Reference number  
DRS  409: 2019  
© RSB 2019  
 
 
 RWANDA 
STANDARD  DRS  
409 
 
First edition  
2019 -mm-dd 
 
School chalk — Specification  
 
  DRS  409: 2019  
©RSB 2019  - All rights reserved                  ii 
 In order to match with technological development and to keep continuous progress in industries, 
standards are subject to periodic review. Users shall ascertain that they are in possession of the latest 
edition  
 
 
 
 
 
 
 
 
© RSB 2019  
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized 
in any form or by any means, electronic or mechanical, including photocopying and microfilm, without 
prior written permission from R SB. 
Requests for permission to reprodu ce this document should be addressed to : 
Rwanda Standards  Board  
P.O Box 7099 Kigali -Rwanda  
KK 15 Rd, 49  
Tel. +250 788303492  
Toll Free: 3250  
E-mail: info@rsb.gov.rw  
Website: www.rsb.gov.rw   
ePortal: www.portal.rsb.gov.rw   DRS  409: 2019  
iii                ©RSB 2019  - All rights reserved  
 Contents  Page 
Foreword  ................................ ................................ ................................ ................................ ............................  iv 
1 Scope  ................................ ................................ ................................ ................................ ......................  1 
2 Normative references  ................................ ................................ ................................ ............................  1 
3 Terms and defi nitions  ................................ ................................ ................................ ...........................  1 
4 Requirements  ................................ ................................ ................................ ................................ .........  1 
4.1 Raw materials  ................................ ................................ ................................ ................................ ........  1 
4.2 General requirements  ................................ ................................ ................................ ...........................  2 
4.3 Specific requirements  ................................ ................................ ................................ ...........................  3 
5 Packaging and Labelling  ................................ ................................ ................................ ......................  3 
5.1 Packaging  ................................ ................................ ................................ ................................ ...............  3 
5.2 Labelling  ................................ ................................ ................................ ................................ .................  3 
6 Sampling  ................................ ................................ ................................ ................................ ................  4 
6.1 General precautions  ................................ ................................ ................................ ..............................  4 
6.2 Scale of sampling  ................................ ................................ ................................ ................................ .. 4 
6.3 Number of test  and criteria for conformity  ................................ ................................ .........................  5 
Annex  A (normative)  Measurement of length and diameter  ................................ ................................ .........  6 
A.1 Procedure  ................................ ................................ ................................ ................................ ...............  6 
A.2 Determination of weight ................................ ................................ ................................ ........................  6 
Annex  B (nor mative)  Determination of weight  ................................ ................................ ...............................  7 
B.1 Procedure  ................................ ................................ ................................ ................................ ...............  7 
B.2 Results  ................................ ................................ ................................ ................................ ....................  7 
Annex  C (normative)  Determination of transverse breaking strength  ................................ .........................  8 
C.1 Apparatus  ................................ ................................ ................................ ................................ ...............  8 
C.2 Procedure  ................................ ................................ ................................ ................................ ...............  8 
Annex  D (normative)  Determination of resistance to abrasion  ................................ ................................ .. 10 
D.1 Apparatus  ................................ ................................ ................................ ................................ .............  10 
D.2 Procedure  ................................ ................................ ................................ ................................ .............  11 
Annex  E (normative)  Determination of free moisture content  ................................ ................................ .... 13 
E.1 Procedure  ................................ ................................ ................................ ................................ .............  13 
E.2 Calculation  ................................ ................................ ................................ ................................ ...........  13 
Annex  F (normative)  D etermination of calcium sulphate content ................................ ..............................  14 
F.1 Reagents  ................................ ................................ ................................ ................................ ..............  14 
F.2 Procedure  ................................ ................................ ................................ ................................ .............  14 
F.3 Calculation  ................................ ................................ ................................ ................................ ...........  14 
Annex  G (normative)  Determination of calcium carbonate content  ................................ ..........................  16 
G.1 General  ................................ ................................ ................................ ................................ .................  16 
G.2 Procedure  ................................ ................................ ................................ ................................ .............  16 
 DRS  409: 2019  
©RSB 2019  - All rights reserved                  iv 
 Foreword   
Rwanda Standard s are prepared  by Technical Committees and approved by R wanda Standards Board (RSB) 
Board  of Directors in  accordance with the procedures of RSB, in compliance with Annex 3 of the WTO/TBT 
agreement on the preparation, adoption and application of standards.  
The main task of technical committees is to prepare national standards.  Final Draft Rwanda S tandards 
adopted by Technical committ ees are ratified  by me mbers of RSB Board of Director s for publication and 
gazettment as Rwanda Standards.  
DRS  409  was prepared by Technical Committee RSB/TC 24, Chemicals and Consumer Products . 
In the preparation of this standard, reference was made to the following standard:  
RS EAS 25 : 2000 School chalk  — Specification  
The assistance de rived from the above source is hereby acknowledged with thanks.  
Committee membership  
The following organizations were represented on the Technical Committee on Chemicals and consumer 
Products (RSB/TC 24) in the preparation of this standard.  
University of Rwanda - College of Science and Technology  
University of Rwanda - College of Education  
Star Construction and Consultancy  Ltd 
AB Chalk  
STB 
Unique Chalk  
Rwanda Investigation Bureau  
Here and Now Group Ltd  
AGROP Y Ltd  
AMEKI Color  DRS  409: 2019  
v                ©RSB 2019  - All rights reserved  
 Standards for Sustainability  
Rwanda  Standards Board  (RSB) – Secretariat  DRS  409: 2019  
©RSB 2019  - All rights reserved                  vi 
 Introduction   
Chalk used in school classrooms comes in slender sticks approximately 0.35 of an inch (nine millimeters) in 
diameter and 3.15 inches (80 millimeters) long. Lessons are often presented to entire classes on chalk -boards 
(or blackboards, as they were original ly called) using sticks of chalk because this method has proven cheap 
and easy.  
As found in nature, chalk has been used for drawing since prehistoric times, when, according to 
archaeologists, it helped to create some of the earliest cave drawings. Later, artists of different countries and 
styles used chalk mainly for sketches, and some such drawings, protected with shellac  or a similar substance, 
have survived. Chalk was first formed into sticks for the convenience of artists. The method was to grind 
natural chalk to a fine powder, then add water, clay as a binder, and various dry colo urs. The resultant putty 
was then rolled into cylinders and dried. Although impurities produce natural chalk in many colo urs, when 
artists made their own chalk they usually added pigments to render these colo urs more vivid. Carbon, for 
example, was used to enhance black, and ferric oxide (Fe 2 O 3 ) created a more vivid r ed.  
Chalk did not become standard in schoolrooms until the nineteenth century, when class sizes began to 
increase and teachers needed a convenient way of conveying information to many students at one time. Not 
only did instructors use large blackboards, b ut students also worked with individual chalkboards, complete 
with chalk sticks and a sponge or cloth to use as an eraser. These small chalkboards were used for practice, 
especially among the younger students. Pens dipped in ink wells were the preferred to ol for writing final copy, 
but these were reserved for older students who could be trusted not to make a mess: paper —made solely 
from rags at this time —was expensive.  
An important change in the nature of classroom chalk paralleled a change in chalk -boards . Blackboards used 
to be black, because they were made from true slate. While some experts advocated a change to yellow 
chalkboards and dark blue or purple chalk to simulate writing on paper, when manufacturers began to fashion 
chalkboards from synthetic m aterials during the twentieth century, they chose the color green, arguing that it 
was easier on the eyes. Yellow became the preferred color for chalk.  
Almost all chalk produced today is dustless. Earlier, softer chalk tended to produce a cloud of dust th at some 
feared might contribute to respiratory problems. Dustless chalk still produces dust; it's just that the dust settles 
faster. Manufacturers accomplish this by baking their chalk longer to harden it more. Another method, used by 
a French company, is to dip eighty percent of each dustless chalk stick in shellac to prevent the chalk from 
rubbing off onto the hands.  
 
