ISO 7404-2:2025

International
Standard
ISO 7404-2
Third edition
Coal — Methods for petrographic
2025-10
analysis —
Part 2:
Method of preparing coal samples
Charbon — Méthodes d'analyse pétrographique —
Partie 2: Méthode de préparation des échantillons de charbon
Reference number
© ISO 2025
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Published in Switzerland
ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Definitions . 1
4 Principle . 1
5 Reagents and materials . 1
6 Apparatus . 2
7 Procedure . 4
7.1 Preparation of the coal sample for making a particulate block .4
7.1.1 Sample.4
7.1.2 Drying .4
7.1.3 Size reduction . . .4
7.1.4 Sample division . . .4
7.2 Preparation of particulate block .4
7.3 Preparation of polished surface of particulate block .4
7.4 Examination of the polished surface .5
7.5 Storage prior to reflectance analysis .5
7.6 Re-examination of a particulate block .5
Annex A (informative) Examples of procedures for the preparation of a polished particulate
block suitable for petrographic analysis from a sample of crushed coal . 6
Bibliography .12

iii
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out through
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with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types
of ISO document should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
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Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 27, Coal and coke, Subcommittee SC 5, Methods
of analysis.
This third edition cancels and replaces the second edition (ISO 7404-2:2009), which has been technically
revised.
The main changes are as follows:
— input from the International Committee for Coal and Organic Petrology (ICCP) has been added;
— clarified that ion-milling equipment is not acceptable for use in preparation of samples for reflected light
microscopy.
A list of all parts in the ISO 7404 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

iv
Introduction
Petrographic analyses have been recognized internationally as important in the context of the genesis,
vertical and lateral variation, continuity, metamorphism and usage of coal. The International Committee
for Coal and Organic Petrology (ICCP) has made recommendations concerning nomenclature and analytical
[1][2][3][4][5][6]
methods and has described in detail the characteristics of a wide range of coals. This document
incorporates many useful comments made by members of the ICCP and by member bodies of ISO/TC 27.
Petrographic analyses of single-seam coals provide information about the rank, the maceral and
microlithotype compositions and the distribution of mineral matter in the coal. The reflectance of vitrinite
is a useful measure of coal rank and the distribution of the reflectance of vitrinite in a coal blend. Together
with a maceral group analysis, it can provide information about chemical and technological properties of the
coal and coal blend. Various other applications, like the characterization of bulk samples, cargoes, etc., and
the precise determination of different rank vitrinites in complex coal blends are in use.
The ISO 7404 series is concerned with the methods of petrographic analysis currently employed in
characterizing coal in the context of its technological or geological use, or both. It establishes a system for
petrographic analysis.
The varied petrographic composition and hardness of coal and the type and amount of included mineral
matter does not permit the formulation of a precise procedure that can be applied with equal success to all
types and ranks of coal. For example, a successful preparation method for use with medium- and high-rank
coals might not be applicable among low-rank coals. Within these limits, therefore, this document allows
the operator to apply individual skills and experience to the preparation of a satisfactory polished surface.
Nevertheless, recommended procedures that have been found applicable to a variety of coals, are given in
the Annex A, which is for information only.
Many processes are involved between the mining of the coal and its preparation for industrial use.
Petrographic analysis can be required at any stage on samples from the coal seam in situ, from borehole cores,
on the raw product from the colliery, on the products from the preparation plant, or on the final product.
The amount and size distribution of the coal being investigated thus varies widely and it is important to
ensure that the sample obtained for petrographic analysis is fully representative.

