes, reeves, and
3. Terminology
cross-grainsaccordingtothequalitydesired.Asfaraspossible,
3.1 For definitions of terms relating to mica refer to Termi- allmarginalcracksaretoberemovedbyrecutting.Theaverage
nology D1711, Part III. area of the pieces for Grade 4 and larger shall be not more than
1.54 times the average area of the largest usable rectangles.
3.2 Definitions of Terms Specific to This Standard:
This would constitute a minimum yield of 65 %. For Grades 5
3.2.1 “A”—a series of rulings or striations intersecting at an
and smaller, the average area of the pieces shall be not more
angle of about 60°.
thantwotimestheaverageareaofthelargestusablerectangles.
3.2.2 blocks—mica thickness of 0.007 in. (0.178 mm) mini-
This would constitute a yield of 50 %.
mum thickness, full-trimmed, unless otherwise designated.
5.2 Half-Trimmed—Forhalf-trimmedmica,followthegrad-
3.2.3 buckle—one or more large depression(s) and eleva-
ingdescribedin5.1fortrimmedsideswithnocracksextending
tion(s). 1
from the trimmed sides, except for sizes 6 and 5 ⁄2 on which
only side must be trimmed free of cracks. On the untrimmed
3.2.4 cracks—irregular fractures within the crystal that are
sides,nodefectsofthedesignatedqualityareacceptablewithin
natural or arise from blasting, rough handling, and so forth.
the minimum rectangle of the designated grade.
3.2.5 full-trimmed mica—mica trimmed on all sides with all
5.3 Natural Block and Thins—Natural block and thins,
cracks, reeves, and cross-grains removed.
muscovite mica shall be graded for size according to the area
3.2.6 haircracks or hairline cracks—minute, irregular
of the rectangle (maximum rectangle for full-trimmed, maxi-
cracks that are barely noticeable until mica is split into films,
mum rectangle of designated quality for half-trimmed) having
resulting in production of torn films.
at least the minimum dimension of one side for the specified
3.2.7 reeves or cross grains—tangled laminations giving
grade. The area within such a rectangle shall meet the
imperfect cleavage, that results in tears or breaks during
requirements of the quality specified by the purchaser as listed
splitting.
in Table 1.
3.2.8 ribboned or rules mica—mica that breaks into narrow
5.4 Method of Grading for Size—TheStandardASTMChart
strips because of parallel fractures.
shown in Fig. 1 shall be used for grading natural block and
thins, muscovite mica according to size. In grading natural
3.2.9 ribs or ridges—crenulations in the form of steps.
block mica and thins for size, all dimensions apply to the
3.2.10 ripple—multiple short waves.
smaller surface measured from the foot of the bevel-trimmed
3.2.11 stains—stains arise from foreign materials, resulting
edge.The specimen to be graded shall be laid upon the chart so
in a partial or total loss of transparency. They are in the form
that it covers point O and has its maximum and minimum
of specks or patches of appreciable area for example, slight
dimensions extending along and covering lines OA and OB,
stain, “vegetable” stain, clay stain, black stain, red stain, black
respectively.Thespecimenshallbeshifteduntiltheusablearea
speckled, light dot or spot, black, red or green dot or spot, etc.
(see “inclusions” in Terminology D1711).
TABLE 1 ASTM Grade Sizes of Muscovite Uncut Mica Block and
3.2.12 stones and stone holes—small embedded crystals or
Thins
holes resulting from them.
Minimum
Area of
Dimen-
3.2.13 thins—knife-dressed mica, 0.002 in. (0.05 mm) to Minimum
ASTM
sion of
Rectangle
Grade
less than 0.007 in. (0.18 mm) in thickness. They are classified
One Side
Sizes
as follows:
Equiva- Equiva-
in. in.
lent, cm lent, mm
3.2.13.1 Thins, 0.002 to 0.004 in. (0.05 to 0.10 mm), and
OOEE Special 100 650 4 100
3.2.13.2 Thick-thins, 0.004 to 0.007 in. (0.10 to 0.18 mm).
OEE Special 80 520 4 100
3.2.14 unmanufactured mica—commercial form of mica EE Special 60 390 4 100
E Special 48 310 4 100
known as blocks, thins, films, and splittings, as described.
A-1 (Special) 36 235 3 ⁄2 89
No. 1 24 155 3 76
3.2.15 waves—alternate elevations and depressions that are
No. 2 15 97 2 51
classified as slight, medium, or heavy.
No. 3 10 65 2 51
No. 4 6 40 1 ⁄2 38
No. 5 3 20 1 25
1 1 7
No. 5 ⁄2 2 ⁄4 15 ⁄8 22
Available fromAmerican National Standards Institute (ANSI), 25 W. 43rd St., No. 6 1 6.5 ⁄4 19
4th Floor, New York, NY 10036, http://www.ansi.org.
D351−18
NOTE 1—This chart is readily extended for Grades OOEE Special, OEE Special, EE Special, and E Special, using the information given in Table 1
of ASTM Specification D351.