 DRS  409: 2019  
1                ©RSB 2019  - All rights reserved  
 School chalks  — Specification  
1 Scope   
This Committee Draft specifies requirements, sampling and test methods for white and coloured chalks 
intended to be used on chalkboards.  
It applies to three types of chalks:  
a) Soft chalks;  
b) Hard chalks; and  
c) Soft dust -free chalks   
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. Fo r undated references, 
the latest edition of the referenced document (including any amendments) applies.  
RS ISO 17318,  Solid fertilisers and soil conditioners  — Determination of arsenic, cadmium. Chromium, lead 
and mercury contents.  
3 Terms and definitions   
For the purposes of this standard, the follow ing terms and definitions apply / the terms and definitions given in 
… and the following apply . 
3.1 
term  
text of the definition  
4 Requirements  
4.1 Raw materials  
4.1.1  The chalk shall be made from good quality calcium sulphate ( calcined gypsum, CaSO 4.½H2O), 
calcium carbonate (limestone, CaCO 3) or a mixture of both types of materials . The total content of the main 
materials shall not be less than 90%.   
©RSB 2019  - All rights reserved                  2 
 4.1.2  Chalks sticks shall not contain poisonous or toxic materials in quantity harmful or injurious to human 
life. 
4.1.3  The material shall be homogeneous mass free from grease, grit and sandy particles liable to scratch 
the surface written upon.  
4.2 General requirements  
4.2.1  Colour  
 4.2.1.1  The white chalks shall be clear without showing any traces of other colors.  
4.2.1.2  The coloured chalks shall be of the declared colour, preferably chosen from the following standard 
colours: blue, brown, green, red and yellow.  
4.2.2  Shape  
The product shall have  a circular cross -section and shall be uniformly tapered.  
4.2.3  Finish  
The finish shall be smooth and at least 75% of the chalks shall be free from external blowholes, depressions 
and signs of crumbling.  
4.2.4  Hardness  
Chalk sticks shall, freely without scratching or otherwise harming the chalkboard, produce visible and distinct 
marks that can be easily erased by a dry eraser such as a piece of dry cloth.  
4.2.5  Writing qualities and freedom from grit  
The chalks shall show no evidence of containing grit, flint, or sandy particles  and shall be capable of making 
continuous marks on a chalkboard.  
 
