ISO 23873:2025

International
Standard
ISO 23873
Second edition
Hard coal — Method for the
2025-09
measurement of the swelling
properties using a dilatometer
Houille — Méthode pour le mesurage des propriétés de
gonflement au moyen d'un dilatomètre
Reference number
© ISO 2025
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 2
5 Materials . 2
6 Apparatus . 2
6.1 Mould and accessories .2
6.2 Dilatometer .4
6.3 Dilatometer furnace .6
6.4 Temperature controller .6
6.5 Temperature recorder .6
6.6 Retort and piston .6
6.7 Means of recording piston movement .7
6.8 Cleaning instruments .7
6.9 Balance .7
7 Procedure . 7
7.1 Temperature calibration .7
7.2 Preparation of test sample and test pieces .8
7.2.1 Test sample .8
7.2.2 Test piece .8
7.3 Determination .9
7.3.1 Number of tests .9
7.3.2 Inspection of apparatus .9
7.3.3 Determination of dilatation .9
7.3.4 Cleaning of the furnace, retort and piston .10
8 Expression of results . 10
9 Precision . 10
9.1 General .10
9.2 Repeatability .11
9.3 Reproducibility .11
10 Test report .11
Bibliography . 14

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
ISO technical committees. Each member body interested in a subject for which a technical committee
has been established has the right to be represented on that committee. International organizations,
governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely
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)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
rights in respect thereof. As of the date of publication of this document, ISO had not received notice of (a)
patent(s) which may be required to implement this document. However, implementers are cautioned that
this may not represent the latest information, which may be obtained from the patent database available at
www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and expressions
related to conformity assessment, as well as information about ISO's adherence to the World Trade
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 second edition cancels and replaces the first edition (ISO 23873:2010), which has been technically
revised.
The main changes are as follows:
— clarified units of measure in Figures 1 and 2 and Table 1;
— added specifications for temperature recorder;
— added balance to Clause 6.
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
The dilatation of coal has been primarily measured by two different standard procedures: the Ruhr
dilatometer and the Audibert-Arnu dilatometer. Both methods are very similar, with the same basic
apparatus and heating rate, but use different initial masses of coal arising from the method of pencil
preparation.
This document combines the two previous procedures by adjusting the equipment specification to define a
single acceptable method.
v
International Standard ISO 23873:2025(en)
Hard coal — Method for the measurement of the swelling
properties using a dilatometer
1 Scope
This document describes a method for the measurement of the swelling properties of hard coal using a
dilatometer.
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 1213-2, Coal and coke — Vocabulary — Part 2: Terms relating to sampling, testing and analysis
ISO 13909-4, Coal and coke — Mechanical sampling — Part 4: Preparation of test samples of coal
ISO 18283, Coal and coke — Manual sampling
3 Terms and definitions
For the purposes of this document, the terms and definitions given in given in ISO 1213-2 and the following 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/
3.1
softening temperature
temperature at which the downward movement of the dilatometer piston is 0,5 mm, also known as
temperature of initial contraction
Note 1 to entry: See temperature θ in Figure 3.
3.2
temperature of maximum contraction
temperature at which the dilatometer piston reaches its lowest point
Note 1 to entry: See temperature θ in Figure 3.
3.3
temperature of maximum dilatation
temperature at which the dilatometer piston reaches its highest point
Note 1 to entry: See temperature θ in Figure 3.
3.4
maximum contraction
maximum downward movement of the dilatometer piston, measured from the zero point
Note 1 to entry: Maximum contraction is expressed as a percentage of the initial test-piece length.

Note 2 to entry: See c in Figures 3 and 4.
3.5
maximum dilatation
maximum upward movement of the dilatometer piston after contraction, measured from the zero point
Note 1 to entry: Maximum dilatation is expressed as a percentage of the initial test-piece length.
Note 2 to entry: See d in Figures 3 and 4. The value can be either positive or negative.
4 Principle
A test piece, in the form of a pencil prepared from powdered coal, is heated at a constant rate in a steel
retort positioned in a furnace. Two reference metals of known melting points have been used to calibrate
the temperature monitoring system. The change in level of a piston resting upon the test piece is observed
continuously, and a record is produced which is characteristic of the swelling properties of the coal.
5 Materials
The following materials are required for temperature calibration (7.1).
5.1 Graphite pencils, 30 mm long, base diameter 7,4 mm, top diameter 6,8 mm, with a small cylindrical
reservoir drilled in the narrow end of each pencil.
5.2 Metal balls, of the following reference metals:
a) lead, of analytical reagent grade, assay (Pb) 99,98 % minimum, melting point 327,0 °C;
b) zinc, assay (Zn) 99,87 % minimum, melting point 419,3 °C.
5.3 Water-based blacking.
6 Apparatus
6.1 Mould and accessories
6.1.1 Mould (see Figure 1), made from steel, case-hardened after machining. The bore shall be polished
after hardening and the bore and uniformity of taper (i.e. 1 in 50) shall conform to the dimensions given in
Table 1.
6.1.2 Mallet, plastics head, mass about 200 g.
6.1.3 Ram (see Figure 1).
6.1.4 Press (see Figure 1).
6.1.5 Load cell (see Figure 1), capable of registering a load of 0 kN to 15 kN.
6.1.6 Pencil gauge (see Figure 1).

Dimensions in millimetres
Key
1 mould cylinder
2 mould plug
3 mould base
4 ram
5 press plunger
6 press mould-support
7 press base-plate
8 load cell
9 pencil gauge
Figure 1 — Mould, press and accessories

Table 1 — Dimensions of mould
Bore
Distance from wide end
mm
mm
(tolerance: −0,00, +0,05)
0 7,4
10 7,2
20 7,0
30 6,8
40 6,6
50 6,4
60 6,2
70 6,0
6.2 Dilatometer
A general arrangement of suitable dilatometer apparatus giving critical dimensions is shown in Figure 2.

Dimensions in millimetres
Key
1 heated block
2 holes for retorts (∅ 15,0 ± 0,1)
3 threaded plug for retort
4 test piece
5 piston
6 retort
7 thermocouple
8 refractory insulation
9 pen mechanism
10 recording-drum stand
11 recording drum with chart
Figure 2 — Typical dilatometer apparatus
6.3 Dilatometer furnace
A furnace capable of heating two or more retorts (6.6) to a temperature of 550 °C at a rate of 3 °C/min is used.
A suitable furnace (for heating three retorts) is illustrated in Figure 2 and consists of a casing fitted with
a base and a top cover. The cover supports a cylindrical block of copper-aluminium alloy CA 104, of 65 mm
diameter and 460 mm long. The block has three holes of 380 mm minimum depth and 15,0 mm ± 0,1 mm
diameter, drilled as shown in Figure 2. The top surface may be insulated by an appropriately shaped
piece of board. The block is heated electrically by an insulated resistance winding, capable of raising the
temperature of the block to 550 °C at a rate of 3 °C/min. The space between the block and the casing is filled
with a thermal insulating material. A suitable temperature sensor, with a precision of at least equal to the
temperature controller described in 6.4, is positioned in the third retort in such a way that the sensor tip
lies centrally 30 mm above the internal base of the retort. The distance of 30 mm is established by using a
graphite pencil (5.1) as a means of measurement.
The furnace shall comply with the following operating conditions.
Heat the furnace to 400 °C, measuring the temperatur
...