FIG. 1ASTM Chart for Grading Natural Muscovite Block and Film Mica
completely covers the largest rectangle, determined by a avoid scratching the mica and accumulating mica dust under
diagonal extending from point O to or beyond a point on any the micrometer surfaces, thereby causing false readings.
of the curves. The number of the curve at the greatest distance
from O cut by the diagonal of the rectangle designates the
7. Classification of Visual Quality of Block Mica
grade of the specimen.
7.1 The classification of the visual quality of muscovite
blocks and thins shall fall into the following thirteen catego-
6. Thickness of Block Mica
ries:
6.1 Measure the thickness with a micrometer as specified in
7.1.1 V-1 Clear,
Test Methods D374. In determining thickness of mica that
7.1.2 V-2 Clear and Slightly Stained,
must be kept within small permissible variations, or where two
7.1.3 V-3 Fair Stai
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: D351 − 14 D351 − 18
Standard Classification for
Natural Muscovite Block Mica and Thins Based on Visual
Quality
This standard is issued under the fixed designation D351; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
This standard has been approved for use by agencies of the U.S. Department of Defense.
1. Scope
1.1 This classification covers the determination of commercially available natural muscovite block mica and is intended to be
independent of the basic color of the mica or its source.
1.2 Muscovite mica is characterized by having an optical axial angle between 50 and 75° (see Appendix X1); and has a weight
loss when heated for 5 min at 600°C not exceeding 0.2 % (based on the weight after drying at 120°C).
1.3 The visual system of classifying the quality of natural muscovite mica covered by this specification is based upon relative
amounts of visible foreign inclusions such as air bubbles, stains, and spots in combination with relative amounts and types of
waviness, as well as other physical properties. In this system, a perfectly clear, transparent, flat specimen of mica is the visual
standard of perfection. Increasing amounts of visual defects lower the visual quality, and a total of 13 levels of visual quality are
covered by this standard. This method of classification, generally known as the Bengal India System, is purely qualitative and is
entirely dependent on personal opinion and judgment.
1.4 The standards for visual quality classification that are covered in this classification are the best commercially available
concept of the various qualities and their relative positions. Variations in the methods of using and applying these standards from
those herein defined may be are specified by the purchaser, or defined by agreement between the supplier and the purchaser.
1.5 Standard size classifications are defined, based upon available usable rectangular areas and the minimum dimensions of the
rectangles that the pieces will yield. Precautions to be taken in making thickness measurements are also described.
1.6 This standard covers the following two definite forms of commercial preparation:
1.6.1 Form 1—Full-trimmed natural block mica, 0.007 in. 0.007 in. (0.178 mm) minimum thickness.
1.6.2 Form 2—Partially-trimmed natural block mica, 0.007 in. minimum thickness.
1.7 The basic color of mica, such as white, ruby, light green, dark green, brownish green, and rum, as well as other colors, and
the method of controlling the color and other problems associated with the basic color, are not a part of this classification.
1.8 The values stated in inch-pound units are to be regarded as standard. The values given in parentheses are mathematical
conversions to SI units that are provided for information only and are not considered standard.
1.9 Section 5 is technically identical to procedures specified in ISO 67-1981.
1.10 Section 6 differs somewhat in procedure from ISO 5972-1978, but data obtained by either should is expected to be
identical.
1.11 Section 7 is technically identical to procedures specified in ISO 2185-1972.
1.12 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
This classification is under the jurisdiction of ASTM Committee D09 on Electrical and Electronic Insulating Materials and is the direct responsibility of Subcommittee
D09.01 on Electrical Insulating Products.
Current edition approved Jan. 15, 2014Nov. 1, 2018. Published March 2014November 2018. Originally approved in 1932. Last previous edition approved in 20082014
ε1
as D351 – 97 (2008)D351 – 14. . DOI: 10.1520/D0351-14. 10.1520/D0351-18.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D351 − 18
2. Referenced Documents
2.1 ASTM Standards:
D374 Test Methods for Thickness of Solid Electrical Insulation (Metric) D0374_D0374M
D1711 Terminology Relating to Electrical Insulation
2.2 ISO Publications:
ISO 67-1981 Muscovite Mica Blocks, Thins, and Films—Grading by Size
ISO 2185-1972 Muscovite Mica Blocks, Thins, and Films—Visual Classification
ISO 5972-1978 Mica Blocks, Thins, Films, and Splittings— Measurement of Thickness
3. Terminology
3.1 For definitions of terms relating to mica refer to Terminology D1711, Part III.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 “A”—a series of rulings or striations intersecting at an angle of about 60°.
3.2.2 blocks—mica thickness of 0.007 in. (0.178 mm) minimum thickness, full-trimmed, unless otherwise designated.
3.2.3 buckle—one or more large depression(s) and elevation(s).
3.2.4 cracks—irregular fractures within the crystal that may be are natural or may arise from blasting, rough handling, etc.and
so forth.