 
 
 DRS  409: 2019  
3                ©RSB 2019  - All rights reserved  
 4.3 Specific requirements  
The product shall comply with the requirements given in table 1 when tested in accordance with the methods 
prescribed therein.  
Table 1 – Specific requirements for s chool chalks  
S/N Parameters  Requirements  Test methods  
Soft Hard  Soft dust -
free 
1 Dimensions, 
mm Length  81.0 ± 2.0 76.0 ± 2.0 78.0 ± 2.0 Annex A  
Diameter  At the base  11.0 ± 1.0 9.5 ± 1.0 9.5 ± 1.0 
At the tip  9.5 ± 1.0 9.5 ± 1.0 9.5 ± 1.0 
2 Weight per 100 sticks, g, min  500 Annex B  
3 Transverse breaking s trength , kg, min.  1,200  1,800  1,200  Annex C  
4 Resistance to abrasion, m3 0.13 – 0.26 0.10 – 0.16 0.08 – 
0.16 Annex D  
5 Moisture content, % by mass, max.  12 12 12 Annex E  
6 Freedom from 
poisonous and 
toxic materials, 
mg/Kg, max  Lead as Pd  10 RS ISO 17318  
Arsenic as As 2O3 2 
Cadmium  7 
Chromium  10 
Mercury  2 
7 Quality of raw 
materials1, % by 
mass, min.  Calcium sulphate 
(as CaSO 4.2H 2O) 90 Annex F  
Calcium carbonate 
(as CaCO 3) 90 Annex G  
1   In case both calcium carbonate and calcium sulphate are used, the total content of the raw material should not be less than 90% and 
the portion should be defined by the manufacturer.   
5 Packaging and Labelling  
5.1 Packaging  
5.1.1  The product shall be packaged in suitable paperboard containers, which offer adequate protection of 
the content during transportation.  
5.1.2  The product shall be tightly packaged in the container and if required by the purchaser saw dust or 
other cushioning materials shall be used to fill up the empty spaces to prevent breakage.  
5.1.3  Unless otherwise agreed to between the purchaser and supplier, each box shall contain 144 chalks.  
5.2 Labelling  
All packages shall be labelled with the following information:  
a) The words “School chalk” followed by the specified colour of the chalks;  
b) Name of the material of the chalk;   
©RSB 2019  - All rights reserved                  4 
 c) Type of chalks;  
d) Number of chalks in the box;  
e) Manufacturer’s name, address and/or recognized trademark;  
f) The batch identification  
6 Sampling  
6.1 General precautions  
6.1.1  Samples shall be drawn from originally unopened and undamaged boxes.  
6.1.2  Samples shall be protected from abnormal exposure to heat, and shall not be allowed to come in 
contact with any liquid.  
6.1.3  Samples shall be handled as little as possible, a nd contact with wet or sweated hands shall be 
avoided.  
6.2 Scale of sampling  
6.2.1  In any consignment, all boxes of school chalks drawn from a single batch of manufacture shall 
constitute a lot. If the consignment is declared  or known to consist of different b atches of manufacture, the 
boxes belonging to the same batch shall be grouped together and each such group shall constitute a separate 
lot. 
6.2.2  For ascertaining the conformity of the lot to the requirements of the specification, tests shall be 
carried ou t for each lot separately. The number of boxes to be selected from lots of different sizes shall be in 
accordance with table 2. From each selected box, 10 chalks shall be picked up at random.  
6.2.3  The boxes shall be selected at random and to ensure random ness of selection, the following 
procedure is recommended for use.  
6.2.4  Count the boxes in a systematic manner as 1,2,3… up to r and so on, where r is the integral part of 
N/n (N and n being the number of boxes in the lot and number of boxes in the lot  a nd number of boxes to be 
selected respectively). Every nth box thus counted shall be selected till the requisite number of boxes is 
obtained.  
6.2.5  From each of the boxes thus selected, 10 chalks shall be picked up blind from different places.  
 