v
International Standard ISO 7404-2:2025(en)
Coal — Methods for petrographic analysis —
Part 2:
Method of preparing coal samples
1 Scope
This document specifies methods for preparing a polished particulate block from a sample of crushed
coal for analysis by reflected light microscopy. These methods can also be applied to the preparation of a
polished, embedded lump of coal.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
requirements of this document. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
ISO 3310-1, Test sieves — Technical requirements and testing — Part 1: Test sieves of metal wire cloth
ISO 7404-1, Coal — Methods for petrographic analysis — Part 1: Vocabulary
ISO 18283, Coal and coke — Manual sampling
International Committee for Coal and Organic Petrology (ICCP), International Handbook of Coal Petrography
3 Definitions
For the purposes of this document, the terms and definitions given in the ICCP International Handbook of
Coal Petrography and ISO 7404-1 apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
4 Principle
A representative sample of air-dried coal is crushed to a specified particle size and mixed with a suitable
binder. The mixture is formed into a particulate block, one face of which is ground and polished to give a
relief-free and scratch-free surface for analysis by reflected light microscopy.
5 Reagents and materials
5.1 Binder, used to hold the particles of crushed coal together as a particulate block, or to embed a lump
of coal.
The properties of the binder shall be such that:
a) there shall be no chemical reaction with the coal or immersion medium;

b) for liquid binders such as polyester resin, the curing temperature required to make the particulate block
should not exceed 100 °C and a temperature of less than 60 °C is preferable;
c) for thermoplastic mounting materials such as polymethyl methacrylate (PMMA) powder, a temperature
of about 120 °C is required for proper annealing, which makes it inadequate for low-rank coals;
d) the surfaces of the coal particles shall be easily wetted and there shall be good penetration of pores
and cracks;
e) the coal particles shall be held securely during grinding and polishing;
f) there shall be a marked contrast with the coal particles when immersed in medium and focused under
the reflected light microscope;
g) the hardness shall be comparable with that of the coal so that a flat, relief-free and scratch-free surface
can be obtained by grinding and polishing;
h) there shall be no large volume changes during curing, which can cause possible damage to the coal
particles;
i) the viscosity of liquid binder shall be such that the tendency of coal grains to segregate due to density
and size is minimized.
5.2 Mould release agent, that does not affect the coal and mounting compound, nor damage the mould.
5.3 Grinding abrasives, consisting of silicon carbide papers of decreasing grain size [53,5 µm (240 grit or
P280), 23,6 µm (400 grit or P800), 16,0 µm (600 grit or P1200)].
Metal-bonded, diamond-embedded 15 µm grinding disks may be used as a substitute for the smaller-grain-
size silicon carbide paper.
5.4 Polishing abrasives, consisting of metal oxide powders, colloidal silica suspension, or diamond pastes
of decreasing grain size.
A polishing abrasive having a maximum particle size not exceeding 0,05 µm shall be used for the final
polishing stage.
The number of polishing stages depends on the grain size of the abrasive used at the final stage of grinding
and on the grain size of the polishing abrasives available. It is recommended that aluminium oxide powders
be used throughout and that an abrasive having a maximum particle size of 0,3 µm be used for the
penultimate polishing stage.
5.5 Lap cloths, made of cotton, silk or synthetic fabric with a minimum of nap.
6 Apparatus
6.1 Test sieve, having an aperture 1,00 mm, in accordance with the requirements of ISO 3310-1, with a
suitable lid and receiver.
6.2 Grinding mill or mortar and pestle, suitable for crushing 0,3 kg to 0,45 kg of coal to pass through the
test sieve (6.1), with the minimum production of fines.
The grinding mill may be manually or electrically operated.

