7.3.2.3 Procedure
First, the porcelain crucible were annealed in the muffle furnace at 700 °C for 1 h,
cooled again in the desiccator and weighed. About 2 g of each sample was weighed
into the porcelain crucible (triple determination).
The crucibles were pre-incinerated in the rapid incineration until no more smoke
occurred and the colour of the samples had changed to grey. Then, the samples were
annealed in the muffle furnace at 700 °C until the constant weight is reached and the
colour of the samples had changed to light grey or white.
Consequently, the samples were cooled in the desiccator, weighed and finally, the ash
was calculated according to eq. 2.