 DRS  409: 2019  
5                ©RSB 2019  - All rights reserved  
 Table 2 – Scale of sampling and permissible number of defectives  
Number of boxes in the 
loy (N)  Number of boxes to be 
selected n  Total number of chalks 
to be selected (n)  Permissible number of 
defective chalks  
Up to 5  1 10 0 
6 to 50  3 30 1 
51 to 100  5 50 2 
101 to 500  10 100 4 
More than 500  15 150 6 
 
6.3 Number of test and criteria fo r conformity  
The lot shall be deemed to comply with the requirements of the standard if, after inspection and testing, the 
samples taken are found to comply with all the requirements of the standard.  
 
  
©RSB 2019  - All rights reserved                  6 
 Annex  A 
(normative)  
 
Measurement of length and diameter  
A.1 Procedure  
Measure the length and diameter at two ends of only full -length chalk by means of callipers and a foot -rule 
calibrated in millimetres or with sliding calliper gauge.  
A.2 Determination of weight  
Weigh 10 chalks, drawn from the same box at a time and compute the mass per 144 chalks for each box.  
The mass computed for each box selected for tests shall not differ  by more than two percent from the 
average.  
 
  DRS  409: 2019  
7                ©RSB 2019  - All rights reserved  
 Annex  B 
(normative)  
 
Determination of weight  
B.1 Procedure  
Weigh 10 chalks, drawn from the same box at a time and compute the weight per 100 chalks for each box.  
B.2 Results  
The weight computed for each box selected for tests shall not differ by more than 2 percent from the average.   
©RSB 2019  - All rights reserved                  8 
 Annex  C 
(normative)  
 
Determination of transverse breaking strength  
C.1 Apparatus  
Metal apparatus , of such construction that a chalk can be supported on two parallel  edges while a load is 
applied through an anvil positioned midway between and parallel to the two suppor ts (figure C1), and capable 
of applying a steadily increasing load of 0 kg to 2 kg to the chalk under test, and of indicating the applied load.  
 
Figure C1 – Determination of transverse breaking strength  
C.2 Procedure  
C.2.1  Test, in turn, each chalk reserved i n terms of determination of transverse breaking strength.  
C.2.2  So secure the supporting edges that the distance between them is appropriate to the product being 
tested.  
C.2.3  Centre a chalk on the two supporting edges, and lower the loading anvil until it  touches the specimen 
under test.  
DRS  409: 2019  
9                ©RSB 2019  - All rights reserved  
 C.2.4  Increase the load steadily at a rate of 50 g to 100 g per second and record the load at which the 
specimen breaks.  
C.2.5  Determine whether  the lot complies with the requirements.   
©RSB 2019  - All rights reserved                  10 
 Annex  D 
(normative)  
 