3.2.5 full-trimmed mica—mica trimmed on all sides with all cracks, reeves, and cross-grains removed.
3.2.6 haircracks or hairline cracks—minute, irregular cracks that are barely noticeable until mica is split into films, resulting
in production of torn films.
3.2.7 reeves or cross grains—tangled laminations giving imperfect cleavage, that results in tears or breaks during splitting.
3.2.8 ribboned or rules mica—mica that breaks into narrow strips because of parallel fractures.
3.2.9 ribs or ridges—crenulations in the form of steps.
3.2.10 ripple—multiple short waves.
3.2.11 stains—stains arise from foreign materials, resulting in a partial or total loss of transparency. They may be are in the form
of specks or patches of appreciable area for example, slight stain, “vegetable” stain, clay stain, black stain, red stain, black
speckled, light dot or spot, black, red or green dot or spot, etc. (see “inclusions” in Terminology D1711).
3.2.12 stones and stone holes—small embedded crystals or holes resulting from them.
3.2.13 thins—knife-dressed mica, 0.002 in. (0.05 mm) to less than 0.007 in. (0.18 mm) in thickness. They may be are classified
as follows:
3.2.13.1 Thins, 0.002 to 0.004 in. (0.05 to 0.10 mm), and
3.2.13.2 Thick-thins, 0.004 to 0.007 in. (0.10 to 0.18 mm).
3.2.14 unmanufactured mica—commercial form of mica known as blocks, thins, films, and splittings, as described.
3.2.15 waves—alternate elevations and depressions that may be are classified as slight, medium, or heavy.
4. Significance and Use
4.1 The properties included in this standard are those required to control the visual quality, usable area, thickness, hardness, and
stiffness.
5. Grading for Size
5.1 Full-Trimmed—All full-trimmed mica blocks and thins shall be fully trimmed to remove all cracks, holes, reeves, and
cross-grains according to the quality desired. As far as possible, all marginal cracks should are to be removed by recutting. The
average area of the pieces for Grade 4 and larger shall be not more than 1.54 times the average area of the largest usable rectangles.
This would constitute a minimum yield of 65 %. For Grades 5 and smaller, the average area of the pieces shall be not more than
two times the average area of the largest usable rectangles. This would constitute a yield of 50 %.
5.2 Half-Trimmed—For half-trimmed mica, follow the grading described in 5.1 for trimmed sides with no cracks extending from
the trimmed sides, except for sizes 6 and 5 ⁄2 on which only side must be trimmed free of cracks. On the untrimmed sides, no
defects of the designated quality are acceptable within the minimum rectangle of the designated grade.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Available from American National Standards Institute (ANSI), 25 W. 43rd St., 4th Floor, New York, NY 10036, http://www.ansi.org.
D351 − 18
5.3 Natural Block and Thins—Natural block and thins, muscovite mica shall be graded for size according to the area of the
rectangle (maximum rectangle for full-trimmed, maximum rectangle of designated quality for half-trimmed) having at least the
minimum dimension of one side for the specified grade. The area within such a rectangle shall meet the requirements of the quality
specified by the purchaser as listed in Table 1.
5.4 Method of Grading for Size—The Standard ASTM Chart shown in Fig. 1 shall be used for grading natural block and thins,
muscovite mica according to size. In grading natural block mica and thins for size, all dimensions apply to the smaller surface
measured from the foot of the bevel-trimmed edge. The specimen to be graded shall be laid upon the chart so that it covers point
O and has its maximum and minimum dimensions extending along and covering lines OA and OB, respectively. The specimen
shall be shifted until the usable area completely covers the largest rectangle, determined by a diagonal extending from point O to
or beyond a point on any of the curves. The number of the curve at the greatest distance from O cut by the diagonal of the rectangle
designates the grade of the specimen.
6. Thickness of Block Mica
6.1 Measure the thickness with a micrometer as specified in Test Methods D374. In determining thickness of mica that must
be kept within small permissible variations, or where two or more specimens are to be measured at once, use Method A of Test
Methods D374. Where the maximum accuracy is not required, Method B may be used.is suitable.
6.2 Because of the abrasive nature of mica, test the micrometer frequently for conformity to the requirements specified in Test
Methods D374. Clean the anvil and spindle as frequently as necessary to prevent the accumulation of mica dust on the surfaces
and to preserve the accuracy of the measurements. To clean, close the micrometer lightly on a clean sheet of bond paper and move
the paper between the surfaces.
6.3 Be careful, when moving from one measurement location to another, to maintain the surfaces of the anvil and spindle
parallel to the surfaces of the specimen at all times, so as to avoid scratching the mica and accumulating mica dust under the
micrometer surfaces, thereby causing false readings.
7. Classification of Visual Quality of Block Mica
7.1 The classification of the visual quality of muscovite blocks and thins shall fall into the following thirteen categories:
7.1.1 V-1 Clear,
7.1.2 V-2 Clear and Slightly
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