The dish is placed on a silica triangle in a hood and
heated with the lean flame of a Fisher burner until the
sample ignites and burns. The heating is continued,
checking that the sample burns at a uniform and
moderate rate and only ash and carbon remain at the
end. One ml of concentrated nitric acid is added drop
by drop over the whole surface of the sample, which is
gently heated until complete elimination of the brown
nitrogen dioxide vapour. Afterwards the dish is heated
at high temperature for a further 1 min.