Determination of resistance to abrasion  
D.1 Apparatus  
D.1.1  Sliding platform A, of width 190 mm, of length 330 mm, fitted with clamps 250 mm apart and 
suitable for clamping down the standard abrasive paper (see C.1.5), and with a stroke of 230 mm.  
D.1.2  Bracket B, of length 18 0 mm, fitted to the base on which the platform slides, and with two holes in 
each upright so that the spindle of the balance arm C can be supported either 38 mm or 57 mm above the top 
of the sliding platform.  
D.1.3  Balance arm C, fitted with platform D and  an adjustable counter -mass E, and that is capable of 
sliding on its spindle along the length of the bracket B.  
D.1.4  Holder F, secured to the centre of the lower surface of platform D, and in which a chalk can be 
securely clamped, the clamp being so const ructed that it will cont damage the test specimen.  
D.1.5  Abrasive paper , standard waterproof silicon carbide, of grade No. 400.  
D.1.6  Graduated measuring cylinder , of capacity 100 cm3. 
  
Key: 
A sliding platform  
DRS  409: 2019  
11                ©RSB 2019  - All rights reserved  
 B bracket  
C balance arm  
D  platform  
E adjust able counter -mass  
F holder  
Figure D1 – Apparatus or determining resistance to abrasion  
D.2 Procedure  
D.2.1  Cut the appropriate specimens to a length of 57 mm of chalks. Abrade both ends of each specimen 
on a piece of abrasive paper until the ends are flat and square, then determine the mass of each specimen to 
the nearest milligram.  
D.2.2  Clamp the abrasive paper to the sliding platform. With the balance arm installed at the appropriate 
height given in table D1, secure the test specimen vertically in holder F ( in case of tapered chalks, clamp the 
thick end), and set the balance arm in equipoise by adjusting the position of the counter -mass. So adjust the 
specimen that the entire tip is in contact with the abrasive paper. If this is not possible, repeat D.2.1.  
D.2.3 Move the balance arm to one extreme of its travel on its spindle, load the platform D with the 
appropriate mass given in table D1 and draw the sliding platform out to its limit at a uniform rate and in such a 
way that the stroke is completed in 4 s to 5 s. so move the balance arm on its spindle that the end of the chalk 
rests on a clean part of the abrasive paper and next to the line formed by the previous stroke. Push the sliding 
platform in at a uniform rate and in such a way that the stroke is comple ted in 4 s to 5 s. Repeat this 
procedure until 10 strokes have been made.  
Table D1 – Resistance to abrasion  
Product  Height of balance arm 
(above sliding platform)  Mass to be placed on 
platform  
Chalks (soft, hard and 
soft dust -free) 57 113 
D.2.4  Remove the test specimen from the holder, determine the loss in mass by again determining the 
mass to the nearest milligram and set it aside for the determination of apparent density.  
D.2.5  Repeat the test on each specimen in the lot.  
D.2.6  Determine, to the near est tenth of a gram, the combined mass of the abraded specimens and set 
them aside in terms of D.2.4 and D.2.5. Determine the volume of each specimen as follows:  
a) Soft and hard chalks : immerse four specimens in water for 1 h. Wipe off the excess water with a 
filter paper. Pour sufficient water into a 100 cm3 measuring cylinder to ensure that the specimens will 
be fully immersed. Record the volume of water, then immerse the specimens, ensuring that no air  
©RSB 2019  - All rights reserved                  12 
 bubbles are present, and determine the volume (in cubi c centimetres)  of water displaced by the 
specimens.   
b) Dust -free chalks : using callipers, measure the length and diameter or cross -section of each 
specimen and calculate its volume, in cubic centimetres.  
D.2.7  In the case of chalks, calculate the apparent de nsity of the specimens by dividing their total mass by 
their total volume. Using the figure for the mass of chalk abraded from each test specimen (see D.2.4), 
calculate the average mass of chalk abraded from each specimen and divide this by the apparent de nsity to 
obtain the average volume, in cubic centimetres, abraded from each specimen.  DRS  409: 2019  
13                ©RSB 2019  - All rights reserved  
 Annex  E 
(normative)  
 
Determination of  free moisture content  
E.1 Procedure  
Grind in a mortar about four chalks so as to pass through 1.00 mm sieve. Weigh accurately about 10 g of the  
ground sample and spread it in a thin layer in a Petri dish. Place it in an oven maintained at 100 ± 2 ºC till 
constant mass is obtained.  
E.2 Calculation  
Free  moisture ,% by mass =100 (𝑚−𝑚1)
𝑚 
where,  
 m = mass in g of the material taken for the test, and  
 m1 = mass in g of the material after drying.   
©RSB 2019  - All rights reserved                  14 
 Annex  F 
(normative)  
 