6.3 Press, for use when pressure is required during curing, for example when using Polymethyl
methacrylate (PMMA).
1)
It shall be capable of producing a pressure of up to 21 MPa and may be a simple hand operated lever, a
torque-wrench, or a hydraulic press.
6.4 Moulds, to hold the mixture of coal and binder during the curing process.
In simple moulding, these can be made from heavy-gauge aluminium foil, but reusable moulds can be made
from silicone rubber, flexible plastic, aluminium or steel. For pressure moulding, a cuboid or cylindrical steel
mould equipped with a removable base and cap or other means of removing the block from the mould after
curing. The interior surfaces of the metal mould should have a ground finish.
Metal moulds for use in pressure moulding shall be capable of withstanding double the pressure normally
applied in making the particulate block. The internal dimensions of the mould shall be such that the face of
the block that will be polished has a surface area of at least 500 mm .
NOTE For reflectance analysis, if the coal is deficient in vitrinite, it can be necessary to make more than one block
of minimum size.
6.5 Pelletizing machine, consisting of an automatic mounting press that can be pre-programmed for
mould size, type of binder (thermosetting, thermoplastic), heating time, cooling time, initial temperature,
and curing pressure.
NOTE These have been found to be time-saving and produce blocks of consistent quality. By employing a spacer in
a tall mould, two pellets can be made during one cycle.
6.6 Containers, disposable, suitable for mixing the required amounts of coal and binder.
NOTE Wax-coated containers are unsuitable for liquid binders.
6.7 Machine for grinding and polishing, either with stationary or rotating laps for manual polishing,
or automatic grinder/polishers which have been found to save time, equipped with interchangeable lapping
discs for each of the grinding and polishing stages. Ion-milling polishing and grinding machines are not an
acceptable means for the preparation of coal samples for analysis by reflected light microscopy.
The machine shall be fitted with a contra-rotating specimen holder of the type in which the specimen is
held rigidly and is not free to rotate independently of the holder. The specimen holder shall have a means of
varying the load on the specimen.
6.8 Sample cleaner, consisting of a means of cleaning the surface of the particulate block between the
successive stages of grinding and polishing. This can consist of an ultrasonic cleaning bath.
Jets of tap water and distilled water shall be used in the cleaning process. If necessary, a water filter shall be
used to remove solid particulates from the water supply before it is used in cleaning and polishing.
6.9 Desiccator, consisting of an airtight container which maintains an atmosphere of low humidity
through the use of a suitable drying agent such as silica gel beads which occupy the bottom part of the
desiccator.
6.10 Reflected light microscope, equipped with a dry objective lens at a magnification of approximately
100x to 250x.
6 2
1) 1 MPa = 10 N/mm = 145 psi.

7 Procedure
7.1 Preparation of the coal sample for making a particulate block
7.1.1 Sample
Obtain a representative sample of the coal being examined. For most purposes, it is convenient to take this
sample after the first stage in the preparation of the laboratory sample for general analysis in accordance
with the requirements of ISO 18283.
7.1.2 Drying
Air-dry the sample (7.1.1) in accordance with the requirements of ISO 18283 to facilitate crushing and sample
division and to avoid interference with the curing of the binder in the preparation of the particulate block.
7.1.3 Size reduction
Reduce the size of the particles to an upper limit of 1 mm.
The reduction in the size of the coarse particles should be carried out using a grinding mill (6.2) adjusted
to give a product crushed with minimum production of fines. If a mortar and pestle (6.2) is used, sieve and
grind the oversize repeatedly until all the coal passes the specified size.
7.1.4 Sample division
Divide the sample using a riffle or small rotary sample divider to obtain a laboratory sample of 50 g to 100 g
of coal in accordance with the requirements of ISO 18283. The laboratory sample may be stored in a sealed
container prior to preparation.
7.2 Preparation of particulate block
The objective is to prepare a suitably thick particulate block that, when polished, exposes a surface
comprised of at least 50 % coal.
NOTE 1 A polished surface with this percentage of coal reduces the time of analysis and any tendency towards the
segregation of particles due to size and density.
The precise procedure for preparing a particulate block depends on the type of binder, mould and whether a
press is used. Provided that the materials and apparatus conform with the requirements of Clauses 5 and 6,
the steps in the procedure may be chosen by the operator.
NOTE 2 In the case of blocks made with epoxy resin, an elevated temperature can be used to promote curing of the
binder. When rapid curing is not required, curing can be carried out at ambient temperature.
7.3 Preparation of polished surface of particulate block
For cylindrical-shaped samples prepared by pressure moulding, grind and polish one end face of the
particulate block, by hand or by using a grinding and polishing machine (6.7) and a series of abrasives of
decreasing particle size. The block may be held manually or by means of a specimen holder.
Particulate blocks made with liquid binders and cured in the absence of pressure under ambient conditions
may show gravity (density) stratification, with larger particles along the bottom or base of the block,
and finer material at the top. Grind and polish the sides of such stratified blocks, that is, the surfaces
perpendicular to the stratification.
Suitable materials for both grinding and polishing are described in 5.3 to 5.5, and A.2.3 to A.2.5. Carry out
the final polish with an abrasive having a maximum particle size not exceeding 0,05 µm.

Thoroughly wash the surface
...