Ash content is a measure of the inorganic filler or coating content of paper, including calcium carbonate, china clay and titanium dioxide.
Ash content is determined by Smithers Pira on oven dried samples which are heated to 525°C (or 900°C) to ensure that all the combustible cellulose material is consumed. This leaves only the inorganic fraction of the material measured as the ash content, which is express as a percentage (%) of the original oven dried weight.
The grit content of a material is determined from the ash remains. The ash is filtered using water through sieves of different mesh sizes to trap the fine grit particles present in the material. These are then dried, weighed and expressed as a weight percentage.