Determination of calcium sulphate content  
F.1 Reagents  
F.1.1  Quality of reagents  
Pure chemicals and distilled water shall be used in the test.  
NOTE  Pure chemicals shall mean chemicals that do not contain impurities, which affect the result of analysis.  
F.1.2  Ammonium acetate solution  
Dissolve 454 g of ammonium acetate in two litres of water. Add aqueous ammonia solution to make the 
solution distinc tly ammoniacal.  
F.1.3  Diatomaceous silica  
Wash kieselguhr f ree from soluble matter and dry to constant mass at 105 ºC to 110 ºC. 
F.1.4  Ammonium hydroxide , 20% by mass  
F.1.2  Ammonium wash solution  
Dilute 100 ml of aqueous ammonia solution to one litre with water.  
F.2 Procedure  
Grind the sample in a mortar so as to pass  through a 250 micro sieve. Place about 4 g of the sample, 
accurately weighed, in a 600 ml beaker. Add 350 ml of ammonium acetate solution stirring the mixture 
thoroughly so as to loosen the matter from the bottom of the beaker. Add about 0.2 g of diatomac eous silica, 
accurately weighed, to the mixture and heat at 70 ºC on a steam bath for 30 minutes with frequent stirring. 
Keep the solvent distinctly ammoniacal during this period by adding aqueous ammonia solution, if necessary. 
Filter the mixture through a weighed Gooch crucible applying suction, if  necessary. Stir frequently to keep the 
diatomaceous earth suspended in the liquid. Wash the residue with five 10 ml portions of hot ammonium 
acetate solution, draining thoroughly on each washing. Repeat washing  with eight 10 ml portions  of ammonia 
wash solution. Dry the crucible in an oven at 70 ºC to constant mass.  
F.3 Calculation  
Calcium  sulphate ,% by mass =𝑚−(𝑚1−𝑚2)−m3
𝑚𝑥100 DRS  409: 2019  
15                ©RSB 2019  - All rights reserved  
 Where,  
 m= mass in g of the material taken for the test , 
 m1= final mass in g of the crucible and the contents after drying , 
 m2= initial mass in g of the crucible plus mass in g of diatomaceous silica used as filter, and  
 m3= mass in g of moisture present in the sample.   
©RSB 2019  - All rights reserved                  16 
 Annex  G 
(normative)  
 
Determination of calcium carbonate content  
G.1 General  
G.1.1  The approach is known as a back titration. A measured mass of chalk from the cliff is weighed out 
and then dissolved completely in an excess of hydrochloric acid. The resultant solution is made up to the mark 
in a 250 ml volumetric flask.  
G.1.2  A titration is then carried to determine the concentration of the excess hydrochloric acid in the flask. 
Knowing the excess and the original amount of acid, it can work back (hence back titration) to find the amount 
of acid reacting with the calcium carbonate i n the chalk.  
G.2 Procedure  
G.2.1  Weigh out accurately 1.30 g of a sample of naturally occurring chalk;  
G.2.2  Dissolve this mass of chalk in 50 ml of 1M hydrochloric acid solution in a 250 ml beaker;  
G.2.3  Transfer the contents of the beaker and all washings to  a 250 ml volumetric flask and make up to the 
mark with distilled water;  
G.2.4  By pipette extracts 25 ml of the acid solution from the volumetric flask and transfers  it to a 250 ml 
conical flask;  
G.2.5  Titrate this aliquot of the acid solution with 0.1 M s odium hydroxide solution using phenolphthalein 
indicator: the end point occurs with the first faint pink tinge to the solution in the flask;  
G.2.6  Repeat the titration until you have concordant results, i.e. titres to within 0.1 ml of each other.  
 DRS  409: 2019  
17                ©RSB 2019  - All rights reserved  
 Bibliogr aphy  
[1] SANS 99:2008 , White chalks  
[2] SANS 223:2012 , Writing equipment  — Coloured chalks and pastels  
[3] IS 2694:1963 , Specification for school chalks moulded white  
[4] IS 4222:1967 , Specification for coloured chalks, moulded  
 
 DRS  409: 2019  
ICS 97.180  
 
 
©RSB 2019  - All right s reserved