Section 147(4) in The Coal Mines Regulations, 2011
(4)The incombustible dust used for the purpose of sub-regulation (3) shall be -(i)such that it does not contain more than 5% of free silica;(ii)of such fineness and character, that it is readily dispersible into the air and that, when used in plates which are not directly made wet by water from the strata it does not cake but it is dispersed into the air, when blown upon with mouth or by a suitable appliance; and(iii)as far as practicable light in colour.
